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Predicting a temperature-oxygen critical habitat squeeze for endangered pacific salmon and sculpin in… Kerker, Kathryn 2020

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   PREDICTING A TEMPERATURE-OXYGEN CRITICAL HABITAT SQUEEZE FOR ENDANGERED PACIFIC SALMON AND SCULPIN IN A SMALL, COASTAL MONOMICTIC LAKE  by Kathryn Kerker  B.A.Sc, University of Waterloo, 2017  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Civil Engineering)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)   August 2020  © Kathryn Kerker, 2020 ii  The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the thesis entitled:  Predicting A Temperature-Oxygen Critical Habitat Squeeze for Endangered Pacific Salmon and Sculpin in a Small, Coastal Monomictic Lake  submitted by Kathryn Kerker  in partial fulfilment of the requirements for the degree of Master of Applied Science in Civil Engineering  Examining Committee: Bernard Laval, Civil Engineering Supervisor Daniel Selbie, Fisheries and Oceans Canada, Science Branch Supervisory Committee Member  iii  Abstract Cultus Lake, British Columbia is critical freshwater lake habitat for two species-at-risk, the Cultus Lake Sockeye Salmon (Oncorhynchus nerka, Cultus population) and Cultus Lake Pygmy Sculpin (Coastrange Sculpin, Cottus aleuticus, Cultus population).  This lake ecosystem is degrading and threatened by epilimnetic warming and decreasing hypolimnetic dissolved oxygen concentrations, largely the result of the effects and interactions of ongoing lake eutrophication and climate change.  Exploration of lake modeling and monitoring data (2009-2018) revealed a correlation between Cultus Lake, BC hypolimnetic temperatures and the volume of hypoxic (<6 mg L-1) deep water lake habitat (r2 = 0.73).  As oxygen and water temperatures are important constraints on fish growth and survival, the temperature-hypoxic volume relationship was coupled to the predictive outputs from a 1-D hydrodynamic model (GLM) that estimates lake thermal structure, modeling lake temperature and oxygen conditions to 2100 CE under two climate change scenarios (RCP4.5, moderate emissions scenario; and RCP8.5, extreme emissions scenario). The findings substantiate the intensification of a temperature-oxygen squeeze in Cultus Lake under climate change, and permit estimation of generalized future critical habitat restrictions for endemic species at risk.  Model simulation results forecasted to 2100 CE, predict a significant future temperature-oxygen habitat squeeze, resulting from the encroachment of surface waters of deleterious temperatures (>17ºC; -25% of lake volume currently; -30% (-34%) under RCP4.5 (RCP8.5) by 2100), and hypoxic hypolimnetic waters (< 6mg/L DO; -20% of lake volume currently; -41% (-58%) under RCP4.5 (RCP8.5). With warming temperatures, the predicted temperature-oxygen squeeze (and volumetric optimal habitat loss) is likely to progressively intensify through time, reducing suitable oxythermic habitat conditions for stenothermic, hypoxia-intolerant fish species in Cultus Lake.  If nutrient loading to Cultus Lake iv  remains unabated, our model predicts the likely elimination of optimal critical habitat for both the Cultus Lake Sockeye Salmon and Cultus Pygmy Sculpin, with intensified warming and/or nutrient loading likely to significantly advance such exclusion.  Reducing the known nutrient loadings to Cultus Lake is essential in order to lower lake primary productivity and associated seasonal hypolimnetic oxygen deficits, particularly before significant internal loading of nutrients from sediment stores enhances and more permanently reinforces the eutrophied lake state.      v  Lay Summary Cultus Lake is a small lake in the Pacific Northwest that is of high recreational, economic, and ecological value. Over the past century Cultus Lake has shifted to a more nutrient-rich state, largely because of human development in the region and the effects of climate change. The lake is home to two species at risk of extinction: Cultus Lake Sockeye Salmon and the Cultus Lake Pygmy Sculpin. High temperatures near the lake surface and low dissolved oxygen near the sediments restrict the habitable regions for these species in the lake. Climate change and eutrophication are predicted to further develop these extremes. The aim of this study is to predict how the amount of suitable critical habitat for species at risk in Cultus Lake may change under predictions of future climate change, and to identify and highlight key lake management actions that may improve the persistence of these endangered fish species.   vi  Preface This thesis is the original, unpublished work of Kathryn Kerker. All analysis of field data was completed by myself. Historical data was provided by Fisheries and Oceans Canada’s Lakes Research Program (Dr. Daniel Selbie and Kelly Malange). The physical model in GLM was created by Mark Sumka (Sumka 2017).  vii  Table of Contents Abstract ......................................................................................................................................... iii Lay Summary .................................................................................................................................v Preface ........................................................................................................................................... vi Table of Contents ........................................................................................................................ vii List of Figures ............................................................................................................................... ix Acknowledgements ........................................................................................................................x Chapter 1: Introduction ................................................................................................................1 Chapter 2: Literature Review .......................................................................................................6 2.1 Location .......................................................................................................................... 6 2.2 Cultural Eutrophication ................................................................................................... 7 2.3 Sockeye Salmon .............................................................................................................. 9 2.4 Temperature-Oxygen Squeeze ...................................................................................... 11 2.5 Thesis Objectives .......................................................................................................... 12 Chapter 3: Methods .....................................................................................................................13 3.1 Historical Data .............................................................................................................. 13 3.2 1-D Hydrodynamic Model ............................................................................................ 14 3.3 Simplified Conceptual Model ....................................................................................... 14 3.4 Defining the Degree of Dissolved Oxygen Depletion .................................................. 