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Thermal Tolerance, Herbivory and Tide Pool Distributions of Littorina sitkana and Littorina scutulata… Brownlee, Graham Robert Paul 2018-04-03

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Thermal Tolerance, Herbivory and Tide Pool Distributions of Littorina sitkana and Littorina scutulata: Implications for a Warming World  by  Graham Robert Paul Brownlee   A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF  THE REQUIREMENTS FOR THE DEGREE OF    BACHELOR OF SCIENCE  in  THE FACULTY OF SCIENCE  (Honours Environmental Sciences)   THE UNIVERSITY OF BRITISH COLUMBIA  (Vancouver)  April 3, 2018   I accept this thesis as conforming to the required standard                                 © Graham Robert Paul Brownlee   	 2	Abstract  As a consequence of climate change, both the magnitude and frequency of extreme temperature events are predicted to increase. Tide pools can experience dramatic fluctuations in their temperatures, and therefore may be strongly affected by future extremes. I looked to identify the effects of thermal variation among tide pools on the herbivorous intertidal snails Littorina sitkana and Littorina scutulata at April Point, Quadra Island, British Columbia. Through a thermal tolerance experiment, I found that L. scutulata is more tolerant to extreme temperatures than L. sitkana. I compared herbivory rates of the two species between a warmed (36 °C) and control (32 °C) temperature treatment, and found that each species has a unique mass-adjusted feeding rate, and that both had their rates inhibited by the hotter treatment. Through a tide pool heating experiment, I found that increased average and maximum temperatures did not contribute to changes in the abundances of adult L. sitkana, L. scutulata, the total abundance of both species, or the percentages of L. sitkana in the pools. Monitoring summer temperature and snail abundances in tide pools, the percentage of L. sitkana, the total abundances of snails, and the abundances of L. sitkana and L. scutulata changed through time, but were not influenced by average or maximum temperature, temperature variance, or temperature hours at or above thirty degrees Celcius. Thus, although the thermal tolerance patterns would suggest a shift towards L. scutulata in warmer (or warming) pools, I did not observe this pattern in the field. Nevertheless, the reduced herbivory rates for both L. scutulata and L. sitkana at high but realistic temperatures suggests that ongoing warming may lead to changes in littorine abundance and top-down control of macro-algae and epiphytes, but that other environmental, interactive, biological or behavioural factors may also contribute to how tide pool and the intertidal species respond to future change. 	 3	Table of Contents   Abstract ...................................................................................................................................... 2  Table of Contents ...................................................................................................................... 3  List of Tables ............................................................................................................................. 5  List of Figures ............................................................................................................................ 6  List of Abbreviations ................................................................................................................ 7  Acknowledgements ................................................................................................................... 8  1. Introduction ........................................................................................................................... 9             1.1 Temperature, Climate Change, and Marine Ecosystems .................................... 9             1.2 The Role of Temperature in the Rocky Intertidal ............................................. 10             1.3 Tide Pools ............................................................................................................... 12             1.4 Study Site and Research Goals ............................................................................ 13  2. Methodology ........................................................................................................................ 15             2.1 Experimental Methodology .................................................................................. 15                         2.1.1 Thermal Tolerance Experiment ........................................................... 15                         2.1.2 Herbivory Rate Experiment .................................................................. 17                         2.1.3 Environmental Temperature and Tide Pool Census Study ............... 20                         2.1.4 Tide Pool Heating Experiment ............................................................. 20             2.2 Data Analysis Methodology ................................................................................. 22                         2.2.1 Statistical and Graphical Methods ....................................................... 22                         2.2.2 Thermal Tolerance Experiment ........................................................... 22                         2.2.3 Herbivory Rate Experiment .................................................................. 23                         2.2.4 Environmental Temperature and Tide Pool Census Study ............... 24                         2.2.5 Tide Pool Heating Experiment ............................................................. 24  3. Results .................................................................................................................................. 26             3.1 Thermal Tolerance Experiment .......................................................................... 26             3.2 Herbivory Rate Experiment ................................................................................. 28                         3.2.1 Mass-Adjusted Herbivory Rate ............................................................ 29             3.3 Environmental Temperature and Tide Pool Census Study .............................. 31                         3.3.1 Change in Percent Littorina sitkana ..................................................... 32                         3.3.2 Change in Total Snail Abundance ........................................................ 34             3.4 Tide Pool Heating Experiment ............................................................................ 36                         3.4.1 Average and Maximum Temperature Test ......................................... 36                         3.4.2 Percent Littorina sitkana Response Ratio ............................................ 38   	 4	4. Discussion ............................................................................................................................ 40             4.1 Thermal Tolerance Experiment .......................................................................... 40             4.2 Herbivory Rate Experiment ................................................................................. 43             4.3 Environmental Temperature and Tide Pool Census, and             Tide Pool Heating Experiment .................................................................................. 45                                                          4.4 Implications and Future Directions .................................................................... 49  Bibliography ............................................................................................................................ 51  Appendix .................................................................................................................................. 58             A.1 Herbivory Rate Experiment ................................................................................ 58                         A.1.1 Fucus distichus Mass Loss: Uncorrected ............................................ 58             A.2 Environmental Temperature and Tide Pool Census ........................................ 60                         A.2.1 Change in Littorina scutulata Abundance ........................................... 60                         A.2.2 Change in Littorina sitkana Abundance .............................................. 62             A.3 Tide Pool Heating Experiment ............................................................................ 64                         A.3.1 Total Snail Abundance Response Ratio .............................................. 64                         A.3.2 Littorina scutulata Abundance Response Ratio .................................. 66                         A.3.3 Littorina sitkana Abundance Response Ratio ..................................... 68        	 5	List of Tables  2.1.1 Table of Thermal Tolerance Experiment Treatment Groups ................................... 16  3.1 Thermal Tolerance Experiment: Littorina sitkana and Littorina   scutulata Results ...................................................................................................................... 26  3.2.1 Herbivory Rate Experiment: Mass-Adjusted Littorina sitkana and   Littorina scutulata Results ...................................................................................................... 29  3.3.1 Environmental Temperature and Tide Pool Census Study: Change in   Percent Littorina sitkana Results ........................................................................................... 32  3.3.2 Environmental Temperature and Tide Pool Census Study: Change in   Total Snail Abundance Results .............................................................................................. 34  3.4.1 Tide Pool Heating Experiment: Average and Maximum   Temperature Results .............................................................................................................. 36  3.4.2 Tide Pool Heating Experiment: Percent Littorina sitkana Response Ratio   Results ...................................................................................................................................... 