Seasonal variations in Rates of Net Community and Gross Primary Production on the Northeastern Shelf

The Northeast U.S. continental shelf, an economically and societally important region, has been experiencing increasing temperatures due to climate change. Understanding rates of productivity in this area, and how these rates change on seasonal and annual time-scales, is crucial for predicting the response of the ecosystem in the future. Net community production (NCP) reflects the total amount of CO₂ drawn down by the ocean’s biological pump. Gross primary production (GPP) is a measure of the photosynthetic flux and thus represents the total amount of CO₂ removed by photosynthesis. The ratio of net community production (NCP) to gross primary production (GPP) is a measure of export efficiency - a high ratio implies the system is exporting much of the organic matter it produces and thus recycling little. Gas tracers – namely oxygen to argon ratios and triple oxygen isotopes – can be used to quantify in situ rates of NCP and GPP. As part of the Northeast U.S. Shelf Long Term Ecological Research (NES-LTER) study, gas tracers have been used to quantify rates of NCP and GPP and their ratio in winter, spring, summer, and fall for two years. We have found seasonal changes in rates of productivity, their ratios and the associated onshore/offshore gradients. For example, observed efficiency of the ecosystem in terms of exporting carbon varied with distance offshore but in opposite ways in different seasons. In particular, in winter, we found that the highest rates of NCP and GPP and the largest export ratio occurred on shore while in the summer, the export ratio and productivity rates were higher offshore.