Hypoxia-induced shifts in nitrogen and phosphorus cycling in Chesapeake Bay
Limnol. Oceanogr., 57(3), 2012, 835-850 | DOI: 10.4319/lo.2012.57.3.0835
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.