38  A.1.1 Herbivory Rate Experiment: Uncorrected Fucus distichus  Mass Loss Results ................................................................................................................... 58  A.2.1 Environmental Temperature and Tide Pool Census Study: Change in   Littorina scutulata Abundance Results .................................................................................. 60  A.2.2 Environment Temperature and Tide Pool Census Study: Change in   Littorina sitkana Abundance Results ..................................................................................... 62  A.3.1 Tide Pool Heating Experiment: Total Snail Abundance Response Ratio   Results ...................................................................................................................................... 64    	 6	A.3.2 Tide Pool Heating Experiment: Littorina scutulana Abundance   Response Ratio Results ........................................................................................................... 66  A.3.3 Tide Pool Heating Experiment: Littorina sitkana Abundance   Response Ratio Results ........................................................................................................... 68    List of Figures 1.4 Map of April Point, Quadra Island, BC Field Site ........................................................ 14  2.1.2 Herbivory Rate Experiment Temperature Profiles .................................................... 19  3.1 Thermal Tolerance Experiment: Tolerance Curves for Littorina sitkana and   Littorina scutulata .................................................................................................................... 27  3.2.1 Herbivory Rate Experiment: Effect of Temperature on Mass-Adjusted   Herbivory Rates of Littorina sitkana and Littorina scutulata .............................................. 30  3.3.1 Environmental Temperature and Tide Pool Census Study: Effects of   Temperature on Change in Percent Littorina sitkana ......................................................... 33  3.3.2 Environmental Temperature and Tide Pool Census Study: Effects of   Temperature on Change in Total Snail Abundance ............................................................ 35  3.4.1 Tide Pool Heating Experiment: Effects of Temperature Manipulation   on Average and Maximum Temperature ............................................................................. 37  3.4.2 Tide Pool Heating Experiment: Effects of Temperature Manipulation on   Percent Littorina sitkana Response Ratio ............................................................................. 39  A.1.1 Herbivory Rate Experiment: Effect of Treatment Group and   Temperature Profile on Fucus distichus Mass Lost ............................................................. 59    	 7	A.2.1 Environmental Temperature and Tide Pool Census Study: Effects of   Temperature on Change in Littorina scutulata Abundance ................................................ 61  A.2.2 Environmental Temperature and Tide Pool Census Study: Effects of   Temperature on Change in Littorina sitkana Abundance ................................................... 63  A.3.1 Tide Pool Heating Experiment: Effects of Temperature Manipulation   on Total Snail Abundance Response Ratio ........................................................................... 65  A.3.2 Tide Pool Heating Experiment: Effects of Temperature Manipulation   on Littorina scutulata Abundance Response Ratio ............................................................... 67  A.3.3 Tide Pool Heating Experiment: Effects of Temperature Manipulation   on Littorina sitkana Abundance Response Ratio .................................................................. 69  List of Abbreviations SS: Sum of Squares df: Degrees of Freedom               	 8	Acknowledgements    I would like to thank my supervisor, Dr. Chris Harley for his help in the design and completion of this study. Your expertise and knowledge has been invaluable, and I learn something new every time we talk. I greatly appreciate all the support you provided to help me complete my thesis, which has become the most rewarding part of my undergraduate degree.    Thank you to Cassandra Konecny for all your assistance in learning how to do statistical analyses, plot data, and for letting me bounce all my theories and thoughts off of you at the Biodiv table. Thank you to Olivia Schaefer and Veronika Franzova for assisting me in counting snails for my censuses out in the field, and for making the long drives and ferry rides more entertaining. Thank you to Gabby Doebeli for helping me with implementing my tide pool heating experiment. Thank you to Emily Lim for providing me with last-minute feedback on my draft during a difficult time. Thank you to Norah Brown and Colin Macleod for helping to teach me how to conduct my statistics in R, and thank you to Colin for your help with the incubator system and setting up my herbivory rate experiments. Finally, Thank you to all the other members of the Harley Lab for their contributions and support along the way.          	 9	 1 Introduction  1.1 Temperature, Climate Change, and Marine Ecosystems               While the future of anthropogenic Green House Gas (GHG) emissions is largely still to be determined through present-day and future policy decisions, many repercussions of modern human activity are already apparent. Atmospheric carbon dioxide (CO2) has increased by over 80 parts-per-million over the last 50 years, and 16 of the 17 warmest years on record have occurred since 2001 (NASA & NOAA, 2017). In both near and long-term predictive models, extreme temperature highs and ‘heat waves’, periods of sustained highs, are likely or very likely to increase in frequency, with the absolute value of these temperature extremes being expected to exceed the magnitude of the average global temperature increase (IPCC 2013 Chapter 11; IPCC 2013 Chapter 12; Sillmann et al, 2013). These predictions of future warming also extend to the world’s oceans, with near-term oceanic warming being relatively insensitive to emissions scenarios, and longer-term predictions being more sensitive, due to the timescales required for large-scale oceanic heat circulation (Glecker et al., 2006; Gregory, 2010; IPCC 2013 Chapter 11; IPCC 2013 Chapter 12). The impacts that climate change will have on marine ecosystems will be felt through changes in both oceanic and atmospheric systems, and understanding how these ecosystems will respond to climate change is necessary to inform management and policy practices to reduce their effects (Green et al., 2014).      	 10	1.2 The Role of Temperature in the Rocky Intertidal  The rocky intertidal is an extreme environment for the species that inhabit it. Dramatic fluctuations in temperature occur surrounding day and nighttime tides, and organisms are strongly influenced by the sun’s threat of extreme emersion temperatures and desiccation (Somero, 2002). Thermal stress is largely considered to be a key physiological stressor in the intertidal, and responses to extreme heat are often due to species-specific physiological mechanisms which directly contribute to how species interact, survive, and are distributed throughout the intertidal (Harley, 2003; Hawkins et al., 2008; Somero, 2002; Vasseur et al., 2014). These unique responses can therefore limit both the vertical and geographic ranges that a particular species are able to occupy, through both biological responses as well as interactions (Harley, 2003; Somero, 2002; Vasseur et al., 2014).    Temperature can affect an organism through its lethal limits of extreme temperature, otherwise known as an upper thermal tolerance limit. These upper limits are often visualized with Thermal Tolerance Curves, which quantify the temperatures at which mortality begins to occur for individuals of a species, up until all members of a particular treatment fail to survive. These upper thermal tolerance limits are believed to play an important role in the ranges that a species can occupy, both vertically and latitudinally (Somero, 2002; Sorte, 2011). Intertidal organisms are often at the forefront of responses to changes in climate, as many species occupy regions that experience temperatures bordering on their upper thermal tolerance limits (Somero, 2002).   	 11	A species’ fitness and performance response to environmental temperature often follows a nonlinear Thermal Performance Curve (TPC), which attempts to quantify an organism’s fitness or performance over a range of body temperatures (Sinclair et al., 2016). Along this curve, experiencing an ideal range of temperatures produces the greatest relative performance of an individual, while temperatures experienced outside of this range can lead to dramatic reductions in fitness or performance (Vasseur et al., 2014). Due to the asymmetry commonly found in TPC’s and the imperfect nature of thermal regulation in ecotherms, changes in the variation of colder and warmer environmental temperatures around a thermal optimum can produce vastly different performance responses, a phenomenon demonstrated through a mathematical relationship known as Jensen’s Inequality (Martin & Huey, 2008; Ruel & Ayres, 1999). Increases in mean environmental temperature that could potentially benefit an individual species can be counteracted by an increase in environmental temperature variance; these interactions can produce a variety of responses that may not be expected when examining only a mean increase and relating it to a species’ TPC (Vasseur et al. 2014). Because these TPC’s are species-specific, changes in the variance of environmental temperature within an ecosystem can produce a variety of positive, negative, or neutral performance responses depending on the species that experience them (Bozinovic et al, 2011; O’Connor, 2009; Vasseur et al, 2014). This variety of performance responses to environmental temperature fluctuations can thus shift the balance between interacting and competing species within an ecosystem (Kordas et al., 2011; O’Connor, 2009).   