17 Chapter 4: Results........................................................................................................................18 4.1 Historical Data .............................................................................................................. 18 4.2 Habitat Restriction ........................................................................................................ 22 4.3 Relating Oxygen and Temperature ............................................................................... 24 viii  Chapter 5: Discussion ..................................................................................................................29 5.1 Long-Term Changes ..................................................................................................... 29 5.2 The Validity of a Predicted Linear Temperature Trend................................................ 30 5.3 Implications of Future Limnological Changes on Species at Risk & Critical Habitat . 33 Chapter 6: Conclusion .................................................................................................................35 Bibliography .................................................................................................................................36  ix  List of Figures Figure 2.1: Map of Cultus Lake showing subwatersheds, bathymetry (10 m contours), and key points of interest. The map extends to the Canada-USA border, but the Frosst Creek watershed extends for an additional 16 km2 south of the border. Modified from Putt et al. 2019................... 7 Figure 2.2: Cultus Lake nutrient budget ......................................................................................... 9 Figure 3.1: Model setup and example results of DO diffusion over time..................................... 16 Figure 4.1: Example of annual cycle of (a) temperature, (b) dissolved oxygen, and (c) chlorophyll a (2013 data) .............................................................................................................. 19 Figure 4.2: Long-term annual trends of (a) maximum Schmidt Stability; (b) Stratified period; (c) maximum epilimnetic temperature; and (d) average hypolimnetic temperature .......................... 20 Figure 4.3: (a) Historical temperature and Schmidt Stability; (b) Historical dissolved oxygen concentrations; (c) Historical chlorophyll a concentrations ......................................................... 22 Figure 4.4: Depth of habitat restriction by temperatures above 17C (red) and dissolved oxygen below 6 mg L-1 (blue) ................................................................................................................... 24 Figure 4.5: (a) Linear increasing trend of maximum fraction of volume restriction by DO with bottom temperatures (r2=0.73) (b) Linear decreasing trend of minimum DO at 30 m with hypolimnetic temperatures (r2=0.80). Values from 1928, 1932, and 1934 were excluded from the trendline. ....................................................................................................................................... 25 Figure 4.6: Variation in bottom temperature over time under two climate scenarios .................. 26 Figure 4.7: Future predictions of (a) fraction of volume restriction by low DO; (b) minimum DO at 30 m under two climate scenarios; and (c) temperature-oxygen squeeze under RCP8.5 and RCP4.5, with RCP8.5 showing greater restriction ....................................................................... 28 Figure 5.2: Theoretical trends of oxygen depletion ...................................................................... 32 x  Acknowledgements I would like to thank my supervisors, Dr. Bernard Laval and Dr. Daniel Selbie, for all the time and energy spent guiding me through this research. I would also like to thank the Fisheries and Oceans Canada staff and researchers of the Lakes Research Program at the Cultus Lake Salmon Research Laboratory for demonstrating limnological data-collection techniques and retrieving historical data. Thank you to the UBC Environmental Fluid Mechanics group for your never-ending encouragement and motivation through every stage of this project.    1  Chapter 1: Introduction Cold-water stenothermic fish, such as Pacific salmon (Oncorhynchus spp.), require cool-water habitats (5 – 17°C; Brett, 1971), with sufficient dissolved oxygen (DO; > 6 mg L-1) to sustain critical life functions (Pon et al. 2010). Sockeye Salmon (O. nerka) principally rear in nursery lakes of the North Pacific coast during their early life histories, exposing juveniles to variable oxythermal conditions that can be strongly influenced by anthropogenic stressors (Moss et al. 2011; Smol 2006). In particular, climate change and cultural eutrophication are interactive threats on oxythermal habitat conditions for many fish species (Magee, McIntyre, and Wu 2017; Moss et al. 2011).  Lake surface temperatures are increasing globally in response to climate change (O’ Reilly et al. 2015). This can pose novel stresses on lake-resident fish, but can also serve to reduce lake hypolimnetic DO concentrations, through temperature-dependent reductions in dissolved oxygen solubilities and increases in lake biological oxygen demands (Ficke, Myrick, and Hansen 2007).  Lakes undergoing cultural eutrophication are particularly sensitive to such climatic forcings, as excess nutrient loads serve to increase lake ecosystem reactivities (i.e. biological productivity, organic matter production, and sedimentation) to temperature or temperature-mediated drivers (Moss et al. 2011).  Hypolimnetic oxygen depletion can induce pronounced hypoxic to anoxic conditions throughout key areas of critical habitats in lakes experiencing cultural eutrophication (Edwards, Conroy, and Culver 2005; Müller et al. 2012).  A temperature-oxygen squeeze for freshwater fish species can occur in lakes during the period of thermal stratification, should available rearing habitat be vertically constrained by warming 2  epilimnetic temperatures and decreasing hypolimnetic DO concentrations beyond environmental tolerances (Hyatt and Rankin 1999). For example, Berge (2009) found that salmonids (i.e. Kokanee salmon) abandoned deeper, cold, oxygen-depleted refuge habitats in favour of more shallow lake strata with higher DO, but above-optimal temperatures. This behavioural response to hypolimnetic oxygen depletion not only increases organismal metabolic rates, and hence foraging and oxygen consumption requirements, but also exposes fish to sub-lethal to lethal temperatures and increased predation and disease risks (Karvonen et al. 2010; Scheuerell and Schindler 2003).  Estimating changes in oxygen availability within lakes is crucial to understanding how lentic fish species will be affected by climate change. Water temperature is an important predictor of hypolimnetic dissolved oxygen depletion in lakes, as it controls its mixing, solubility, and reaction rates (Ficke et al. 2007).  The rates and magnitudes of seasonal oxygen depletion are strongly modified by the interactive effects lake nutrient status, with lakes receiving higher nutrient loads, exhibiting enhanced organic matter production and associated seasonal hypolimnetic oxygen losses (Moss et al. 2011).  Heating of surface waters affects the extent and frequency of seasonal mixing within lakes. While lakes are stratified, the hypolimnion is largely isolated from diffusive (i.e. atmosphere) and biotic (i.e. photosynthetic production) sources (Ficke et al. 2007).  