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The upcoming mid-point assessment for the Chesapeake TMDL slated for 2017 will be aided by research and quantitative tools to interpret progress made to date, and anticipated response by 2025. Our proposed research targets questions that will inform our expectations of restoration trajectories for Chesapeake Bay and other eutrophic temperate estuaries where non-linear responses and lags (hysteresis) may occur even as nutrient loads are reduced. Our restoration goals for estuarine water quality are directly tied to the process of oligotrophication; implementing management actions that lead to lower production of organic matter and improved water quality. While the history of increased nutrient loading in systems like the Chesapeake Bay have resulted in greater understanding of the processes of eutrophication, far less experimental work has occurred to understand oligotrophication.
Our key objective is to quantify the lost ecosystem services related to nitrogen cycling under hypoxic conditions. How much is denitrification inhibited under hypoxic bottom water conditions? To what extent does low dissolved oxygen enhance remineralization of organic N and P to the water column? In the context of restoration efforts, we hypothesize that eutrophic estuaries are more resilient and exhibit signs of hysteresis, where efforts to reduce nutrient loading may encounter a restoration trajectory that does not respond linearly to load reductions.
We propose to investigate the resilience of eutrophic estuarine ecosystems subject to hypoxic bottom waters by experimentally manipulating dissolved oxygen at the whole-tributary scale in Chesapeake Bay. Leveraging a unique, large-scale engineered aeration system in the Rock Creek estuary, we describe a field campaign to collect rate measurements and state variables needed to characterize a nutrient budget and quantify nutrient cycling under both hypoxic and normoxic conditions. A series of experiments throughout the year will involve experimentally reducing dissolved oxygen in bottom waters by turning off the large scale aeration system for one-week time periods. Experimental design will follow a modified "before-after-control-impact" design and include measurement of ecosystem responses of nutrient concentrations, chlorophyll-a, dissolved oxygen, primary production, respiration, and sediment fluxes of nitrogen and phosphorus to characterize benthic-pelagic coupling.
Measurements in the Rock Creek estuary will be used to parameterize and validate a biogeochemical model that may then be used in other systems to enumerate the extent to which low oxygen changes nutrient cycling; serving as a quantitative tool to track the cost of hypoxia and to provide a means to explore the process of oligotrophication. The increased efficiency of hypoxic systems, where a single molecule of N is likely to be recycled more frequently before it is lost via denitrification or burial, will be quantified and put into the hands of managers seeking expected responses to restoration actions. Our outreach plan will include direct interaction with environmental engineers, the Chesapeake Bay Program, Maryland DNR, Anne Arundel County, and local citizens' groups to customize our results to management-specific needs and actions at the Rock Creek and Chesapeake Bay scales.
Testa, JM; Kemp, WM; Harris, LA; Woodland, RJ; Boynton, WR. 2016. Challenges and directions for the advancement of estuarine ecosystem science. Ecosystems20(1):14 -22. doi:10.1007/s10021-016-0004-0. UM-SG-RS-2016-23.