Linkages between Phytoplankton and Bottom Oxygen in the Chesapeake Bay

The primary cause of coastal hypoxia is generally attributed to phytoplankton blooms and their subsequent benthic decay. However, direct linkages between phytoplankton and hypoxia have been rarely reported. Here using satellite and field data, we show that such linkages exist in the Chesapeake Bay. For deeper (>10 m) but nonhypoxic stations, bottom dissolved oxygen is significantly correlated with surface algal biomass during the preceding weeks. Optimal correlation exhibits a bimodal seasonal variation with two peaks in April and August, respectively. Significant particulate dislocation effect is also observed: In April, May, August, and September when water column is stratified, the most significant influencer of bottom oxygen is attributable to phytoplankton blooms located down-estuary; during destratified months of October and November, however, the dislocation effect switches to the opposite direction, that is, phytoplankton from up-estuary have the biggest impact. We also notice enhanced phytoplankton growth posterior to lower bottom oxygen, and the strongest correlation is during summer when release of recycled ammonium and hypoxia-induced phosphate is the most intense. These results demonstrate that the decay of phytoplankton exerts a significant impact on bottom oxygen and releases ammonium which sustains further bloom, whereas hypoxia promotes phytoplankton growth by enhancing the efficiency of phosphate recycling. Importantly, the capability of using satellite data to capture these linkages has significant ramifications for improving hypoxia forecasts in coastal systems. It essentially provides a way to characterize and constrain changes in bottom oxygen demand.