Loss of salt marsh plants impacts ribbed mussel (Geukensia demissa) size, density, and influence on sediment nitrogen cycling

The mutualism between Spartina alterniflora (Smooth Cordgrass) and Geukensia demissa (Ribbed Mussels) can increase ecosystem services, including the removal of excess nitrogen via denitrification. However, different responses of these species to sea-level rise and eutrophication can cause mussel beds to persist in areas where cordgrass has been lost to erosion and excessive tidal inundation. The function of these remnant mussel beds, in the absence of cordgrass mutualists, remains unclear. In this study, we sampled an eroding salt marsh in Jamaica Bay (NY, USA) to determine mussel density and size in non-vegetated mussel beds and vegetated marsh. We also collected sediment cores from remnant mussel beds and adjacent mudflats (i.e., former marsh) and performed continuous-flow core incubations to measure benthic nutrient fluxes, sediment oxygen demand, and denitrification. We found that remnant mussel beds were dominated by larger mussels compared to vegetated beds, which suggests less recruitment to remnant beds. In contrast to previous studies, which have found positive effects of mussels on denitrification, we found similar rates in mussel beds and mudflats. Sediment oxygen demand was also higher in remnant mussel beds suggesting that sediments may become more reduced due to mussel respiration. Our study suggests that the presence of mussels does not enhance, and may inhibit, nitrogen removal in eroded marsh sediments.