Strong linkage between phytoplankton and oxygen suggest the value of satellite data for hypoxia forecasting

Hypoxia poses an increasing threat to global aquatic ecosystems. In coastal waters, the primary cause of rising hypoxia is generally attributed to eutrophication which promotes phytoplankton growth and subsequent benthic decay of phytoplankton-derived organic matter. Although the correlation between hypoxia and nutrient loadings was widely demonstrated, direct evidence showing linkages between hypoxia and phytoplankton has been rarely reported. Here we show such linkages in the Chesapeake Bay using high-resolution time series of satellite-derived chlorophyll data obtained with an absorption-based approach that teased out the interference of nonalgal pigmenting agents. We found that for non-hypoxic stations deeper than 10 m, dissolved oxygen in bottom waters of the Bay is significantly correlated with surface algal biomass during the preceding weeks with the degree of correlation varying with period of time. Optimal correlation exhibits a bimodal monthly variation with two peaks in April and August, respectively. Significant particle dislocation effect was not found except for fall season and in particular, October, when bottom DO along the bay mainstem is most correlated with [Chl-a] sampled 46 km upstream and 8−15 weeks before. We also observed feedback effects of bottom hypoxia on phytoplankton; bottom [DO] is also correlated with surface [Chl-a] sampled after it, and the strongest correlation is during summer when bottom hypoxia and the associated release of ammonium and phosphate is the most intense. These results demostrate that surface phytoplankton and bottom hypoxia are strongly linked together by providing ”feul” to each other; whereas phytoplankton generates oxygen-consuming organic matter, hypoxia promotes the production of ammonium and phosphate needed by phytoplankton. The capability of using satellite data to capture those linkages has significant ramifications for improving hypoxia forecasts in coastal systems by providing a way to characterize and constrain changes in oxygen demand in bottom waters.