One of the crucial pathways through which temperature acts within intertidal ecosystems is through herbivory. Warming has been shown to increase per-capita interaction strengths between an herbivore and its food source, and range limits for algae can be determined not only 	 12	by abiotic stressors, but also through biotic interactions like herbivory (O’Connor, 2009; Harley, 2003). Herbivory can also act to enable spatial heterogeneity in community composition, direct trajectories of succession to a greater variety of community outcomes, and facilitate the establishment of important habitat-forming species such as barnacles by feeding on competing microalgae (Kordas, Donohue & Harley, 2017). The effects of temperature can also be more sensitive on different components of herbivory; metabolic rates have been shown to be more sensitive to changes in temperature than ingestion rates, which decreases the overall energy efficiency of intertidal herbivores (Iles, 2014). All of these pathways can contribute to shifts in the interactive balance of species within the intertidal, and complicate the relationship that increasing temperature extremes and heat waves may have with intertidal communities. Because of these complexities, further study needs to be conducted on these relationships to better our understanding of how the environment influences interactions and compositions of intertidal systems.   1.3 Tide Pools   Tide pools are important microhabitats that are found throughout many rocky intertidal ecosystems. They can vary dramatically in their physical structure, provide habitat complexity which contributes to the diversity of the intertidal, and have been found to have a greater richness of taxa than the surrounding emergent rock (Firth et al., 2014; Pastro et al., 2016; Underwood & Skilleter, 1996). Tide pools can support greater survival and more rapid growth of algae due to their lower temperatures and reduction in emersion time when compared to the surrounding rock, while the presence of grazers within a pool can drive algal abundances and diversity (Underwood & Jernakoff, 1984). While studies have been conducted on the structural 	 13	components of tide pools and how they influence species abundances and composition, there has been little research regarding how abiotic stressors impact these factors, how they directly relate to species-specific responses, and how those in turn relate to interactions. My research looked to further explore these important and understudied components of tide pools and the intertidal by examining how temperature can influence the abundances and interactions of species within tide pools through species-specific responses, and what the potential implications of climate change may be on these diverse microhabitats.   1.4 Study Site and Research Goals   The overarching objective of my thesis was to study the effects of temperature on tide pools and the organisms that inhabit them. To do this, I chose to do my study at April Point, on Quadra Island, British Columbia. Quadra Island is located at the northern end of the Strait of Georgia, between Vancouver Island and the mainland. At 35 kilometres in length, it possesses a unique shape, being less than 2 kilometres wide at its narrowest point, and upwards of 15 kilometres at its widest, and has a variety of beaches and rocky intertidal shores throughout its southern coast. On the southwest corner of Quadra, midway down the narrow finger of the island, the April Point (50.0642°N, 125.2344°W) rocky intertidal zone can be found (Fig 1.4). The resort’s shoreline has high, flat edges, with steppes downward towards the low intertidal. My studies were conducted, and individuals collected, from tide pools on a southwest-facing 400m stretch of shoreline.   Tide pools are abundant on the April Point shoreline, and are found in a variety of substrate colours, dimensions and structures, as well as species compositions and abundances. 	 14	The marine gastropods Littorina scutulata and Littorina sitkana were the dominant herbivores in the mid to high intertidal, as well as within the pools. Because of their high abundances, they were chosen to be the subjects of my study, for both the net effect they likely possessed on the April Point intertidal, as well as for their ease of collection for laboratory experiments. For this study, my goals were to identify potential differences in the lethal and sub-lethal responses to extreme temperatures by L. sitkana and L. scutulata, and to determine whether temperature influences the abundances and distribution of the two species within tide pools at April Point. My study hopefully builds upon the limited body of knowledge surrounding tide pools and their abiotic and biotic importance, and offer predictions as to the potential ecological impacts that climate may have on the intertidal, as well as on tide pools specifically.     Fig 1.4 Map of the Strait of Georgia, with April Point, Quadra Island, B.C. (50.0642°N, 125.2344°W) highlighted. 	 15	2. Methodology 2.1 Experimental Methodology 2.1.1 Thermal Tolerance Experiment   L. sitkana and L. scutulata were collected from tide pool populations at April Point, Quadra Island, on May 28, 2017. These individuals were collected from pools far away from pools used for the environmental temperature study, in order to avoid disturbing the pools and potentially influencing their abundances. I then transported the collected snails to the laboratory at the University of British Columbia, and placed them in containers within a recirculating seawater system (~13.4 °C).    To determine the thermal tolerances of the two species and identify any potential differences between them, on June 16, 2017 I removed 7 sets of 15 L. scutulata of similar sizes from the seawater table, and placed them within 7 50 mL falcon tubes, alongside 6 sets of 10 L. sitkana in 6 other tubes; the numbers of each species within each tube was dependent on the number of healthy individuals from the study site available for the trial. These tubes were then filled to the brim with water from the seawater system and sealed with a lid. I then placed the tubes into a mesh tray, alongside a single falcon tube fill with seawater devoid of snails, and the tray within a thermal bath (Thermo Scientific HAAKE A25) set to 13 °C, to be exposed to a range of previously experimentally determined temperatures. The temperature of the thermal bath was then increased by 0.5 °C every 10 minutes, for 11 hours and 20 minutes, until the final experimental temperature of 47 °C had been held for 10 minutes. Each tube corresponding to a particular L. sitkana and L. scutulata temperature treatment was removed after the 10-minute exposure to their final temperature (Table 2.1.1). At each removal period, the seawater-only tube 	 16	was opened, and the temperature within it recorded, using a thermocouple thermometer (Oaktron TEMP100 JTEK). When a treatment group was removed, I removed the individuals within the warmed tube and placed them into separate 50 mL tubes filled with seawater, secured the tubes with a mesh lid, partially submerged the tubes in the seawater system to maintain a constant temperature, and left them to recuperate for 75 hours.    After the 3-day recovery period, I placed the individuals from each temperature treatment group, for both species, upside down within a petri dish. Groups were given 10 minutes to right themselves and begin moving. Individuals who were capable of righting themselves and moving were considered to have survived, while individuals who were deceased or were too inhibited to function normally were considered to be ecologically deceased.   Table 2.1.1 Table of L. sitkana and L. scutulata treatment groups, thermal bath display temperatures, seawater tube internal temperatures, and time spent submerged in the bath.  Treatments Readout T (°C) Seawater Tube T (°C) Time in Bath (hours mins) Si 39 39.00 37.9 8 hours 40 mins Si 40 40.00 38.8 9 hours Si 41 Sc 41 41.00 40.0 9 hours 20 mins Si 42 Sc 42 42.00 40.8 9 hours 40 mins Si 43 Sc 43 43.00 41.9 10 hours Si 44 Sc 44 44.00 42.8 10 hours 20 mins Sc 45 45.00 43.9 10 hours 40 mins Sc 46 46.00 44.9 11 hours Sc 47 47.00 45.8 11 hours 20 mins  	 17	2.1.2 Herbivory Rate Experiment   I collected the L. sitkana and L. scutulata for this experiment from April Point tide pools on September 1, 2017, and transported them back to UBC, where they were placed in containers within the seawater table (~13.4 °C) and starved for the 5 days prior to the start of the trial. Four incubator units and two treatment groups were utilized in the experiment (Panasonic MIR-154-PA Cooled Incubator). These treatment groups were warmed and control temperature profiles, based on temperature data collected from April Point tide pools over the course of the summer. Both of the treatments had the same overnight temperature (12 °C) and light cycle, with the warmed temperature treatment increasing by 4.6°C increments to a maximum of 36.0 °C, and the control treatment by 4.0°C increments to a maximum of 32.0 °C, each reaching their peaks at the same time of day (Fig 2.1.2).    I then collected the rockweed Fucus distichus from Wreck Beach, Vancouver, B.C. (49.2623°N, 123.2615°W) to act as a food source for the snails. 32 pieces of F. distichus were selected from this collection, and each were blotted dry on 2 fresh pieces of paper towel, and weighed using a high-precision scale (Mettler Toledo PB405-S/FACT, ± 0.01g). These pieces of F. distichus were placed in individual falcon tubes, and filled with seawater from the seawater table (~13.4°C). Four different species treatment groups were used for the experiment: a control group devoid of any snails, a L. sitkana group with 4 individuals, a L. scutulata group with 4 individuals, and a group consisting of 2 L. sitkana and 2 L. scutulata. Each incubator possessed 2 replicates for each snail treatment group. I placed snails corresponding to each treatment group in the falcon tubes, and secured the tops with a mesh screen. At 0800 each day, I transferred the F. distichus and snails in each tube from their current tube to a new tube filled with seawater, 	 18	which were placed in the incubators at 0800 each day to ensure that the temperature of each of the tubes were equal at the time of exchange, in order to prevent potential oxygen limitation due to respiration from occurring during the trial. On September 17th, the programs for each of the incubators were started, and once all of the incubators reached the programmed temperature, at 1015, I placed the trays of falcon inside and began the experiment.  