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Factors influencing the distribution of the intertidal chlorophyte, Codium fragile (Suringar) Hariot in Barkley Sound, British Columbia Rodgers, Sherri Ann

Abstract

The purpose of this research was to quantify the occurrence of the chlorophyte, Codium fragile (Suringar) Hariot in Barkley Sound, British Columbia habitats and explore several factors that may be correlated to its patchy distribution. Codium fragile is a unique species for study as it is a coenocytic seaweed found in physically-stressful habitats; specifically, rocky intertidal areas. Many previous studies have focused on the invasive nature of a closely-related subspecies (Codium fragile ssp. tomentosoides), however, few studies have examined the biology and ecology of native C. fragile on NE Pacific shores. I monitored Codium fragile at four different field sites from 1998-2001 and found that plants were unevenly distributed over time (e.g. seasonal and annual population fluctuations) and space (e.g. differed in abundance from one location to another). I hypothesized that herbivory, sand burial, wave exposure (or a combination of these factors) could account for the patchy distribution patterns at Prasiola Point, Second Beach, Seppings Island and Scott's Bay. I conducted feeding preference, absorption efficiency, and macroalgal diet vs. fitness experiments using generalist herbivores (e.g. purple urchins and black chitons) and three common intertidal seaweeds (e.g. Codium fragile, Hedophyllum sessile, Mazzaella splendens). My results show that compared to the two other seaweeds, C. fragile is not preferred by purple urchins or black chitons, it has the lowest (short-term) nutritional value, and did not significantly increase somatic or reproductive growth for either herbivore species. Field observations indicated it was possible that infrequent sand burial events restricted the occurrence of Codium fragile at Scott's Bay. Experimental results show that adult C. fragile plants buried in sand (for five weeks) grew less compared to control plants (no sand). This information provided insight to the fate of C. fragile adults at Scott's Bay, but did not explain the irregular distribution of plants at Prasiola Point or Second Beach (where no sand burial occurred). This is the first study exploring biomechanical properties of a coenocytic algal species from temperate shores. I estimated maximum wave forces for each of my four study sites in order to classify them as wave-protected or waveexposed. I measured the force to remove or break Codium fragile plants from their attachment on rocky substratum, comparing size classes as well as origin of representative C. fragile plants. These experiments showed that C. fragile possesses removal forces comparable to those recorded for other (multi-cellular) algal species. There were no differences in removal forces related to size; however, there were significant differences depending on where C. fragile plants originated. Measured drag coefficients show that C. fragile conforms to increased water velocities (similar to other algal species). Calculations using known data were used to project what water velocities were required to break or dislodge individuals based on size or habitat. Large C. fragile plants are more susceptible to increased velocities than smaller plants; plants growing at Scott's Bay are more likely to break or detach compared to plants from other locations. My research suggests that wave forces limit both size and survival of Codium fragile at wave-exposed habitats (e.g. Prasiola Point), yet many individuals remain healthy and persist over time. A combination of factors (e.g. herbivory and wave forces) might be causative for the irregular occurrence at Prasiola Point. This thesis advances our minimal knowledge of native Codium fragile plants in NE Pacific habitats, compared to the wealth of information on the invasive subspecies, C. fragile ssp. tomentosoides. My research is original as it measures the effects of both biotic and abiotic factors on a coenocytic macroalgal species found in rocky, intertidal habitats. Of the three factors I chose to study, herbivory is widespread throughout published ecological literature whereas effects of sand burial are few. Furthermore, my wave action results contribute to an emerging field of study in marine science increasing our understanding of how seaweeds persist in wave-swept environments.

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