Background/Question/Methods  Increases in the frequency and severity of disturbance events, such as periodic hypoxia and sediment resuspension by trawling, have lead to dramatic changes in benthic invertebrate communities in many coastal regions.  In order to predict the effects of disturbance on ecosystem function, it is necessary to improve our understanding of how changes in invertebrate density and diversity affect benthic geochemical cycling.  We used laboratory microcosms to test the effects of recolonization of previously defaunated sediments by deep-burrowing polychaetes (Nereis virens) and suspension-feeding bivalves (Mya arenaria) on porewater ammonium and soluble-sulfide concentrations as well as benthic fluxes of oxygen and nutrients: dissolved inorganic nitrogen (ammonium and nitrate + nitrite, or DIN) and soluble reactive phosphorus (SRP).  We added single and two-species treatments including a range of densities comparable to densities common for shallow estuaries.  We sampled porewater and measured fluxes under both dark and light conditions one month after organism addition.    
Results/Conclusions Effects of N. virens on porewater chemistry were significantly greater than M. arenaria, but even the lowest density of each organism lowered porewater ammonium and soluble-sulfide concentrations significantly relative to defaunated controls.  However, increasing density had little additional effect on porewater.  Benthic oxygen and nutrient fluxes generally increased between low and high densities.  However fluxes of DIN and SRP were higher in controls than in low-density treatments and fluxes of SRP were significantly lower in N. virens treatments than M. arenaria treatments.  This suggests that whether organisms would increase or decrease nutrient availability depends on density and that the ratio of DIN:SRP release depends on species composition.  Mya arenaria also enhanced benthic gross primary production (GPP), whereas N. virens and two-species treatments had no obvious effects on benthic GPP.  Changes in porewater solute concentrations and benthic flux rates in two-species treatments were generally greater than expected based on single-species treatments, and deviation of observed from expected values increased with density.  This suggests that niche complimentarity could influence the effects of these organisms on elemental cycling and that the effect becomes more important with increasing density.