After 3 days, I removed the trays of falcon tubes from the incubators.  The pieces of F. distichus were then removed from the tubes, and placed within a tray filled with seawater to prevent excess drying from occurring before their weighing. I blotted dry each piece of F. distichus with two fresh pieces of paper towel, and again weighed them on the high-precision scale. For the control treatments devoid of snails, their changes in F. distichus mass differed greatly between the warmed and control temperature treatments. These differences were averaged separately, then added to the F. distichus mass loss for the snail treatment groups within these temperature treatments, in order to better account for differences in F. distichus growth that may have occurred between the two temperature treatments throughout the trial.  Afterwards, the snails from each tube were blotted dry with paper towel and weighed on the scale. I recorded the F. distichus mass lost in the treatment groups with snails, added the average mass change for the controls for their temperature profile, and divided by the total mass of the snails within the tube to determine the mass of F. distichus consumed by the snails, adjusted for F. distichus growth and total snail mass.   	 19	 Fig 2.1.2 Plot of 24-hour temperature profiles, for warmed (red) and control (blue) treatments.    121416182022242628303234360 500 1000 1500 2000 2500Time (Hours)Temperature (°C)ProfileControlWarmed	 20	2.1.3 Environmental Temperature and Tide Pool Census Study   On May 27, I selected 23 tide pools at April Point Resort representative of the variety of depths, exposures, volumes, species presences, and substrate colours of its high intertidal. In order to avoid large fluctuations in CO2 concentrations and pH from photosynthesis and respiration of algae potentially contributing to changes in snail abundances, tide pools with large amounts of algae were not selected to be monitored. To monitor the temperature of the pools, I placed iButton temperature loggers wrapped in parafilm wax within a piece of Z-spar marine epoxy, and secured them to bottom of these selected pools near their deepest points. On June 23, the iButtons were removed from the pools, had their stored data downloaded, and were redeployed and secured within their original pools.    From May to August, I conducted a census of the abundances of adult and juvenile L. sitkana and L. scutulata every month during the lowest tide series (May 28, June 23, July 23, August 20). Juveniles were considered to be any individuals smaller than approximately 2 millimetres in length, and adults longer than 2 millimetres. After the final census was conducted August 20, I removed the iButton loggers, brought them back to the lab, and downloaded their data to determine whether temperature contributed to changes in abundances or pool species compositions observed over the summer.   2.1.4 Tide Pool Heating Experiment   On August 20 at April Point Resort, I selected 15 tide pools within a relatively close vicinity of one another for the experiment. 10 of the pools chosen were used previously in the 	 21	census and environmental temperature study, and the 5 other pools were not previously monitored. I created three treatment groups: a no-water control; where no water was supplied to the pool, a water-control; where ambient seawater was supplied to the pool, and a heat-water group; where heated seawater was supplied to the pool. I divided the 15 pools into 3 groups by their location along the intertidal, and randomly assigned treatments; 4 of the pools in a group were assigned to two of the treatment groups, and the last pool was assigned to the third group. This allowed for the treatments to be spatially representative of the pools used in the study, while still allowing for the random assignment of treatments.    To supply water to the pools, I placed a turkey deep fryer on a lawn uphill of the tide pools, with a stacked bucket beside it. Ambient seawater was collected from the shore and supplied the stacked bucket, which then supplied water to the fryer through an adjustable hose. The heated water was distributed to the pools directly from the fryer, and the control water from the stacked buckets, using siphon pumping via 12.7-millimetre diameter polyurethane irrigation lines. I punched holes at the ends of the lines in the pools to allow for more even distribution of water within the pools. Due to the length of the lines used, and the heat loss observed in pilot trials, the water in the fryer was heated to higher temperatures than desired within the pools; once the water reached the pools, it was at temperatures more indicative of those recorded throughout the summer.    Prior to the start of the trial, I conducted a census of L. sitkana and L. scutulata in the pools on August 20, following the methodology outlined in the environmental temperature and tide pool census study. On August 24, the treatments were applied to the pools from 1020 to 	 22	1520, with the temperature of each pool, as well as the temperature of the supplies of ambient and heated seawater, being recorded with a thermocouple thermometer (Oaktron TEMP100 JTEK) every hour. On September 1, I conducted another census on the pools, to allow for the determination of any changes in individual species abundances or tide pool compositions that may have occurred as a result of the heating.   2.2 Data analysis Methodology 2.2.1 Statistical and Graphical Methods   All statistical analyses for the above experiments were performed using R Studio 1.0.153 (RStudio Inc, 2017). All figures were created using ggplot2 2.2.1, all linear models were created using the R Statistics Package, all linear mixed effect models were created using Linear and Nonlinear Mixed Effects Models 3.1-131, and all ANOVAs were conducted using the Companion to Applied Regression 2.1-5.  2.2.2 Thermal Tolerance Experiment   For the thermal tolerance analysis, I utilized the seawater temperatures recorded from within the seawater-only falcon tube; there was an approximately 0.5°C difference between the readout temperature given by the thermal bath and the water temperature measured in the falcon tubes, which was more indicative of the temperatures individuals within the trials were experiencing.   	 23	 To determine the effects of temperature on L. sitkana and L. scutulata survival, I calculated the proportion survived at the end of the trial for each treatment group and species. From this, a linear regression and two-way ANOVA was used to determine if temperature had an effect on survival, if species had unique tolerances to extreme temperatures, and whether they have differential responses to these extreme temperatures (n = 1).   2.2.3 Herbivory Rate Experiment   For the warmed and control temperature profiles, I measured the control Fucus distichus treatments for their changes in blotted wet mass before and after the completion of the trial. The averages of these changes in mass were calculated separately for the warmed and control temperature treatments, and were added to the other treatment groups changes in mass for the corresponding treatments. I did this in order to account for potential growth or loss of mass during the course of the experiment, and did it separately for each temperature treatment, due to differences in mass changes observed in the controls between the two profiles.    These corrected changes in F. distichus masses for each of the treatment groups were divided by the sum of the blotted wet mass of the individuals within each tube, to produce a mass-adjusted herbivory rate that was also corrected for snail-independent factors. To determine the effects of species and temperature profiles on L. sitkana and L. scutulata growth rates, I conducted a linear regression and two-way ANOVA to compare mass adjusted herbivory rates between temperature profiles and treatment groups (n = 4).   	 24	2.2.4 Environmental Temperature and Tide Pool Census Study   From the beginning to the end of each census period (May 26 – June 23, June 24 – July 23, July 24 – Aug 20), the differences between the individual abundances for L. sitkana and L. scutulata, total snail abundances, and percent L. sitkana as a function of the total abundances of adult L. sitkana and L. scutulata were calculated. I counted juveniles separately, and ultimately excluded them from any analyses due to extreme fluctuations in their abundances over the course of the summer.    I then selected tide pools with complete temperature data at the end of the final census period (Aug 20) for the analysis. For each census period, the average, maximum and variance in temperature were calculated, as well as the number of hours each tide pool experienced equalling to or surpassing 30 °C, which was a threshold temperature beyond which many European Littorina species were found to succumb to heat comas (Clarke et al., 2000). I then compared the changes in abundance variables to each of these temperature variables using linear regressions with mixed effects, accounting for the random effect of tide pools, and two-way ANOVAs (n = 9).   2.2.5 Tide Pool Heating Experiment   For the experiment, I recorded each pool’s environmental temperature every hour, and calculated average and maximum temperatures over the course of treatment applications, and linear regressions and two-way ANOVAs were utilized to determine the effect of treatment group on these temperature variables (n = 5).  	 25	 Utilizing the census data collected the day prior to, and 11 days after, the application of the treatments, total L. sitkana and L. scutulata abundances, the total abundance of snails and change in percent L. sitkana as a function of the total abundance of snails were calculated. Due to a non-normal distribution of my final dataset, I computed the log response ratio for each variable, and compared these results to the average and maximum temperatures for each pool using linear regressions and two-way ANOVAs (n = 5).                  	 