Sediment biogeochemistry along an oyster aquaculture chronosequence

Abstract

Oyster aquaculture is expanding globally, and there has been a recent research surge examining how oyster farms alter coastal ecosystems. Yet, until now, the role of time in these studies has largely been missing. Here we used an in situ chronosequence approach to determine how the presence of oyster aquaculture (Crassostrea virginica) altered sediment nitrogen (N), oxygen (O2), and phosphorus (P) cycling. Overall we found that the sum of nitrogen fluxes increased significantly following addition of aquaculture, and switched from net N consumption (i.e. net nitrogen fixation: -14.41 µmol N m-2 h-1) to production (i.e. net denitrification: 553.57 µmol N m-2 h-1). Ammonium (NH4+) fluxes did not differ between bare sediment and oyster aquaculture. Additionally, both the magnitude of N2 and NH4+ fluxes oscillated on an annual scale of aquaculture age, but not predictably so. We observed significantly more variance (σ2) in dinitrogen and NH4+ fluxes in sediments beneath aquaculture, indicating increased non-linearity. O2 fluxes increased from Years 4 to 6, before returning to baseline conditions. There were no differences in sediment P cycling. This study demonstrates that sediment biogeochemical processes can become non-linear under the pressure of oyster aquaculture, and this non-linearity likely has important implications for ecosystem function.