Warming and nutrient loading interact to shift N gases from the eastern oyster Crassostrea virginica

Bivalves harbor nitrogen-transforming microbes. Warming and excess nutrients may alter coastal nitrogen cycling by affecting activities of bivalves and these microbes. N 2 and nitrous oxide (N 2 O) gas production from the eastern oyster Crassostrea virginica were compared under 2 temperatures (18 and 24°C) and a range of nitrogen (N) loads (65 to 140 µM ammonium nitrate) for up to 2 mo. Data were analyzed with multi-factor ANOVAs, principal component and factor analyses, and partial least squares regression. Denitrification (DNF) rates were greater at the warmer temperature (p = 0.05) by 5 d. By 2 mo, DNF rates were significantly affected by N additions (p < 0.01). Specifically, net DNF shifted among warmed oysters (24°C) to net nitrogen fixation (NF) above 110 µM ammonium nitrate. DNF rates were moderately related to temperature, pH, and dissolved oxygen. NF rates had moderately strong linkages to N addition levels (p = 0.02) and temperature (p = 0.03). Potential N 2 O production was low and did not significantly vary with treatments. This study suggests warming and N loading may stimulate nitrogen removal by oysters in the short term, whereas over longer periods additional internal nitrogen inputs may increase.