Hypolimnetic water temperatures and corresponding oxygen solubilities at the onset of summer stratification determine the initial oxygen concentrations prior to seasonal depletion (Stefan et al. 1996). Under climate change, the seasonal intensity and duration of lake thermal stratification is 3  projected to increase (Adrian et al. 2009; O’ Reilly et al. 2015; Sumka 2017). Protraction of the thermally stratified period in lakes delays diffusive- and mixis-mediated oxygen recharge of the lower water column (Foley et al. 2012), which results in reduced hypolimnetic oxygen concentrations over a longer seasonal window. These constraints can strongly influence freshwater fish habitat use, growth, condition, and survival, particularly if these habitats are thermal refugia for cold-water stenotherms (Magee et al. 2017).  Temperature is an important predictor of chemical and microbial reaction rates influencing hypolimnetic oxygen concentrations. Typically, microbial and chemical reaction rates double with an increase in temperature of 10C (Horne and Goldman 1994). In lake hypolimnia, sediment oxygen demand (SOD) and water-column respiration are the principle factors regulating dissolved oxygen depletion, and are both temperature-mediated processes (Edwards et al. 2005). SOD is the rate of oxygen consumption from the water directly above the sediment by the decay of settled organic matter and chemical oxidation of reduced species (e.g. Fe2+, Mn2+, S2-; Price, Cerco, & Gunnison 1994). Aerobic respiration by plankton, bacteria, and other living organisms in the water-column increases with temperature through increased enzymatic activity, consuming hypolimnetic oxygen (Mitchell and Gu 2010). Reaction rates in both the homogeneous aerobic upper sediment layer (under an oxic water column) and the anoxic-anaerobic lower sediment layer are controlled by temperature (Walker and Snodgrass 1986). The quantity of settling organic matter also affects SOD, but the response is delayed due to the limited depth of the aerobic layer (Giorgio and Williams 2005). Amounts of settling organic matter from the water column are projected to increase as rates of phytoplankton growth and 4  nutrient recycling rise with temperature under climate change, which serves to increase SOD of lakes through time (Wilhelm and Adrian 2008).  Here I investigate the relation between hypolimnetic oxygen depletion and temperature in a small monomictic lake, Cultus Lake, British Columbia, which is critical habitat for two fish species at risk, the Cultus Lake Sockeye Salmon (Oncorhynchus nerka, Cultus population) and Cultus Lake Pygmy Sculpin (Coastrange Sculpin, Cottus aleuticus, Cultus population).  Cultus Lake is currently stressed by the effects and interactions of cultural eutrophication and climate change (Putt et al. 2019; Shortreed 2007), which are expected to exacerbate hypolimnetic oxygen depletion in the future without nutrient abatement (Moss et al. 2011; Putt et al. 2019). Cultus Lake’s extended monitoring record permits long-term modeling of lake conditions, which may provide insight into lake outcomes of similar systems of the Pacific Northwest. Future predictions of vertically-resolved lake water temperatures under two climate change scenarios predicted by a regionally-downscaled global climate model (RCP 4.5, RCP 8.5; Moss et al., 2010) are modelled in the General Lake Model (GLM; Hipsey, Bruce, & Hamilton, 2014; see Sumka, 2017). These modelled lake temperatures along with the current empirical relationship between hypolimnetic dissolved oxygen and water temperature, derived from decadal monitoring of Cultus Lake, are used to predict future trends in hypolimnetic oxygen depletion. The maximum annual habitat volume restriction by decreasing hypolimnetic DO and increasing surface water temperatures, using defensible estimates for cold-water stenothermic fish (i.e. > 17oC; < 6 mg L-1 DO; Brett, 1971; Pon, Tovey, Bradford, Maclellan, & Hume, 2010; U.S. EPA, 1986), is estimated to assess changes in suitable Sockeye Salmon habitat in Cultus Lake over 5  time, constrained by the temperature-oxygen squeeze. The potential impacts on lake physics, ecological structure and functioning, and the persistence of species at risk are discussed. 6  Chapter 2: Literature Review 2.1 Location Cultus Lake is a warm monomictic freshwater lake located approximately 10 km south of Chilliwack, British Columbia. It has a maximum depth of 44 m and an area of 6.3 km2 (Shortreed 2007). Due to its urban proximity, the lake receives up to three million visitors per year (Fraser Valley Regional District 2011). There are several communities and seasonal dwellings surrounding the lake: Cultus Lake Community, Lindell beach, and Cultus Lake Provincial Park. Its popularity and location in proximity to the Fraser Valley has led to cultural eutrophication resulting from excess nutrient loading from agriculture, atmospheric deposition, and septic leachate (Putt et al. 2019).    Cultus Lake is critical habitat for two species-at-risk, the endangered Cultus Lake Sockeye Salmon and Cultus Lake Pygmy Sculpin. The Cultus Lake Sockeye population is currently listed as endangered under COSEWIC due to severe declines in adult escapement since the 1960’s, declining from tens of thousands returning historically down to hundreds (COSEWIC 2003; DFO 2018). The Cultus Lake Pygmy Sculpin is currently listed as endangered under COSEWIC, and as threatened under SARA.   Putt et al. (2019) identified four primary subwatersheds of disparate hydrology, using hierarchical clustering, each transmitting unique creek, overland flow, and groundwater water contributions to Cultus Lake: Columbia Valley, International Ridge, Vedder Mountain, and Smith Falls Creek (Figure 2.1). The largest tributary, Frosst Creek, runs through the predominantly forested Columbia Valley subwatershed. Many small creeks run down the 7  relatively undisturbed steep slopes of International Ridge and Vedder Mountain. The Smith Falls Creek subwatershed includes a flat wetland area that adds some flow routing and nutrient attenuation (Putt et al. 2019). There is one channelized lake outflow, Sweltzer Creek, which travels northward to the Chilliwack River, a tributary of the Fraser River.   Figure 2.1: Map of Cultus Lake showing subwatersheds, bathymetry (10 m contours), and key points of interest. The map extends to the Canada-USA border, but the Frosst Creek watershed extends for an additional 16 km2 south of the border. Modified from Putt et al. 2019.  2.2 Cultural Eutrophication Eutrophication is the gradual increase in lake productivity that occurs naturally as lakes age and fill with nutrient-rich sediment (Ansari and Singh Gill 2014). A lake’s trophic state is an 8  indication of its productivity, and ranges from oligotrophy (low productivity) to eutrophy (high productivity). Oligotrophic lakes are clear, have low nutrient contents, and often support cold-water fish species. Eutrophic lakes have lower water clarity due to algal growth responses to higher lake nutrient concentrations, and are often warm with low oxygen levels in their hypolimnia (Ansari and Singh Gill 2014).  