Hypoxia‐induced shifts in nitrogen and phosphorus cycling in Chesapeake Bay

Abstract

We investigated interactions between hypoxia and nutrient cycling in Chesapeake Bay using quantitative analysis of long‐term monitoring data covering the periods 1965‐1980 and 1985‐2007. The data included vertical water column profiles of temperature, salinity, NH+4, PO3‐4, and O2, as well as rates of total nitrogen (TN) and total phosphorus (TP) loading to the bay from the Susquehanna River. We investigated the hypothesis that a doubling of the volume of hypoxic (O2 < 62.5 µmol L−1) water generated per unit TN load in the past 25 yr is related to enhanced water‐column and sediment recycling of NH+4 and PO3‐4 under low O2, and that this increased nutrient recycling creates a feedback that further generates hypoxia. We found that bottom water in the upper bay region, where seasonal hypoxia first develops, was enriched in NH+4 and PO3‐4 relative to other regions. Evidence of the positive feedback effect of low O2 on nutrient recycling was found in the fact that bottom‐water pools of NH+4 and PO3‐;4 per unit TN and TP loading, respectively, were significantly and positively related to hypoxic volume in upper bay regions during June. Similarly, NH+4 pools generated per unit TN load were significantly higher during 1985‐2007, when hypoxic volume had been approximately doubled, relative to 1965‐1980. This positive feedback effect on nutrient recycling may help to explain the persistence of extensive hypoxia in June, even during years of reduced N loading.