26	3. Results 3.1 Thermal Tolerance Experiment  In my laboratory thermal tolerance trials, I found a significant negative relationship between temperature and the proportions of L. sitkana and L. scutulata survived (2-way ANOVA, temperature effect: F1= 83.24, p < 0.0001) (Table 3.1). Furthermore, both species had differential thermal tolerance curves observed in the trial (2-way ANOVA, species effect: F1= 27.13, p < 0.001) (Table 3.1, Fig 3.1). There was a nonsignificant interaction between temperature and species (F1= 0.0855, p = 0.7766) (Table 3.1, Fig 3.1).   Table 3.1 Table of two-way ANOVA results for the linear model of L. sitkana and L. scutulata proportions survived. Bolded p-values represent significant values less than 0.05. n = 1.  Variable SS df F Value p Temperature 1.71046 1 83.24 0.000007634 Species 0.55735 1 27.13 0.0005579 Temperature: Species 0.00176 1 0.0855 0.7766 Residuals 0.18493 9    	 27	   Fig 3.1 Thermal tolerance curves for L. sitkana and L. scutulata, showing the relationship between environmental temperature within falcon tubes, and proportion of the treatment group that survived. 0.00.20.40.60.81.038 40 42 44 46Temperature (°C)Proportion SurvivedL. scutulata L. sitkana	 28	 3.2 Herbivory Rate Experiment   When comparing growth-corrected changes in F. distichus mass, with no adjustments for snail mass, I observed significant relationships between change in mass and species treatment group (2-way ANOVA, Treatment Group effect: F3 = 8.28, p < 0.001), temperature profile (2-way ANOVA, Temperature Profile effect: F1= 20.47, p < 0.001), and the interaction between species treatment group and temperature profile (2-way ANOVA, Treatment Group: Temperature Profile effect: F3= 3.62, p < 0.05) (Table A.1.1, Fig A.1.1). I found that the snail-occupied treatment groups significantly differed in their mass-adjusted herbivory rates (2-way ANOVA, Treatment Group effect: F1= 4.126, p < 0.05) (Table 3.2.1, Fig 3.2.1). The warmed and control temperature profiles also significantly differed in their mass-adjusted herbivory rates (2-way ANOVA, Temperature Profile effect: F1= 12.57, p < 0.005) (Table 3.2.1, Fig 3.2.1). There was no significant interaction between treatment groups and temperature profiles (2-way ANOVA, Treatment Group: Temperature Profile effect: F1= 1.185, p = 0.3284) (Table 3.2.1, Fig 3.2.1).          	 29	 3.2.1 Mass-Adjusted Herbivory Rate  Table 3.2.1 Table of two-way ANOVA results, from the linear model of L. sitkana and L. scutulata mass-adjusted herbivory rates. Bolded p-values represent significant values less than 0.05. n = 5.  Variable SS df F Value p Treatment Group 0.57770 2 4.126 0.0335 Temperature Profile 0.087979 1 12.57 0.002314 Treatment: Profile 0.016597 2 1.185 0.3284 Residuals 0.126021 18   	 30	 Fig 3.2.1 Box plots of mass-adjusted herbivory rates for control and warmed treatments of 2 L. sitkana and 2 L. scutulata, 4 L. scutulata, and 4 L. sitkana, corrected for snail-independent effects of temperature.   0.00.10.20.30.4L. sitkana/L. scutulata L. scutulata L. sitkanaFucus distichus Mass Lost/Snail Mass (g/g)Control Warmed	 31	3.3 Environmental Temperature and Tide Pool Census Study  A significant relationship between change in percent L. sitkana and census period was found in the statistical tests for temperature hours greater than or equal to thirty degrees Celcius (2-way ANOVA, Census Period effect: F2,13= 6.521, p < 0.05), temperature variance (2-way ANOVA, Census Period effect: F2,13= 6.732, p < 0.01), average temperature (2-way ANOVA, Census Period effect: F2,13= 6.604, p < 0.05), and maximum temperature (2-way ANOVA, Census Period effect: F2,13= 7.158, p < 0.01) (Table 3.3.1, Fig 3.3.1). I found no significant effects on change in percent L. sitkana by any of the tested temperature variables, and there were no significant interactions between census period and any of these temperature variables (Table 3.3.1, Fig 3.3.1).    I found trends towards relationships between changes in total snail abundance, as well as L. scutulata abundances, between census periods for maximum temperature, temperature variance, and temperature hours greater than or equal to thirty degrees Celcius (Table 3.3.2, Fig 3.3.2; Table A.2.1, Fig A.2.1). There were no significant effects found on the changes in L. sitkana abundances by census period or any temperature variable (Table A.2.2, Fig A.2.2).          	 32	3.3.1 Change in Percent Littorina sitkana  Table 3.3.1 Table of two-way ANOVA results, from the linear model of change in percent L. sitkana, with average temperature, maximum temperature, temperature variance, and temperature hours greater than or equal to thirty degrees Celcius. Bolded p-values represent significant values less than 0.05. n = 9. Temperature Variable Statistical Test Numerator df Denominator df F Value p Average Temperature Census Period 2 13 6.604 0.0105 Average Temperature 1 13 0.176 0.6820 Census Period: Average Temperature 2 13 0.251 0.7817 Maximum Temperature Census Period 2 13 7.158 0.0080 Maximum Temperature 1 13 0.0978 0.7595 Census Period: Maximum Temperature 2 13 1.198 0.3329 Temperature Variance Census Period 2 13 6.732 0.0098 Temperature Variance 1 13 0.991 0.3376 Census Period: Temperature Variance 2 13 0.0533 0.9483 Temperature Hours >= 30°C Census Period 2 13 6.52 0.0109 Temperature Hours >= 30°C 1 13 0.245 0.6289 Census Period: Temperature Hours >= 30°C 2 13 0.07859 0.9249  	 33	 Fig 3.3.1 Scatter plots of the relationships between the change in each tide pool’s percent L. sitkana, from the beginning to the end of each census period, and maximum temperature, average temperature, temperature variance, and temperature hours greater than or equal to thirty degrees Celcius. Grey bars represent standard errors, and blue lines represent linear trendlines.  −20−10010203016 17 18 19 20Average Temperature (°C)∆ % L. sitkana−20−10010203030.0 32.5 35.0 37.5 40.0Maximum Temperature (°C)∆ % L. sitkana−20−10010203020 30 40 50 60Temperature Variance (°C2)∆ % L. sitkana−20−1001020300 20 40 60Temperature Hours >=30 °C (Hours)∆ % L. sitkanaPeriodJuly − AugustJune − JulyMay − June	 34	3.3.2 Change in Total Snail Abundance   Table 3.3.2 Table of two-way ANOVA results, from the linear model of change in total snail abundance, with average temperature, maximum temperature, temperature variance, and temperature hours greater than or equal to thirty degrees Celcius. Bolded p-values represent significant values less than 0.05. n = 9.  Temperature Variable Statistical Test Numerator df Denominator df F Value p Average Temperature Census Period 2 13 2.739 0.1017 Average Temperature 1 13 0.194 0.6672 Census Period: Average Temperature 2 13 0.0987 0.9067 Maximum Temperature Census Period 2 13 3.142 0.0771 Maximum Temperature 1 13 0.978 0.3407 Census Period: Maximum Temperature 2 13 1.279 0.3112 Temperature Variance Census Period 2 13 3.329 0.0680 Temperature Variance 1 13 0.952 0.3470 Census Period: Temperature Variance 2 13 2.024 0.1717 Temperature Hours >= 30°C Census Period 2 13 2.834 0.0952 Temperature Hours >= 30°C 1 13 0.5657 0.4654 Census Period: Temperature Hours >= 30°C 2 13 0.2817 0.7590   	 35	   Fig 3.3.2 Scatter plots of the relationships between the change in each tide pool’s total snail abundance, from the beginning to the end of each census period, and maximum temperature, average temperature, temperature variance, and temperature hours greater than or equal to thirty degrees Celcius. Grey bars represent standard errors, and blue lines represent linear trendlines.    −100001000200016 17 18 19 20Average Temperature (°C)∆ Total Snail Abundance−100001000200030.0 32.5 35.0 37.5 40.0Maximum Temperature (°C)∆ Total Snail Abundance−100001000200020 30 40 50 60Temperature Variance (°C2)∆ Total Snail Abundance−10000100020000 20 40 60Temperature Hours >=30 °C (Hours)∆ Total Snail AbundancePeriodJuly 24− Aug 20June 24− July 23May 26− June 23	 36	3.4 Tide Pool Heating Experiment   For my tide pool heating experiment, I found a significant effect of treatment group on average (Two-way ANOVA, Treatment Group effect: F2 = 8.461, p < 0.05) and maximum temperature (Two-way ANOVA, Treatment Group effect: F2= 10.01, p < 0.005) (Table 3.4.1, Fig 3.4.1). I observed no significant relationships between change in response ratio of percent L. sitkana, total snail abundance, change in L. scutulata abundance, or change L. sitkana abundance with average or maximum temperature (Table 3.4.2, Fig 3.4.2; Table A.3.1, Fig A.3.1; Table A.3.2, Fig A.3.2; Table A.3.3, Fig A.3.3).    3.4.1 Average and Maximum Temperature Test  Table 3.4.1 Table of two-way ANOVA results, from the linear model of treatment group and average temperature and maximum temperature. Bolded p-values represent significant values less than 0.05.  n = 5.  Temperature Variable Statistical Test df SS Mean SS F Value p Average Temperature Treatment 2 74.667 37.333 8.461 0.005103 Residuals 12 52.596 4.413   Maximum Temperature Treatment 2 156.433 78.217 10.01 0.002769 Residuals 12 93.756 7.813       	 37	 Fig 3.4.1 Box plots of average and maximum temperatures of control water, no water, and warmed water treatment groups.   20.022.525.027.530.0Control Water No Water Warmed WaterAverage Temperature (°C)24283236Control Water No Water Warmed WaterMaximum Temperature (°C)Control Water No Water Warmed Water	 38	3.4.2 Percent Littorina sitkana Response Ratio  Table 3.4.2 Table of two-way ANOVA results, from the linear model of the  log response ratio of percent L. sitkana, with treatment group and average temperature and maximum temperature. Bolded p-values represent significant values less than 0.05. n = 5.   Temperature Variable Statistical Test df SS F Value p Average Temperature Treatment 2 0.02528 0.0481 0.9533 Average Temperature 1 0.06922 0.2631 0.6203 Treatment: Average Temperature 2 0.20305 0.3859 0.6905  Residuals 9 2.36760   Maximum Temperature Treatment 2 0.03622   0.0677 0.9350 Maximum Temperature 1 0.16718 0.6249 0.4496 Treatment: Maximum Temperature 2 0.06472 0.1209 0.8875  Residuals 9 2.40797     	 39	 Fig 3.4.2 Scatter plots of the relationships between each tide pool’s percent L. sitkana log response ratio, from the day prior to treatment application and eight days later, and average and maximum temperature. Grey bars represent standard errors, and blue lines represent linear fits.  −0.50.00.51.020.0 22.5 25.0 27.5 30.0Average Temperature (°C)% L. sitkana Response Ratio (lnRR)−0.50.00.524 28 32 36Maximum Temperature (°C)% L. sitkana Response Ratio (lnRR)TreatmentControl WaterHot WaterNo Water	 40	4. Discussion   Studying the effects that temperature has on intertidal communities, as well as making predictions of future community shifts, requires study of the intra and interspecific variations in lethal and sublethal effects of temperature, the life-history strategies of the species who experience them, and the how environmental temperature is changing, both locally and at larger geographic scales. Increases in temperature variation have been linked to an increase in the positive skewness of both minimum and maximum temperatures, which changes the shape of a temperature distribution towards the warmer region (Donat & Alexander, 2012). Climate warming isn’t expected to just increase mean temperatures or variances around the globe, however; extreme temperature events such as heat waves are considered ‘likely’ or ‘very likely’ to increase in frequency by both near and long-term predictive models (IPCC Report, Chapter 11, 2013; IPCC Report, Chapter 12, 2013). Researching how species may react to changes in environmental temperature regimes and an increase in heat wave frequency through these variations in responses and life-history strategies is necessary to gain a greater understanding of how intertidal systems may change with climate warming in the future.    4.1 Thermal Tolerance Experiment    Through the thermal tolerance experiment, I found that April Point L. scutulata was able to survive in higher proportions to higher temperatures than L. sitkana, and thus possessed a higher thermal tolerance. The slopes of the tolerance curves for L. sitkana and L. scutulata were also similar, though they differed in the magnitudes of temperature that resulted in lethal responses for the two species. Upper thermal tolerance limits can play an important role in how a 	 41	species responds to warming, and differences in these tolerances can contribute to larger-scale shifts in abundances and distributions. Along the Iberian Peninsula in southwestern Europe, gastropods such as the limpet Siphornaria pectinata, the periwinkle Littorina saxatillis, and the whelk Nucella lappilus were found to have significant negative correlations between mean Sea Surface Temperature (SST) and their abundance, and had observed northward range shifts between studies conducted in the early 20th and 21st centuries. The most abundant gastropods along the Iberian Peninsula, such as Melarhaphe neritoides, Patella depressa, Gibbula umbilicalis, and Osilinus lineatus were not found to have significant correlations between SST and their abundances, with no significant range shifts detected in these species in recent history (Rubal et al., 2013). I observed that April Point tide pools already experience temperatures that are within the lethal range of L. sitkana, and as temperatures of this magnitude or greater become more frequent, larger reductions in the abundance of L. sitkana, and smaller reductions in L. scutulata abundance, may be observed in the future due to these differential thermal tolerances.   Through a mathematical model incorporating thermal performance data with climate projections, the impact that increasing temperature variance has on an individual species was found to depend on that species’ TPC, as well as where the mean temperature falls along this curve. Both positive or negative changes in performance can be obtained depending on how the mean temperature and temperature variance fits the curve, and changing the mean or variance independently of one another can produce different performance responses (Vasseur et al., 2014). If climate warming leads to an in an increase in temperature variation, alongside a positive skew towards higher maximum temperatures and lower minimum temperatures at April Point, it may affect the L. sitkana and L. scutulata populations differently due to their different 	 42	upper thermal tolerance limits, and subsequently contribute to shifts in the abundances and distributions of both species in April Point tide pools.    These differences in upper thermal tolerances may relate to the current geographic ranges of both species, with L. sitkana’s southern limit reaching Washington State and L. scutulata’s reaching down to and beyond southern California, and may potentially coincide with differences in range shifts in a manner similar to those observed in the Iberian Peninsula (Hohenlohe, 2004; Rubal et al., 2013; Yamada, 1976). Species like L. sitkana who more frequently experience temperatures within their lethal range may be observed to shift northward more rapidly than a species such as L. scutulata, who are able to better withstand these higher temperatures. However, latitudinal variations in upper thermal tolerance limits in the whelk Nucella canaliculata along the central and northern California and Oregon coast have been observed, with populations from the more exposed and extreme Oregon coast having higher thermal tolerances than their Californian counterparts (Kuo and Sanford, 2009). The thermal tolerances that were observed in the April Point L. sitkana and L. scutulata populations therefore may not necessarily be representative of the thermal tolerances of populations across their entire latitudinal range.   Life history strategy and capability for dispersal can also play an important role in a species’ ability to cope with extreme temperatures. In the northwest Atlantic, cold-adapted northern populations of Littorina littorea had a reduced ability to acclimate to warmer conditions than southern populations, but because of L. littorea’s pelagic larvae, which releases free-floating larvae into the water, the more tolerant southern populations of L. littorea were able to 	 43	‘rescue’ less tolerant northern populations to a greater extent than other species with crawl-away larval stages (Sorte, Jones, and Miller, 2011). In the case of L. sitkana and L. scutulata, L. scutulata has a pelagic, free-floating larval stage, while L. sitkana has a benthic, crawl-away larval stage. L. scutulata may possess a greater ability for their April Point populations to be supported through dispersal from other regions possessing more tolerant individuals. April Point L. sitkana, on the other hand, may have a diminished rescue effect from other populations around April Point, which may contribute to their lower overall abundances when compared to L. scutulata. Across British Columbia and Washington State, low amounts of genetic variation were found both within and across L. sitkana populations in these regions, while L. scutulata was found to high levels of gene flow among their populations (Kyle & Boulding, 2000). This may indicate that even if other L. sitkana populations were able to disperse to April Point, they may not possess enough genetic variability in order to potentially rescue the local population with more tolerant individuals. Localized variations in upper thermal tolerance limits occurring within L. sitkana and L. scutulata populations should be investigated in the future, along with those of other populations around the region, in order to better understand the capabilities that either species may have for dispersal, rescue and local adaptation in a warming climate.     4.2 Herbivory Rate Experiment   For the mass-adjusted herbivory rate analysis, I found that the L. scutulata treatment group had the highest herbivory rate on average for both temperature profiles, L. sitkana had the lowest herbivory rate on average for both profiles, and that the treatment group with two individuals of each species was between the two for both temperature profiles. I also found no 	 44	unique effects of treatment group with temperature profile; this indicates that the effect of temperature on herbivory rates was not amplified or buffered depending on the species.    The net effect that warming will have on the April Point intertidal will, in part, be felt indirectly through its effects on the interactions that L. sitkana and L. scutulata have with other members of the community. Density effects can impact the overall effect that a species has on its community through the changing of abundances of the species (Kordas et al., 2011). If L. sitkana and L. scutulata become less abundant due to experiencing lethal temperatures more frequently, reduced herbivory pressure on the epiphytes and algae that they consume may occur as a result. Indirect effects of warming can also occur through per-capita effects, where an individual has a reduced pressure on their food source through physiological and behavioural mechanisms. L. sitkana already experiences lethal temperatures in April Point tide pools, and both species have reduced mass-adjusted herbivory rates under warmed conditions, so an overall net effect of reduced herbivory pressure on primary producers may be observed within the pools they occupy (Kordas et al., 2011). Herbivory also plays an important role in resisting changes in composition due to temperature within rocky intertidal zones. Limpets have been shown to facilitate the settlement of barnacles in primarily algae-rich zones through their herbivory of the algae, which can subsequently provide habitat for a number of other consumer species (Kordas, Donohue & Harley, 2017). If the net effect that L. sitkana and L. scutulata have on the algal and epiphyte species they consume in April Point pools declines because of warming, other important habitat modifiers or consumers may be less able to establish themselves within the pools, potentially further altering interactions, abundances, and species compositions within them (Kordas, Donohue & Harley, 2017).  	 