Zostera marina (eelgrass) growth and survival along a gradient of nutrients and turbidity in the lower Chesapeake Bay

Survival of transplanted Zostera marina L. (eelgrass), Z. marina growth, and environmental conditions were studied concurrently at a number of sites in a southwestern tributary of the Chesapeake Bay to elucidate the factors limiting macrophyte distribution in this region. Consistent differences in survival of the transplants were observed, with no long-term survival at any of the sites that were formerly vegetated with this species but that currently remain unvegetated. Therefore, the current distribution of Z. marina likely represents the extent of suitable environmental conditions in the region, and the lack of recovery into historically vegetated sites is not solely due to lack of propagules. Poor long-term survival was related to seasonally high levels of water column light attenuation. Fall transplants died by the end of summer following exposure to levels of high spring turbidity (K-d \u3e 3.0) Accumulation of an epiphyte matrix during the late spring (0.36 to 1.14 g g(-1) dry wt) may also have contributed to this stress. Differences in water column nutrient levels among sites during the fall and winter (10 to 15 mu M dissolved inorganic nitrogen and 1 mu M dissolved inorganic phosphates) had no observable effect on epiphyte accumulation or macrophyte growth. Salinity effects were minor and there were no symptoms of disease. Although summertime conditions resulted in depressions in growth, they did not alone limit long-term survival. It is suggested that water quality conditions enhancing adequate seagrass growth during the spring may be key to long-term Z. marina survival and successful recolonization in this region

How do nutrient conditions and species identity influence the impact of mesograzers in eelgrass-epiphyte systems?

Coastal eutrophication is thought to cause excessive growth of epiphytes in eelgrass beds, threatening the health and survival of these ecologically and economically valuable ecosystems worldwide. Mesograzers, small crustacean and gastropod grazers, have the potential to prevent seagrass loss by grazing preferentially and efficiently on epiphytes. We tested the impact of three mesograzers on epiphyte biomass and eelgrass productivity under threefold enriched nutrient concentrations in experimental indoor mesocosm systems under summer conditions. We compared the results with earlier identical experiments that were performed under ambient nutrient supply. The isopod Idotea baltica, the periwinkle Littorina littorea, and the small gastropod Rissoa membranacea significantly reduced epiphyte load under high nutrient supply with Rissoa being the most efficient grazer, but only high densities of Littorina and Rissoa had a significant positive effect on eelgrass productivity. Although all mesograzers increased epiphyte ingestion with higher nutrient load, most likely as a functional response to the quantitatively and qualitatively better food supply, the promotion of eelgrass growth by Idotea and Rissoa was diminished compared to the study performed under ambient nutrient supply. Littorina maintained the level of its positive impact on eelgrass productivity regardless of nutrient concentrations