Positive and Negative Feedbacks Within Zostera marina Beds Within the Chesapeake Bay, Virginia.

摘要

Particulate levels within marine, estuarine and freshwater vegetated shallows are often lower than in nearby open water, although most of the studies quantifying this trend are from non-tidal, freshwater systems. The potential positive feedbacks between vegetation, water clarity, and zooplankton clearance and the potential negative feedback from microbially-mediated sulfide production were investigated in several eelgrass (Zostera marina) beds in the lower York River and Mobjack Bay, Chesapeake Bay, Virginia and through the construction of a computer simulation model.;Paired automated chlorophyll a and turbidity sensors were deployed for eight one-week periods to compare particulate levels inside and outside of eelgrass beds. The vegetated estuarine shallows monitored appeared to behave differently than those in freshwater vegetated systems, in that they were not able to consistently maintain improved water clarity relative to adjacent, unvegetated areas. Predictive equations for particulate levels inside the eelgrass beds were developed by regressing chl a and turbidity against wind and tidal influences for use in a Zostera simulation model.;Zooplankton were sampled during two summer seasons to quantify their impact on water clarity. In 2006, zooplankton densities were significantly higher in vegetated than non-vegetated areas, but not in 2007. Zooplankton densities were significantly higher at night, both inside and outside of the vegetated beds. Overall, the zooplankton densities encountered within the SAV beds had the potential to filter approximately 2-6% of the water column per day, much less than typically encountered in freshwater.;Eelgrass density, sediment organic content and porewater sulfide levels were quantified in situ in several SAV beds throughout spring and summer. There was no significant difference in [S] between vegetated and unvegetated areas, [S] was not correlated with eelgrass cover or sediment organic levels, but field results demonstrated that porewater [S] above 900-1000 muM inhibited eelgrass growth within the study area. An iron enrichment experiment demonstrated some potential for iron to positively affect Z. marina growth and survival, but responses were site specific and highly variable.;Finally, a computer simulation model was constructed that incorporated positive and negative effects within Z. marina beds, including tidal- and wind-induced particulate loading, resulting attenuation of light, particulate removal due to biological and physical filtration, temperature stress and sulfide toxicity. Modeled Z. marina responded to reduced light with approximately proportional reductions in year-end shoot and root/rhizome biomass. The model was less sensitive to increased sulfides; increases of 1.5, 2.0 and 2.5 times background sulfide levels resulted in incremental reductions of year-end shoot biomass by 20-25% and root/rhizome biomass by 15-20%. The model was most sensitive to temperature; a 1°C increase reduced year-end shoot and root/rhizome biomass by 41%; sulfide and temperature stress combined reduced shoot and root/rhizome biomass by 64%. With eelgrass in the Chesapeake Bay growing near its southern limits, model results indicate that either sulfide or temperature stress may limit restoration efforts and induce continued losses of eelgrass. Internal feedbacks reduce some of the stress caused by light limitation, but do not compensate for a 1°C increase in temperature or increases in sulfide levels.