45	  The effects that warming may have on rocky intertidal species aren’t limited to herbivores, however. Changes in temperature and pH have been shown to indirectly affect herbivores and their herbivory rates through changes in the palatability of algal tissues via mechanisms such as nutritional quality and chemical defenses (Harley et al., 2012; Poore et al., 2012). Increased temperature and carbon dioxide (CO2) can increase the primary production and biomass of algae through temperature-dependent physiological rates (Harley et al., 2012; Koch et al., 2013; Petchey et al., 1999). The reduction of top-down pressure on primary producers through the density and per-capita effects of temperature on L. sitkana and L. scutulata, coupled with potential increases in primary production physiological mechanisms, may lead to a shift in the April Point tide pool communities further towards a greater abundance of algae and epiphytes and away from L. sitkana and L. scutulata.    4.3 Environmental Temperature and Tide Pool Census, and Tide Pool Heating Experiment   From the start to the end of each census period, I observed changes in the shifts in both total abundance of L. sitkana and L. scutulata, as well as in percent L. sitkana. These results indicate that over the course of the summer where temperature and abundances were monitored within April Point tide pools, the composition of the pools changed in relation to these two species, but I did not observe any significant relationships between these changes and any of the temperature variables I tested. Large, single-species reproductive events occurred throughout the summer within the pools included in the study, and I attempted to account for large changes in abundances in a pool due to reproduction by counting the juvenile and adult L. sitkana and L. 	 46	scutulata separately, and conducting analyses one each group separately, as well as together, but did not find any results that differed in their findings.    Because my methodology for the tide pool heating experiment was novel in its development and implementation, I first wanted to determine if it was a viable method to significantly alter the temperature of water within tide pools before conducting analyses on the census results. I found that both the average and maximum temperatures of the warmed water treatment group were significantly higher than those in the control and warmed water treatment groups. These results indicate that this method of heating may be viable for future warming experiments. Because of the long distances the water had to travel from the fryer to the tide pools a large amount of heat loss occurred in the water while it travelled from the pot to the pool in pilot studies. To counteract this, I had to increase the water temperature within the fryer to temperatures approximately 15-20°C higher than my desired range of temperatures at the end of the line, and these elevated temperatures within the pot may have contributed to a greater degree of evaporation within the water source than would be expected when heated ambiently. Therefore, salinity should be another environmental variable that is monitored should this methodology be applied in future studies.     Based on the results of the thermal tolerance experiment, I predicted that there would be a negative relationship between temperature and the change in percent L. sitkana for a tide pool, so that as temperature increases for a pool, the more tolerant L. scutulata would see an increase in relative abundance, while the less tolerant L. sitkana would see a decrease. But from the tide pool heating experiment, I did not observe any significant effects of average or maximum 	 47	temperature on changes in response ratios for total snail abundance, each species’ abundance, or for percent L. sitkana. In the tide pool heating experiment, the peak temperature measured for any of the pools (35.5°C) did not reach a lethal magnitude for either species based on the tolerance experiment results. Therefore, the changes in abundances that I observed may be due to other biotic and abiotic events that occurred within the time period between censuses. Of the pools that I monitored temperature in at April Point, and had complete data for the entire summer on without iButton failure, only one of the pools reached a temperature (40°C) that was found to be lethal for either species through the thermal tolerance experiment (L. sitkana).   Because the immersion temperature that individuals within these pools experienced throughout the summer months only once reached directly lethal levels for either species, extreme temperature events may not have had occurred in high enough magnitudes, frequencies, or durations to directly manipulate the abundances of L. sitkana or L. scutulata through lethal responses. The nature of the substrate, in combination with behavioural responses of L. sitkana and L. scutulata to temperature, may also play a role in exposure to extreme temperature and shifts in abundances. Deeper portions of tide pools and cracks in the rock may provide opportunities to escape more extreme temperatures that may be present within shallower sections of the pool when heated by the sun. Crevices provide microhabitats for marine gastropods such as limpets and snails, and their environmental conditions have an outsized influence on an individual’s body temperature as well as their survivability during high tide (Gray & Hodgson, 2003; Tepler, Mach, and Denny, 2011). Movement between tide pools may also occur when the tide is in; snails caught emersed during low tides were very often exhibiting refuge behaviour by hiding within crevices, or attaching to the rock through a mucus strand to avoid direct contact 	 48	with the hot substrate; therefore, successful traversal between pools by these snails would be unlikely during low tide. A potentially counterintuitive flux of snails into tide pools may also occur during extreme heat events. Snails located on emergent rock surrounding the hottest of these tide pools may attempt to seek refuge from direct contact with this rock or from the threat of desiccation by moving into the tide pools, thus increasing the abundance of snails within them due to this influx of refugees. During high tide, wave action or regular traversal may also lead to individuals moving between pools. While I was unable to explore how L. sitkana and L. scutulata moved between pools, this is an important avenue to explore further, to gain a greater understanding about the types of refuge responses that L. sitkana and L. scutulata may possess to escape extreme temperatures.    A potential confounding factor that may have affected the abundances of L. sitkana and L. scutulata, beyond the factors discussed above and the temperature variables tested, is the responses of the two species to other abiotic stressors, as well as interactions between these responses. These interactions play an important role in determining the winners and losers of ecosystems, and can affect a species in a number of ways: they can be additive; where the combined effect of the stressors is not changed by the presence of multiple responses, they can be antagonistic; where the combined effect of the stressors is reduced through interactions between the responses of one stressor with another, or they can be synergistic; where the two stressors act as a single event that can overwhelm physiological responses (Gunderson, Armstrong, & Stillman, 2016). Tide pools are extreme environments in ways beyond temperature as well: salinity, pCO2 and pH, and oxygen availability are other environmental variables that can vary dramatically within intertidal and tide pool systems, and can also 	 49	contribute to a species’ overall response to its environment. Differences in shell morphological characteristics of the gastropod Littorina littorea have been observed under warmed, elevated CO2, and combined conditions. These results were found alongside changes in ATP concentration, which was shown to be positively correlated with shell thickening and weight, indicating that changes in shell morphology may be due to metabolic disruption (Melatunan et al., 2013). In the larvae of the intertidal gastropod Nassarius festivus, significant interactive effects of temperature and pCO2 on mortality and respiration were observed, but were reduced with higher salinity in mortality, while swimming speed increased with temperature and salinity, but decreased with pCO2 (Sarmento et al., 2014). Interactions between multiple stressors have also been shown to have effects on the survival and performance of many other taxa (Tedengren et al., 1999; Sarmento et al., 2016; Findlay et al., 2010). Improving our understanding of the impacts of climate change on intertidal ecosystems will require further study of the interactions between multiple stressors and how these interactions will affect the performance on individual species, and how these performance responses affect interspecific interactions, which influence abundances and distributions within the competition-intense intertidal.   4.4 Implications and Future Directions  The results of this study suggest that April Point L. sitkana and L. scutulata possess inherently different lethal and sublethal responses to temperature, through thermal tolerance and herbivory rate experiments, but that they are inhibited by extreme temperatures in a similar manner; one species did not have amplified or buffered responses to temperature over the other. If April Point experiences increases in the mean and variance of its environmental temperature, and if extreme heat events begin falling within the inhibitory ranges observed in this study more 	 50	frequently, a potential overall decline in abundances of snails, as well as species-dependent declines, may contribute to shifts in April Point tide pool compositions. Alongside this, an overall reduction in top-down pressure on the algal and epiphyte species that L. sitkana and L. scutulata feed on may occur, through changes in density or per-capita effects. Through the census and tide pool heating experiment, however, I was unable to attribute the changes in abundances of the two species I observed in the field to any of the environmental temperature variables studied. My study highlights the effect that temperature alone can have on individual species through their unique responses, and what the potential implications of these responses may be in a warmer future, but it also offers a cautionary tale in attempting to make ecological conclusions or predictions based entirely on lab-based observations. Results like those found in my thermal tolerance would appear on their face to support clear predictions as to how warming will affect the L. sitkana and L. scutulata that inhabit April Point tide pools, but those were not supported with by the results from my field study or experiment. The impact that climate change will have on intertidal communities may also be felt through changes in sea level, salinity, oceanic circulation, oxygen concentrations, pCO2, and pH, all of which are shifting alongside temperature, which on its own may not be enough to describe the patterns in abundances I observed in nature. Alternatively, my lack of results may simply be due to an artefactual reason, such as the movement of individuals into or out of tide pools, and that temperature really is the driving force behind shifts in snail abundances at April Point. Further study that prioritizes the incorporation of these types of complexities, both in laboratory and field experiments, as well as using lab-based experiments as a support for field-based observations is necessary to better our understanding of how climate change has affected, and will continue to affect, the rocky intertidal zone, as well as other ecosystems.   	 