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