Abstract
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.
Highlights
Anthropogenic eutrophication of lakes, estuaries, and coastal waters is a difficult process to reverse [1,2]
sediment oxygen demand (SOD) for all of the stations peaked at 22°C which accounted for 46 % of the total annual SOD. 20 % of the annual oxygen demand occurred at15°C, and 19 % at 28°C
Our results show that for the Hampton River tributary in the early spring time, even before hypoxic events, the sediments demand 0.58 to 0.95 g O2 m-2 d-1
Summary
Anthropogenic eutrophication of lakes, estuaries, and coastal waters is a difficult process to reverse [1,2] It is typically first established by excessive loading of a combination of inorganic nutrients (N & P) and organic matter [3]. That newly released N and P from the decaying organic matter, along with N and P from the watershed promote blooms of phytoplankton [8,9] When those algae die and sink, it adds to the biological oxygen demand of the bottom waters and sediments. As with the organic matter washing in from the watershed, the settled algae degrade and release N and P Circulation of these regenerated nutrients to sufficiently illuminated surface waters stimulates another round of excessive phytoplankton growth [4, 10]. Suggesting that in these highly stressed ecosystems, “cascading” ecological phenomena can be
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