Cultural eutrophication is the response of a lake to artificial additions of nitrogen and phosphorous arising from human sources. Cultus Lake has been undergoing cultural eutrophication since at least the 1950’s (Gauthier et al. 2020). As the surrounding area continued to grow, so did the number of yearly visitors (Gauthier et al. 2020). The primary anthropogenic sources of nutrients include agricultural runoff, atmospheric deposition, and septic leachate, as shown in Figure 2.2 (Putt et al. 2019). Agricultural runoff from fields in the Columbia Valley enters Frosst Creek and contributes a large percentage of total nutrient loading (especially nitrogen) to the lake (Putt et al. 2019). Atmospheric deposition from upwind agricultural and urban areas is a major contributor of both nitrogen and phosphorous via direct lake-surface deposition as well as runoff from all watersheds including International Ridge, Vedder Mountain, and Smith Falls which have little other human influences (Putt et al. 2019). Septic leachate is a large contributor of phosphorous but could be largely rectified if waste were diverted to a sewage treatment plant. Other sources of nutrients include gull guano and decomposing salmon carcasses (Putt et al. 2019). Although Cultus Lake is currently classified as oligotrophic, it is transitioning to an oligo-mesotrophic state (Shortreed 2007) which affects the future of the species-at-risk as well as the lake’s value as a recreational site (fishing, algal growth, water quality, etc.). 9   Figure 2.2: Cultus Lake nutrient budget  2.3 Sockeye Salmon The Fraser River watershed is the primary habitat for Sockeye Salmon in BC, and contains over 100 distinct populations, however the majority are contained in nine major stock groups (Fisheries and Oceans Canada, 1998). Cultus Lake Sockeye Salmon are genetically distinct from all other Sockeye populations, including those elsewhere in the Fraser River, with which they share common ancestry (COSEWIC, 2003). Attempts to transplant non-native Sockeye were unsuccessful due to the unique adaptations of the Cultus Lake population which evolved over thousands of years (COSEWIC, 2003). The Cultus Lake Sockeye population is currently listed as endangered under COSEWIC (COSEWIC, 2003), and is currently under re-consideration by the Minister for listing under Schedule 1 of Canada’s Species at Risk Act (SARA).  The typical lifespan of Sockeye Salmon is four years (Burgner 1991).  In the past, spawning occurred in the shallow gravel area of Lindell Beach (COSEWIC, 2003), but now occurs in deeper waters (Bradford et al. 2011). The eggs mature over the winter and emerge as fry in the 10  spring, spending 1-3 years in Cultus lake before migrating to the ocean between March and June as smolts (COSEWIC, 2003; Grant, 2018). Adult Sockeye spend two years in the Gulf of Alaska with other Sockeye populations before returning to Cultus Lake as reproductively mature adults to spawn (COSEWIC, 2003).   Sockeye fry often perform diurnal vertical migrations within nursery lake water columns (Scheuerell and Schindler 2003), a behaviour that is conserved in Cultus Lake. During the day juveniles reside in cooler deep waters, which in Cultus Lake is in close proximity to the lake sediments (DFO Lakes Research Program, unpublished data). At dusk fry ascend to the upper hypolimnion and epilimnion to feed on zooplankton, which aggregate at depths between 5m and 15m associated with a deep chlorophyll maximum within the lake (Cultus Sockeye Recovery Team, 2005), but spend the night near the thermocline. They rise once more to feed at dawn before retreating back to the bottom waters (Brett 1971; Scheuerell and Schindler 2003). Vertical migrations are undertaken by juvenile Sockeye Salmon to maximize feeding, avoid predators who identify prey by sight, and for energy conservation (Cultus Sockeye Recovery Team 2005; Scheuerell and Schindler 2003). With lower oxygen in the hypolimnion and higher epilimnetic temperatures, the extent and benefits of diurnal vertical migrations are diminished, posing further stresses that may influence persistence.   Cultus Lake Pygmy Sculpin (CPS) are largely associated with profundal (deep) waters within Cultus Lake, making nighttime sojourns into the upper water column to feed (COSEWIC 2010; Ricker 1960).  Spatially-resolved seasonal trapping of CPS has indicated that they may no longer frequent the deeper portions of Cultus Lake, deemed to be an important part of their critical 11  habitat (Chiang et al. 2015; Ricker 1960), during the early- to mid-fall, when profundal oxygen concentrations are lowest in the lake (Loudon 2020).  2.4 Temperature-Oxygen Squeeze Cultus Lake is a warm monomictic lake, meaning that it becomes thermally stratified during the summer and is continuously mixed throughout the winter (Shortreed 2007). Thermal stratification during summer is characterized by a warm, upper epilimnion overlying a cool, lower hypolimnion, with a region of rapidly varying temperature in between known as the metalimnion (Wetzel 2001). The onset of stratification occurs when atmospheric heating, primarily due to an increase in incident solar radiation, warms the surface waters of the lake to create a density difference strong enough to resist mixing by wind stress (Yeates 2007). This density difference prevents the transfer of dissolved substances by turbulent mixing though the metalimnion. This restriction combined with nutrient uptake by phytoplankton in the epilimnion and oxygen consumption by respiration and aerobic decomposition leads to a decrease in nutrient concentration in the epilimnion and oxygen in the hypolimnion for as long the period of thermal stratification persists. As hypolimnetic oxygen decreases, cold-water fish species like Sockeye Salmon may be forced out of their thermal refuge into warmer waters above (Pon et al. 2010). A temperature-oxygen squeeze occurs when hypoxic conditions restrict deeper habitats, while sub-lethal to lethal temperatures restrict prolonged access to surface habitats, confining stenothermic fish species to a narrow region of the water column (Berge 2009; Ficke et al. 2007). This may cause an increased risk of predation and may make accessing food sources more energy consuming (Berge 2009; Martins et al. 2012). The metabolic rates of fish scale with temperature, causing them to require more food and oxygen for survival at warmer water temperatures (Cultus 12  Sockeye Recovery Team 2005). As hypolimnetic oxygen depletion proceeds throughout the season, the habitable region narrows until late fall, when the density difference is no longer great enough to resist mixing by wind stress. At mixis, dissolved substances are vertically redistributed, creating uniform concentrations of oxygen and nutrients throughout the water column.   2.5 Thesis Objectives Past studies have looked at the effects of nutrient loading, increasing temperatures linked to climate change, and the health of the Sockeye Salmon population in Cultus Lake. This study aims to bridge the gap between the physical, chemical, and biological limnology of Cultus Lake to illustrate the effect of the evolution of temperature and oxygen on species-at-risk. The following research questions are considered: 1. How can hypolimnetic oxygen be predicted? 2. How is hypolimnetic oxygen expected to change under future conditions? 3. How will changes in temperature and oxygen affect species at risk? These are addressed by observing trends in historical data and analyzing correlations between the extent of oxygen depletion and hypolimnetic temperature. These observations are coupled with results from a 1-dimensional hydrodynamic model (GLM) which predicts temperature in Cultus Lake under two climate scenarios to the year 2100 to estimate future hypolimnetic oxygen concentrations. The results are used to predict the potential temperature-oxygen squeeze for species at risk in Cultus Lake under future climate change.  