Science Serving Maryland's Coasts

R/EH-17

Quantifying Changes to Nutrient Cycling and Nitrogen Removal in an Estuary as a Consequence of Aeration

Principal Investigator: 

Lora A. Harris

Start/End Year: 

2016 to 2018

Institution: 

Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science

Co-Principal investigator: 

Jeremy Testa, Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science

Strategic focus area: 

Resilient ecosystem processes and responses

Description: 

 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.

Impact/Outcome: 

RelevanceThe Patapsco River in Baltimore, is a highly polluted estuary inlet of the Chesapeake Bay. In 1988, engineers installed an aeration system in one of its tributaries, Rock Creek, to improve water quality. The system pumps air into the water, which mixes with oxygen-rich surface waters to prevent low-oxygen conditions. Researchers are studying Rock Creek to determine how phytoplankton growth and nitrogen and phosphorus recycling changes with alterations to the oxygen concentrations, and how fast the system responds to low-oxygen events. Before aeration, Rock Creek, which runs through Anne Arundel County, had too little oxygen and too much algae, which fueled the growth of plankton. When the plankton died, the bacteria used all available oxygen to decompose the dead plankton, and then used sulfate to continue decomposition. The byproduct of the resulting reactions was hydrogen sulfide, which caused a rotten-egg smell that residents found unpleasant. Fish could not survive in the waters. The aerators changed that, pumping 15,000 liters of air per minute into the water.

ResponseIn 2017, Maryland Sea Grant supported researchers who wanted to quantify the benefits and determine to what extent the level of dissolved oxygen changed how nitrogen and phosphorus were recycled in the water column and sediments. If nutrient recycling were to increase under low oxygen conditions, each molecule of nutrient discharged into the creek would have more opportunities to grow plankton, stalling restoration efforts. Researchers from the University of Maryland Center for Environmental Science Chesapeake Biological Lab conducted field experiments, turning off the aeration systems for a day and up to a week to monitor biological activity in Rock Creek as oxygen conditions collapsed. The researchers have since added a carbon component to their study, to examine how the metabolism (or breathing in and out) of the estuary changes with aeration.

ResultsThey found that oxygen disappeared almost entirely within 24 hours after the aerators were turned off, and the area of depleted oxygen expanded far beyond the aerated region. Managers throughout the Chesapeake Bay seeking to increase oxygen in tributaries and reduce nitrogen could expand the tools available to do so. The researchers are advising Anne Arundel County managers as they work on a new aeration system, and their information is intended to help the Rock Creek community apply for additional restoration funding to further clean up the creek.

Related Publications: