Sedimentary Oxygen Demand and Orthophosphate Release: Sustaining Eutrophication in a Tributary of the Chesapeake Bay
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Sedimentary Oxygen Demand and Orthophosphate Release: Sustaining Eutrophication in a Tributary of the Chesapeake Bay

  • Published Date:

    2018

  • Source:
    Journal of Water Resources and Ocean Science. Vol. 7, No. 3, 2018, pp. 42-48
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  • Description:
    Beginning in the mid 20th Century the Chesapeake Bay began to show the first signs of eutrophication, with seasonal depletion of free oxygen in bottom waters (hypoxia). Eutrophication is driven largely by external loading of phosphorus (P) and nitrogen (N). These nutrients maintain high levels of phytoplankton productivity and subsequent transfer of fixed carbon to the sediments. That carbon fuels heterotrophs that uptake free oxygen in the bottom waters at a faster rate than it can be replenished during seasonal stratification, resulting in periods of persistent hypoxia and anoxia. Aerobic and anaerobic decomposition of the settled plankton and detritus drives the release of remineralized nutrients such as orthophosphate (P). Episodic and seasonal mixing events transport the N and P to better illuminated surface waters where it supports blooms of phytoplankton, which will settle and continue the positive feedback loop of eutrophication. To better understand the role of sediments in the ongoing stress caused by eutrophication in the Chesapeake Bay we incubated sediment cores at temperatures to model an in situ seasonal cycle. We measured oxygen concentrations and P levels to estimate the release of orthophosphate to the overlying waters under various oxygen conditions. During oxic conditions the net flux of orthophosphate was from the water column into the sediments. Anoxia drove P flux from the sediments back to the water column. These results indicate internal P loading during periods of anoxia by the sediments to the water column may lead to continued eutrophication.
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