13  Chapter 3: Methods 3.1 Historical Data Profiles of dissolved oxygen were measured monthly between June 1927 and September 1929 by Dr. Foerster from water samples at 25-foot intervals (Ricker 1937). Biweekly profiles of water samples at 5 m intervals were measured by W.E. Ricker between January 1927 and May 1935. Analysis for DO was done on water samples using the Winkler titration method (Ricker 1937). However, because of data gaps, annual oxygen minima could only be estimated in the following years: 1928, 1932, and 1934. Profiles of temperature were consistently measured biweekly from 1932-1936 using a reversing thermometer, with some additional measurements in 1927-1929. This data was summarized by Shortreed (2007), who originally compared it to current-day observations.  Monthly limnological data collection on Cultus Lake has been performed by Fisheries and Oceans Canada’s (DFO) Lakes Research Program since February 2009. These data include profiles of nitrate-nitrogen, dissolved oxygen, chlorophyll A, and temperature, as well as hypolimnetic and euphotic zone averages of numerous chemical species (including NO3-, NH3, SRP, TP, TDP; see methods in Shortreed 2007). Additional temperature profiles were measured from April 2001 to March 2003. Dissolved oxygen was measured using a YSI Pro ODO optical DO meter at 1 m to 5 m intervals. Chlorophyll A was derived by first filtering water through 0.45 μm Millipore HA filters which were frozen before macerating in 90% acetone. A Turner Designs Model 10-AU fluorometer was then used to find the chlorophyll concentration (Shortreed 2007). Temperature was measured using an Applied Microsystems Ltd. MicroCTD and an Applied 14  Microsystems STD-12 Plus recording at 1 Hz.  Data from the lake’s central station (CL3) was used for this study.  3.2 1-D Hydrodynamic Model A one-dimensional hydrodynamic model (General Lake Model, GLM) was used by Sumka (2017) to simulate the thermal structure of Cultus Lake under future climate scenarios predicted by a regionally-downscaled global climate model. Sumka performed monthly data collection using a Seabird SBE 19plus v2 Profiler CTD, which when supplemented with the DFO monthly data created a bi-weekly dataset. Sumka also installed thermal moorings in three locations for the year 2016. The model was calibrated using the bi-weekly temperature profiles and thermal mooring data for 2016 and was validated with DFO historical monthly profiles from 2001-2003 and 2009-2015. Lake temperature was then predicted until 2100 under two climate scenarios: moderate emission (RCP4.5) and extreme emission (RCP8.5) outcomes, based on downscaled outputs of the Canadian Regional Climate Model (Can-RCM4). Future predictions see a delay in the breakup of stratification by 16 days (45 days) for RCP4.5 (RCP8.5) by 2100, with some years having persistent stratification throughout the winter (Sumka 2017).   3.3 Simplified Conceptual Model A simplified conceptual model of quantifiable processes in one spatial dimension (i.e. the vertical) was created to identify expected trends of oxygen depletion and confirm key assumptions. In this model, the processes of hypolimnetic oxygen depletion are simplified by looking only at sediment oxygen demand. This simplification ignores any water-column losses due to respiration. It has been shown that the contribution of water-column respiration increases 15  with lake productivity and hypolimnion depth, with lakes of similar characteristics to Cultus Lake having <40% of total respiration in the water-column (Cornett and Rigler 1987).  Assuming SOD can be modelled as a diffusive process in the hypolimnion with a constant diffusivity, then the diffusion equation can be used to calculate the effect of SOD on the water-column. The one-dimensional diffusion equation is: 𝑑𝐶𝐷𝑂𝑑𝑡= 𝐷𝜕2𝐶𝐷𝑂𝜕𝑧2 (1) where CDO is the concentration of dissolved oxygen, D is the diffusivity (here set as 10-5 m2 s-1 for an approximate order of magnitude (Edwards et al. 2005)), z is the elevation above the sediment, and t is time. Using a uniform initial condition, constant flux boundary at the sediment, and infinite upper boundary, the analytical solution to the diffusion equation is: 𝐶𝐷𝑂 = 𝐶𝐷𝑂0 − 𝐽𝑆𝑂𝐷 [2√𝐷𝑡𝜋exp (−𝑧24𝐷𝑡) − 𝑧 (1 − erf𝑧2√𝐷𝑡)] (2) where CDO0 is the uniform dissolved oxygen initial concentration and JSOD is the mass flux of oxygen to the sediment (sediment oxygen demand). Figure 3.1 outlines the model setup and shows example model results with a constant SOD. As time passes, oxygen is consumed from higher in the water-column in a nonlinear fashion. The last timestep of the simulation has the lowest oxygen concentrations, similar to the oxygen concentration measured just prior to turnover.  16   Figure 3.1: Model setup and example results of DO diffusion over time, where blue indicates low oxygen  To relate oxygen depletion to water temperature, the effect of temperature on the flux of oxygen to the sediment (JSOD) is considered. Theoretically, JSOD can be related to temperature using the simplified Arrhenius equation: 𝐽𝑆𝑂𝐷 = 𝐽𝑆𝑂𝐷20𝐶𝜃𝑇−20 (3) where 𝜃 is the simplified Arrhenius temperature coefficient, and 𝐽𝑆𝑂𝐷20𝐶 is the flux of oxygen to the sediment at a standard temperature of 20C.    17  3.4 Defining the Degree of Dissolved Oxygen Depletion The degree of oxygen depletion can be defined in multiple ways. For example, it can be defined as oxygen concentration at a specific depth, or as oxygen concentration suitable for a particular species. Oxygen concentration at a specific depth is a simple method to observe changes over time. Dissolved oxygen at a depth of 30 m was used in order to compare 2009-2017 with 1928, 1932, and 1934, as per Shortreed (2007). Using the method of suitable oxygen concentration, a threshold was set under which Sockeye Salmon are negatively impacted. Little is known of the aerobic scope of the Cultus Pygmy Sculpin. For this study, the low-DO threshold was set at 6 mg L-1 based on the level at which growth rate is affected and most members exhibit oxygen distress (Pon et al. 2010; U.S. EPA 1986). This definition allows an optimal habitat depth range, and subsequent optimal habitat volume, to be defined. For each year (2009-2017) the duration of optimal habitat volume reduction, as well as the seasonal-maximum sub-optimal habitat volume, allow a single value to represent the severity of oxygen depletion for that year. As Sockeye Salmon are also affected by high temperatures, a temperature threshold was also defined. The threshold of 17C was chosen because above this level food conversion is inefficient (Brett 1971). This method could not be used for 1928-1934 observations as thresholds were rarely exceeded but indicates an important reference point for change in temperature and oxygen levels since this time period.  