51	Bibliography  Bozinovic, F., Bastías, D.A., Boher, F., Clavijo-Baquet, S., Estay, S.A. & Angilletta Jr., M.J. (2011). The Mean and Variance of Environmental Temperature Interact to Determine Physiological Tolerance and Fitness. Physiological and Biochemical Zoology, 84(6), 543-552. doi: 10.1086/662551 Donat, M.G. & Alexander, L.V. (2012). 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Effects of patch-size on the structure of assemblages in rock pools. Journal of Experimental Marine Biology and Ecology, 197(1), 63-90. doi: 10.1016/0022-0981(95)00145-X  Vasseur, D.A., DeLong, J.P., Gilbert, B., Greig, H.S., Harley, C.D.G., McCann, K.S., Savage, V., Tunney, T.D. & O’Connor, M.I. (2014). Increased temperature variation poses a greater risk to species than climate warming. Proc. R. Soc. B, 281, 1-8. doi: 10.1098/rspb.2013.2612 Yamada, S.B. (1976). Geographic Range Limitation of the Intertidal Gastropods Littorina sitkana and L. planaxis. Marine Biology, 39(1), 61-65. doi: 10.1007/BF00395594        	 58	Appendix A.1 Herbivory Rate Experiment A.1.1 Fucus distichus Mass Loss: Uncorrected  Table A.1.1 Table of two-way ANOVA results, from the linear model of uncorrected losses in Fucus distichus mass with treatment group and temperature profile. Bolded p-values represent significant values less than 0.05. n = 5. Variable SS df F Value p Treatment Group 0.172240 3 8.280 0.0005901 Temperature Profile 0.141911 1 20.47 0.0001393 Treatment: Profile 0.075364 3 3.623 0.02745 Residuals 0.166410 24    	 59	 Fig A.1.1 Box plots of uncorrected changes in F. distichus mass for control and warmed treatments of 2 L. sitkana and 2 L. scutulata, 4 L. scutulata, and 4 L. sitkana. 0.00.10.20.30.40.5L. sitkana/L. scutulata L. scutulata L. sitkana ControlFucus distichus Mass Lost (g)Control Warmed	 60	A.2 Environmental Temperature and Tide Pool Census A.2.1 Change in Littorina scutulata Abundance  Table A.2.1 Table of two-way ANOVA results, from the linear model of change in L. scutulata abundance, with average temperature, maximum temperature, temperature variance, and temperature hours greater than or equal to thirty degrees Celcius. Bolded p-values represent significant values less than 0.05. n = 9.  Temperature Variable Statistical Test Numerator df Denominator df F Value p Average Temperature Census Period 2 13 3.076 0.8875 Average Temperature 1 13 0.1435 0.7109 Census Period: Average Temperature 2 13 0.1226 0.8856 Maximum Temperature Census Period 2 13 3.584 0.0576 Maximum Temperature 1 13 1.017 0.3315 Census Period: Maximum Temperature 2 13 1.452 0.2697 Temperature Variance Census Period 2 13 3.687 0.0539 Temperature Variance 1 13 0.869 0.3682 Census Period: Temperature Variance 2 13 1.884 0.1913 Temperature Hours >= 30°C Census Period 2 13 3.170 0.0756 Temperature Hours >= 30°C 1 13 0.5262 0.4811 Census Period: Temperature Hours >= 30°C 2 13 0.2581 0.7764  	 61	 Fig A.2.1 Scatter plots of the relationships between each tide pool’s change in L. scutulata abundances, from the beginning to the end of each census period, and maximum temperature, average temperature, temperature variance, and temperature hours greater than or equal to thirty degrees Celcius. Grey bars represent standard errors, and blue lines represent linear trendlines.  −100001000200016 17 18 19 20Average Temperature (°C)∆ L. scutulata Abundance−100001000200030.0 32.5 35.0 37.5 40.0Maximum Temperature (°C)∆ L. scutulata Abundance−100001000200020 30 40 50 60Temperature Variance (°C2)∆ L. scutulata Abundance−10000100020000 20 40 60Temperature Hours >=30 °C (Hours)∆ L. scutulata AbundancePeriodJuly 24− Aug 20June 24− July 23May 26− June 23	 62	A.2.2 Change in Littorina sitkana Abundance   Table A.2.2 Table of two-way ANOVA results, from the linear model of change in L. sitkana abundance, with average temperature, maximum temperature, temperature variance, and temperature hours greater than or equal to thirty degrees Celcius. Bolded p-values represent significant values less than 0.05. n = 9.  Temperature Variable Statistical Test Numerator df Denominator df F Value p Average Temperature Census Period 2 13 1.195 0.3339 Average Temperature 1 13 0.5154 0.4855 Census Period: Average Temperature 2 13 0.02465 0.9757 Maximum Temperature Census Period 2 13 1.272 0.3128 Maximum Temperature 1 13 0.7345 0.4101 Census Period: Maximum Temperature 2 13 0.6201 0.5531 Temperature Variance Census Period 2 13 1.513 0.2567 Temperature Variance 1 13 1.281 0.2782 Census Period: Temperature Variance 2 13 2.511 0.1197 Temperature Hours >= 30°C Census Period 2 13 1.254 0.3178 Temperature Hours >= 30°C 1 13 0.7158 0.4128 Census Period: Temperature Hours >= 30°C 2 13 0.4563 0.6434  	 63	 Fig A.2.2 Scatter plots of the relationships between each tide pool’s change in L. sitkana abundances, from the beginning to the end of each census period, and maximum temperature, average temperature, temperature variance, and temperature hours greater than or equal to thirty degrees Celcius. Grey bars represent standard errors, and blue lines represent linear trendlines.   −500050016 17 18 19 20Average Temperature (°C)∆ L. sitkana Abundance020040030.0 32.5 35.0 37.5 40.0Maximum Temperature (°C)∆ L. sitkana Abundance−500050020 30 40 50 60Temperature Variance (°C2)∆ L. sitkana Abundance−50005000 20 40 60Temperature Hours >=30 °C (Hours)∆ L. sitkana AbundancePeriodJuly 24− Aug 20June 24− July 23May 26− June 23	 64	A.3 Tide Pool Heating Experiment  A.3.1 Total Snail Abundance Response Ratio  Table A.3.1 Table of two-way ANOVA results, from the linear model of the log response ratio of total snail abundance, with treatment group and average temperature and maximum temperature. Bolded p-values represent significant values less than 0.05. n = 5.  Temperature Variable Statistical Test df SS F Value p Average Temperature Treatment 2 0.59662 0.8792 0.4479 Average Temperature 1 0.19041 0.5612 0.4729 Treatment: Average Temperature 2 0.42835 0.6312 0.5539  Residuals 9 3.05363   Maximum Temperature Treatment 2 0.69498 1.046 0.3906 Maximum Temperature 1 0.29149 0.8771 0.3735 Treatment: Maximum Temperature 2 0.38978 0.5864 0.5762  Residuals 9 2.99112    	 65	 Fig A.3.1 Scatter plots of the relationships between each tide pool’s total snail abundance log response ratio, from the day prior to treatment application and eight days later, and average and maximum temperature. Grey bars represent standard errors, and blue lines represent linear trendlines. −1.5−1.0−0.50.020.0 22.5 25.0 27.5 30.0Average Temperature (°C)Total Snail Abundance Response Ratio (lnRR)−1.5−1.0−0.50.024 28 32 36Maximum Temperature (°C)Total Snail Abundance Response Ratio (lnRR)TreatmentControl WaterHot WaterNo Water	 66	A.3.2 Littorina scutulata Abundance Response Ratio   Table A.3.2 Table of two-way ANOVA results, from the linear model of the log response ratio of L. scutulata abundance, with treatment group and average temperature and maximum temperature. Bolded p-values represent significant values less than 0.05. n = 5.  Temperature Variable Statistical Test df SS F Value p Average Temperature Treatment 2 0.5512 0.5118 0.6159 Average Temperature 1 0.0000 0.0000 0.9977 Treatment: Average Temperature 2 0.6697 0.6218 0.5585  Residuals 9 4.8467   Maximum Temperature Treatment 2 0.5928 0.5576 0.5912 Maximum Temperature 1 0.0187 0.0352 0.8553 Treatment: Maximum Temperature 2 0.7130 0.6706 0.5352  Residuals 9 4.7846     	 67	 Fig A.3.2 Scatter plots of the relationships between each tide pool’s L. scutulata abundance log response ratio, from the day prior to treatment application and eight days later, and average and maximum temperature. Grey bars represent standard errors, and blue lines represent linear trendlines. −1.5−1.0−0.50.020.0 22.5 25.0 27.5 30.0Average Temperature (°C)L. scutulata Abundance Response Ratio (lnRR)−1.5−1.0−0.50.024 28 32 36Maximum Temperature (°C)L. scutulata Abundance Response Ratio (lnRR)TreatmentControl WaterHot WaterNo Water	 68	 A.3.3 Littorina sitkana Abundance Response Ratio   Table A.3.3 Table of two-way ANOVA results, from the linear model of the log response ratio of L. sitkana abundance, with treatment group and average temperature and maximum temperature. Bolded p-values represent significant values less than 0.05. n = 5.  Temperature Variable Statistical Test df SS F Value p Average Temperature Treatment 2 0.4309 0.5894 0.5747 Average Temperature 1 0.4892 1.338 0.2771 Treatment: Average Temperature 2 1.1815 1.616 0.2514  Residuals 9 3.2899   Maximum Temperature Treatment 2 0.7167 0.973 0.4146 Maximum Temperature 1 0.9002 2.443 0.1525 Treatment: Maximum Temperature 2 0.7445 1.01 0.4019  Residuals 9 3.3160     	 69	 Fig A.3.3 Scatter plots of the relationships between each tide pool’s L. sitkana abundance log response ratio, from the day prior to treatment application and eight days later, and average and maximum temperature. Grey bars represent standard errors, and blue lines represent linear trendlines. −1.0−0.50.00.520.0 22.5 25.0 27.5 30.0Average Temperature (°C)L. sitkana Abundance Response Ratio (lnRR)−1.0−0.50.00.524 28 32 36Maximum Temperature (°C)L. sitkana Abundance Response Ratio (lnRR)TreatmentControl WaterHot WaterNo Water

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