Impacts of oysters on eelgrass (Zostera marina L.) : importance of early life history stages in response to aquaculture disturbance

Graduation date: 2007The importance of seagrasses (families Posidoniaceae, Zosteraceae,\ud Hydrocharitaceae and Cymodoceaceae) to estuarine communities is widely accepted and\ud this, combined with their continued decline throughout the world, have resulted in a need\ud to better understand the factors affecting their growth and reproduction. Conservation and\ud restoration of seagrasses will require an understanding of their population biology\ud including the role of seeds in maintaining populations following disturbance. On the US\ud west coast, shellfish aquaculture can co-occur with protected eelgrass (Zostera marina\ud L.). Many aquaculture practices constitute a pulse perturbation, and a key question\ud concerns the ability of eelgrass to recover. This thesis addresses the impacts of oyster\ud aquacultural disturbance on these early life history stages.\ud I studied seed production, germination, and seedling growth and survival of\ud eelgrass under different oyster aquaculture practices: dredging and off-bottom longline\ud culture. Germination of experimentally added seeds was highest in dredged areas, where\ud adult shoot densities were lowest. Seedlings survived better and were bigger in plots\ud where adult plants had been removed. Natural seedling recruitment and seed production\ud were highest in dredged beds compared to longline beds and reference areas. From these\ud data, I propose that the greater recruitment in dredged beds is due to both enhanced seed\ud densities as well as removal of neighboring adult plants. Low success in longlines may be\ud due to a combination of physical factors including increases in sediment accretion and\ud significantly lower redox values. Dredging can enhance or at least maintain seed density\ud and seed germination, but longline aquaculture appears to significantly reduce eelgrass\ud recruitment.\ud I also address seed production and seed germination patterns across five sites in\ud Willapa Bay, WA and discuss the relative roles of seed supply, physical factors, and\ud biological factors in driving those patterns. Some of the variation in natural recruitment\ud could be explained by patterns of flowering and seed production with some sites that had\ud higher seedling numbers producing significantly more propagules. Recruitment was\ud highest under colder water temperatures, while other physical factors (redox potential,\ud sediment grain size, and sediment accretion) were less important. To understand what\ud may be influencing seedling density, I conducted a seed addition experiment that showed\ud differences in germination and early seedling survival were controlled in part by number\ud of adult neighbors, although adult densities at the larger scale did not predict germination\ud success. The results of this bay-wide study suggest that Z. marina recruitment patterns in \ud Willapa Bay are driven by seed abundance, but differences in germination success were also important