18  Chapter 4: Results 4.1 Historical Data I start by first looking at the observed typical annual cycle of temperature, oxygen, and chlorophyll a in Cultus Lake, followed by an analysis of interannual variations. Being a warm monomictic lake, on average Cultus Lake mixes from December to March (i.e. the turnover period) and is stratified from April to November (Figure 4.1a). The hypolimnion remains cool throughout the summer while the epilimnion exceeds 20C. Dissolved oxygen (Figure 4.1b) is elevated throughout the water column during turnover, and increases further in the later stages of mixis, likely due to photosynthetic production, as light intensifies in the spring. Once the lake thermally stratifies, the lake hypolimnion is largely isolated from diffusion of atmospheric oxygen sources, and oxygen depletion commences near the lake sediments. A seasonal deep oxygen maximum evolves throughout the summer period, centered around ~10 m water depth (i.e. within the thermocline), and associated with the development of a deep chlorophyll maximum (i.e. deep phytoplankton productivity) (Figure 4.1c).  This deep chlorophyll maximum results from rapid seasonal depletion of epilimnetic nutrients, coupled with high light penetration in the water column, and an abundance of bioavailable nutrients in the metalimnion and upper hypolimnion (Leach et al. 2018). These annual cycles in temperature, oxygen, and chlorophyll a are seen in all contemporary years of record, with some inter-annual variation and apparent long-term trends (Figure 4.3). 19   Figure 4.1: Example of annual cycle of (a) temperature, (b) dissolved oxygen, and (c) chlorophyll a (2013 data) Inter-annual variation is observed in temperature, DO, and chlorophyll a series over the period of record (Figure 4.3). Complete historical temperature data includes the years 1932-1937, 2001-2003, and 2009-2018 (Figure 4.3a). Over the period 1932-2018 there has been a significant increase in epilimnetic temperatures in Cultus Lake (Shortreed 2007; Sumka 2017), and a lengthening of the stratified period (Sumka 2017). The stratified period is defined as the number of days the Schmidt Stability is greater than 30 J m-2 (Engelhardt and Kirillin 2014), where Schmidt Stability is defined from Idso (1973) as the amount of work required to mix a water body to a uniform density. The maximum annual Schmidt Stability has increased over the measured period, from 2530 J m-2 to 2900 J m-2, indicating a rise in the strength of stratification and resistance to mixing (Figure 4.2a). Since the 1930s, the onset of stratification has advanced 20  by two weeks (Figure 4.2b) and maximum epilimnetic temperatures have risen from 21C to 23C (Figure 4.2c; Sumka, 2017). One-tailed t-tests were performed for each parameter to determine if the present-day mean is greater than the mean in 1920-1930 by a statistically significant amount. All of the trends listed above (maximum Schmidt Stability, stratified period, and maximum epilimnetic temperature) were significant. There is no statistically significant increasing trend in hypolimnion temperature (t = 0.47 < tcrit = 1.83), as the inter-annual variation exceeds any increase (Figure 4.2d).  Figure 4.2: Long-term annual trends of (a) maximum Schmidt Stability; (b) Stratified period; (c) maximum epilimnetic temperature; and (d) average hypolimnetic temperature  21  The annual cycle of dissolved oxygen between 1928 and 1935 (Ricker 1937) indicates complete historical oxygenation of the water column throughout the mixing period of December to May (Figure 4.3b), with oxygen concentrations nearly saturated and constant with depth. A gradual decline of hypolimnetic oxygen occurs during the stratified period with the lowest concentration near the lake sediments at 5.73 mg L-1 on October 1, 1934; an annual minimum much higher than is observed today. A slight oxygen subsurface maximum occurred during that period, centered at ~15 m depth, most likely due to phytoplankton photosynthesis (Shortreed 2007), however primary production and chlorophyll a data are not available during this period. Oxygen profiles from 2009 to 2018 show a similar cycle but with greater extremes. Higher photosynthetic rates induce supersaturated conditions near ~10 m depth, which generally correspond with deep chlorophyll maxima. The difference in depth of the subsurface maxima between the 1930s and the past decade likely reflects a change in the vertical distribution of nutrients, a change in thermocline depth, and reduced water clarity (Leach et al. 2018; Shortreed 2007). In these years, oxygen overlying the lake sediments becomes severely low (i.e. < 2 mg L-1), often producing anoxic conditions near the sediment-water interface. This reflects a substantial increase in the intensity of oxidative processes (microbial decomposition) due to excess amounts of organic matter reaching the hypolimnion as a consequence of lake eutrophication (Putt et al. 2019; Shortreed 2007).  More recently, non-uniform water column concentrations of DO during turnover likely occur due to continuous decomposition near the sediment-water interface and oxygen-debts accumulated over anoxic periods.  There is substantial inter-annual variation in Cultus Lake chlorophyll a (Figure 4.3c). Most years exhibit an initial increase in chlorophyll a in February-March during turnover, forming a 22  relatively intense spring bloom, typically lasting from late-March to late-May (Figure 4.1). The main bloom occurs in late summer to fall. The summer blooms with highest chlorophyll a concentration occurred in 2015 and 2016, which are the years with above-average temperatures and the greatest hypolimnetic oxygen depletion.  Figure 4.3: (a) Historical temperature and Schmidt Stability; (b) Historical dissolved oxygen concentrations; (c) Historical chlorophyll a concentrations 4.2 Habitat Restriction The annual cycle of temperature and oxygen limits the optimal habitat depth available for Sockeye Salmon in Cultus Lake. Using an oxygen threshold of 6 mg L-1 and a temperature threshold of 17C, the suboptimal depth ranges are highlighted in Figure 4.4. At any given time, quality Sockeye Salmon habitat is restricted to the water column volume between regions of low oxygen and high temperature (represented as white space in Figure 4.4). The depth range of 23  suboptimal habitat induced by low DO increases throughout the stratified period and peaks before turnover. Similarly, the depth range of suboptimal habitat induced by high temperatures also expands throughout stratification as the surface mixed layer deepens. The maximum restriction by low DO slightly lags that of high temperature, as surface temperatures decrease below the threshold of 17C with ambient cooling before lake turnover occurs.  Based upon the duration and extent of habitat restriction by DO, 2016 was identified as being the most restrictive year in the instrumental series (2009-2017), with 2011 the least restrictive year. The year 2018 was excluded as it was incomplete at the time of analysis. Cultus Lake habitat restriction by temperature varied in duration but little in depth over the measured period. The duration of habitat restriction by temperature varied from 95 days to 133 days, whereas the maximum restricted depth only varied between 8.7 m and 10.5 m. In contrast, the duration of consecutive DO restriction varied from 80 days to 226 days, and the maximum restricted depth ranged from 6.5 m to 21 m. I focus on habitat restriction by DO due to its large hypolimnetic variation between years, and the importance of cool, dimly-lit profundal refuges in juvenile Sockeye diel vertical migration (Brett 1971; Scheuerell and Schindler 2003), which are the primary critical habitat of the Cultus Lake Pygmy Sculpin (CPS; Fisheries and Oceans Canada 2017). Depths of habitat restriction are translated into suboptimal habitat volume using the hypsograph for Cultus Lake to refine estimated Sockeye Salmon and CPS volumetric habitat loss.  24   Figure 4.4: Depth of habitat restriction