Influence of simulated bivalve biodeposition and microphytobenthos on sediment nitrogen dynamics: A laboratory study

Suspension-feeding eastern oysters, Crassostrea virginica, were once abundant in Chesapeake Bay and may then have exerted top-down control on phytoplankton and also reduced turbidities, thereby increasing light available to benthic plants. Alternatively, oysters may have simply recycled inorganic nutrients rapidly back to the water column, with no long-lasting reduction in phytoplankton biomass resulting from oyster feeding activity. To help distinguish between these scenarios, we explored changes in nitrogen fluxes and denitrification in laboratory incubations of sediment cores held under oxic and anoxic conditions in response to loading by pelletized phytoplankton cells, an experimental analog for oyster feces and pseudofeces. When organics were regenerated under aerobic conditions, typical of those associated with oyster habitat, coupled nitrification–denitrification was promoted, resulting in denitrification of ;20% of the total added nitrogen. In contrast, under anoxic conditions, typical of current summertime conditions in main-stem Chesapeake Bay where phytoplankton is microbially degraded beneath the pycnocline, nitrogen was released solely as ammonium from the added organics. We postulate that denitrification of particulate nitrogen remaining in oyster feces and pseudofeces may enhance nitrogen removal from estuaries. In aerobic incubations with sufficient light (70 mmol m22 s21), a benthic microalgal/cyanobacterial community grew that not only absorbed the inorganic nitrogen released from the added organics but also fixed N2. This result suggests that an ecosystem dominated by benthic primary production may develop in shallow waters when reduced turbidity associated with bivalve feeding increases light penetration to a level that can sustain benthic microalgal production. The Chesapeake Bay, in common with many estuaries worldwide, is subject to extreme anthropogenic nutrient enrichment that promotes high levels of phytoplankton production (Malone 1992). Phytoplankton ungrazed by metazoan consumers is advected to the bottom of the deep central channel in the middle and upper Chesapeake Bay (Kemp and Boynton 1992), where it is subject to microbial degradation. The resulting intense microbial respiration exceeds the rate at which surface oxygen is mixed to the bottom, so anaerobic microbial processes predominate in bottom waters in summer months (Boicourt 1992). Because bacterially mediated coupled nitrification–denitrification (Henriksen and Kemp 1988; Koike and Sorensen 1988) is inhibited under such anoxic conditions, nitrogen is regenerated solely as NH , which, when mixed into the euphotic zone, stimulates 4 further phytoplankton production. We hypothesize that abundant stocks of benthic suspension feeders will (1) consume phytoplankton in eutrophic coastal waters, thereby reducing the amount of material available to be regenerated through water-column microbial processes and (2) reduce turbidity by filtering phytoplankton and inorganic particles .;3 mm from the water column and transferring in their feces and pseudofeces (collectively 1 Corresponding author ([email protected]).