VARIATION IN BRYOZOAN FEEDING RATES ALONG A COASTAL PRODUCTIVITY GRADIENT

Human-induced nutrient enrichment has significantly enhanced coastal primary productivity, resulting in the formation of an extensive “dead zone” in the Gulf of Mexico and many other locations globally. This rapid increase in productivity may profoundly influence marine community composition and functional diversity. For this study, we investigated feeding rates of suspension-feeding marine animals in the Gulf of Mexico and how they vary along a productivity gradient in the northern gulf. The characteristics of cupuladriid bryozoans, a group of colonial invertebrates that inhabit tropical continental shelf environments, may vary in response to changes in primary productivity. The size of autozooids, the calcified feeding structures in the bryozoan colony, correlate with the colony’s feeding rate and are hypothesized to vary with productivity levels. In the Gulf of Mexico, larger autozooids are expected in less productive environments (e.g., Florida, FL), and smaller autozooids in more productive environments (e.g., Alabama, AL). This hypothesis can be evaluated by analyzing autozooid areas; larger autozooid areas correlate with larger feeding organs, enabling higher feeding rates and, thus, more rapid food intake. More rapid food intake may benefit organisms in less productive environments, allowing for ongoing food consumption in areas where food supply is limited. Field-collected dead bryozoan colonies were categorized by genus and reproductive strategy and assessed through a combination of light and scanning electron microscopy. A total of 737 colonies and 31,137 autozooids were analyzed. Statistical analyses conducted in R revealed significant differences in median autozooid area, with larger sizes offshore of FL and smaller sizes offshore of AL. This raises the question of whether bryozoans will be able to adapt to human-induced increases in nutrients. If so, we expect a reduction in autozooid size in increasingly eutrophic environments, as lower feeding rates are adequate to meet energy demands. However, severe eutrophication that results in the development and expansion of hypoxic waters may limit the occurrence of bryozoans and other benthic fauna, more broadly. Future work could use radiocarbon dating to determine the timing of live-dead shifts in autozooid size within these regions.