by temperatures above 17C (red) and dissolved oxygen below 6 mg L-1 (blue) 4.3 Relating Oxygen and Temperature The severity of oxygen depletion during stratification is positively correlated with the temperature of hypolimnetic waters (i.e. here taken as deeper than 30 m). This is likely due to the increased rates of oxygen consumption that result from higher enzymatic activity in aerobic bacteria responsible for the majority of organic matter (OM) decomposition (Mitchell and Gu 2010), and lower oxygen solubility at higher temperatures (Foley et al. 2012). Figure 4.5a highlights the increasing trend of habitat volume restriction by DO with increasing average temperatures below 30 m (R2 = 0.73). A similar relationship (Figure 4.5b) was observed between decreasing minimum DO at 30 m and increasing average temperatures below 30 m (R2 = 0.80). A trend was also expected between the duration of volume restriction by DO and the stratified period, as thermal stratification is associated with isolation of the hypolimnion from oxygen sources (Wetzel 2001). However, no significant trend was observed in the measured data. This may be due to additional factors such as cool fall precipitation and groundwater intrusions adding oxygenated water to the hypolimnion, decoupling the temperature-DO relationship.  25   Figure 4.5: (a) Linear increasing trend of maximum fraction of volume restriction by DO with bottom temperatures (r2=0.73) (b) Linear decreasing trend of minimum DO at 30 m with hypolimnetic temperatures (r2=0.80). Values from 1928, 1932, and 1934 were excluded from the trendline.  The relationship between hypolimnetic water temperatures and dissolved oxygen concentrations, presented in Figure 4.5, is expected to change under different nutrient loadings, associated organic matter sedimentation rates, and dissolved oxygen conditions within the water column, and at the sediment-water interface. Limited historical data (1928-1934) similarly highlights an apparent negative relationship between hypolimnetic water temperatures and dissolved oxygen, but also the maintenance of a highly-oxic hypolimnion (i.e. > 8 mg/L DO) at a time when state-(a) (b) 26  based limnological hind-cast modeling suggests Cultus Lake was likely ultra-oligotrophic to oligotrophic (Putt et al. 2019).     Future hypolimnetic oxygen concentration predictions were necessarily predicated upon the assumption that lake nutrient loading (e.g. autochthonous and internal) and OM production remain constant, which it is unlikely to be the case without enactment of targeted nutrient abatements (Putt et al. 2019). Thus, the predictions of future reductions of optimal habitat volume and its exclusion are likely conservative, presenting a best-case scenario. Conversely, should targeted nutrient abatement be successful, it is likely the opposite would be true.  Using similar methods to Sumka (2017) I modelled lake temperatures under the two climate scenarios from the results of GLM (Figure 4.6). Future hypolimnetic temperatures (at 30m) trended to 7.9C and 6.7C in scenarios RCP8.5 and RCP4.5 respectively. These predictions were applied to the observed correlations between bottom temperature and oxygen depletion (Figure 4.5) and are presented in Figure 4.7.   Figure 4.6: Variation in bottom temperature over time under two climate scenarios 27  Using the epilimnetic temperature and hypolimnetic dissolved oxygen limits of 17oC and 6 mg L-1 DO respectively, the maximum predicted habitable volume restriction by DO alone trends toward 58% and 41% by 2100 in climate scenarios RCP8.5 and RCP4.5 respectively (Figure 4.7a). Increasing habitable volume restriction by DO may extinguish rearing juvenile Sockeye Salmon or force them higher in the water column where there is significant light penetration, leaving them more vulnerable to predation and elevated water temperatures. The minimum DO at 30 m is projected to be 3.5 mg L-1 and 4.8 mg L-1 by 2100 in RCP8.5 and RCP4.5 respectively (Figure 4.7b). These are well below harmful oxygen thresholds for aquatic life (Brett and Blackburn 1981), and render the entire volume of critical habitat below 30 m as suboptimal for Cultus Sockeye Salmon for a protracted period annually. In some future years, such as 2084 CE, where the hypolimnion is modelled to approach 9C, the entire hypolimnetic volume becomes severely oxygen restricted (< 6 mg L-1 DO) across the late stratified period. The increasing volumetric habitat restriction by both high temperatures and low oxygen leads to a temperature-oxygen squeeze, where Sockeye Salmon habitable volume becomes progressively more limited. Figure 4.7c shows that by 2100 suitable habitat is severely reduced, and in some years completely eliminated. It is important to note that the maximum restriction by low DO is limited to the hypolimnion, as the thermocline acts as a barrier to oxygen regeneration from the oxic mixed layer.   28   Figure 4.7: Future predictions of (a) fraction of volume restriction by low DO; (b) minimum DO at 30 m under two climate scenarios; and (c) temperature-oxygen squeeze under RCP8.5 and RCP4.5, with RCP8.5 showing greater restriction   29  Chapter 5: Discussion 5.1 Long-Term Changes  Cultus Lake is on a eutrophication trajectory, forced largely by external, anthropogenic nutrient sources, transitioning from ultra-oligotrophy/oligotrophy in the early 20th century to conditions currently approaching mesotrophy (Gauthier et al. 2020; Putt et al. 2019; Shortreed 2007). It is estimated that total nitrogen and phosphorous loadings have doubled since the development of the region (Putt 2014).  Model estimates are supported by sediment core analyses indicating large increases in sediment algal pigments since the ca. 1940’s to 1950’s (Gauthier et al. 2020), reflective of overlying algal production during the period of deposition (Leavitt and Hodgson 2001).  Over the coming century, changes to hypolimnetic organic matter loading within Cultus Lake are expected, either enhanced by further lake eutrophication and/or reduced by lake, watershed and airshed mitigation measures promoting lake oligotrophication (Putt et al. 2019), that are not captured in the period of record, and thus are difficult to predict.  The directional nature and magnitude of these nutrient and associated organic matter forcings will have strong consequences for the lake’s annual hypolimnetic oxygen depletion trends, and persistence of species at risk, particularly under the climate warming forecasted for southern British Columbia (Moss et al. 2010).  The slope of the relationship between hypolimnetic DO and temperature will change proportionally in response to organic matter loading to the hypolimnion, as near-sediment mineralization rates and associated aerobic decomposition vary with trophic status at equivalent temperatures (Gudasz et al. 2015). The heat content and inter-annual thermal inertia of Cultus Lake are predicted to increase substantially over the coming century (Sumka 2017). This makes 30  reductions in nutrient loadings to Cultus Lake essential to preserving or reversing seasonal hypolimnetic oxygen depletion trends imperilling endemic species at risk, and averting oxygen-mediated non-linear limnological responses, such as runaway eutrophication associated with lake internal loading (Putt et al. 2019).  Time is of the essence in reducing nutrient loads to Cultus Lake, as studies have shown a delayed response between nutrient reductions from active mitigation measures and water quality improvements (Giorgio and Williams 2005). Since only a small layer of sediment is exposed to the oxygenated water column at a time, and the reactions are diffusion-limited (Santschi et al. 1990), the reduction in sediment oxygen demand (SOD) following a reduction in organic loading can be delayed on the order of a decade (Snodgrass 1987). Internal loading of nutrients, especially phosphorous under low DO conditions at the sediment-water interface, can hinder nutrient mitigation outcomes (Forsberg 1989). It is estimated that lake sediments take many decades to recover after phosphorous loading is reduced depending on the amount of accumulation, allowing lake productivity and associated organic matter loading to be sustained long after nutrient loading has been reduced (Søndergaard, Jensen, and Jeppesen 2003).   5.2 The Validity of a Predicted Linear Temperature Trend The simplified conceptual model introduced in section 3.3 was used to justify the linear trend between hypolimnetic temperature and habitat restriction by low DO or DO at 30m. In this section we explore expected temperature-DO trends while ignoring changes in trophic status. The simplified model indicates the expected trends between JSOD and oxygen depletion for our two parameters: oxygen threshold volume and minimum DO at 30 m. 31  The simplified Arrhenius equation (3) shows that the rate of oxygen consumption from the sediment increases exponentially with temperature, as shown in Figure 5.1a. The oxygen threshold depth is calculated by setting DO and time constant (6 mg L-1 and 100 days are used as constant values for comparison) in equation (2) and solving for depth (z) at different values of SOD. As shown in Figure 5.1b, the depth of threshold oxygen is nonlinear with JSOD. However, when the corresponding depths are plotted with temperature the trend is linear (Figure 5.1c). DO threshold depth is related to volume using the hypsograph for Cultus Lake, which can be approximated as linear (Figure 5.1d). Therefore, the sub-optimal habitat volume under the threshold oxygen also varies linearly with temperature. The minimum DO at 30 m is taken as the concentration 12 m above the sediment after the set time period. At a given depth and time, DO should decrease linearly with JSOD (Figure 5.1e). When plotted against temperature which increases exponentially with JSOD, the minimum DO at 30 m decreases exponentially with temperature (Figure 5.1f).  32   Figure 5.1: Theoretical trends of oxygen depletion The duration of stratification, oxygen solubility, and water-column respiration likely contribute to the deviation in the measured data from the predicted trend. Based on this simplified model, a linear trend for the volume below the threshold oxygen is anticipated, validating our approach. For the minimum DO at 30 m, an exponential decrease with temperature is expected. However, due to the limited range of available data, using an exponential trend for extrapolation to higher hypolimnetic temperatures than measured cannot be justified. A linear correlation predicts the conservative, best-case scenario by predicting a higher minimum oxygen concentration. Also, the overall temperature change is small as the hypolimnion is not affected by warming temperatures to the same extent as the epilimnion, with the average change in hypolimnetic temperature from now until 2100 being within 4C. Due to the reasons stated above, linear correlations for the parameters drawn from the measured data are appropriate. However, there is a limit to this linearity when extrapolating over large temperature ranges. The linear trend for the volume 33  below the threshold oxygen concentration is confined by the depth of the hypolimnion. Thus, the sub-optimal habitat volume due to low oxygen can only grow until it reaches the thermocline, which is a no-flux boundary, although the depth of this layer varies inter- and intra-annually. Above this level oxygen is replenished by primary production and atmospheric diffusion. In Cultus Lake the maximum sub-optimal volume is 70% of the total lake volume based on the current thermocline depth (but 100% suboptimal habitat when temperature is considered). Changes in the predicted trend would occur as 70% restricted volume is approached, which necessarily limits the time-period for estimates based on these trends.   5.3 Implications of Future Limnological Changes on Species at Risk & Critical Habitat Sockeye Salmon require cool, oxygen-rich environments within nursery lakes (Burgner 1991). The predicted progressive limitation of suitable habitat within Cultus Lake will likely force juvenile salmon into sub-optimal environments (i.e. high temperature or low DO), as a function of their diel vertical migration. By 2100, approximately 58% of the lake volume could be sub-optimal habitat due to low dissolved oxygen concentrations during lake stratification under RCP8.5. Prolonged exposure to higher temperatures (i.e. > 17 oC) interferes with normal growth through reduced bioenergetic efficiency (Brett 1971). Altered residence depths in the water column may impose a higher risk of predation, as shallower depths have higher light levels. Sockeye fry in Cultus Lake typically reside in darkness near the lake sediments during the day to avoid visual predators, but may no longer be able to access these critical cool-water refuge habitats if hypoxic to anoxic (Cultus Sockeye Recovery Team 2005).  Such an outcome may have particular consequence for fish, as the period of maximum volumetric habitat restriction by DO largely coincides with the hottest periods of the year.  34  Low hypolimnetic oxygen may also impact the Cultus Lake Pygmy Sculpin, as it is a largely bottom-dwelling species (Chiang et al. 2015). Their abundance is unknown but may show a slight decline based on by-catch data of salmon trawls (Chiang et al. 2015; Fisheries and Oceans Canada 2017; National Recovery Team for Cultus Pygmy Sculpin 2007). The predicted decline in hypolimnetic oxygen over the coming decades will likely have a detrimental effect on the Cultus Lake Pygmy Sculpin due to their heavy dependence on benthic and profundal environments (Loudon 2020).  This has been a key consideration in their COSEWIC up-listing to endangered status. 35  Chapter 6: Conclusion This study presents a simple method of predicting changes in hypolimnetic oxygen depletion to assess a lower bound for the potential impact on species at risk, but assumes nutrient loading does not increase with time. Results suggest that even if the trophic status of Cultus Lake remains constant, owing to its elevated nutrient content, hypolimnetic oxygen will become severely depleted during the stratified period by 2100 as a result of hypolimnetic warming by climate change. Suitable Sockeye Salmon habitat is predicted to be progressively reduced, and in some years eliminated completely. Targeted nutrient abatement will improve lake water quality (Putt et al. 2019) and will reduce the extent and rates of seasonal hypolimnetic oxygen depletion in Cultus Lake. However, persistent benthic anoxia, resulting in enhanced internal loading of sediment-stored nutrients into the lake water column, may lead to a delay in improved water quality. Worse, a rapid and self-reinforcing eutrophication trajectory could occur, a condition that would be very difficult to reverse, and likely terminal for lake species at risk.  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