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‘Day-Night Swings’ of Low Oxygen Impact Shallow Water Habitats and Animals: Past and Future Investigations

Hypoxia is a major concern for many of the nation’s waters
Hypoxia occurs when dissolved oxygen (DO) in the water becomes too low to support most life or compromises the growth, reproduction and immune responses of organisms.  Although some hypoxic zones can develop naturally, many such zones have worsened (and many others initiated) by excess nutrients from agriculture, human waste disposal, and the burning of fossil fuels.  Additionally, changes in oxygen can affect the pH1 of coastal waters which has further implications regarding the reproduction, growth, disease/parasite immunity and overall survival of animals.

Less research attention is focused on shallow-water oxygen depletion
Most of the current research on the ecosystem impacts of hypoxia focuses on deep-water areas such as the “dead zone” systems in the northern Gulf of Mexico, Baltic Sea, coastal Washington and Oregon, and the mainstem area of the Chesapeake Bay.  Less is known about the effects of shallow-water hypoxia, which is also a prevalent estuarine phenomenon.  Deep-water hypoxia is usually persistent (i.e., weeks-months-seasons) and even permanent.  However, shallow-water hypoxia is usually diel-cycling (diel: daily, pertaining to a 24-hr period). Diel-cycling hypoxia drives primarily by the 24 hour daytime-nighttime cycle of production and respiration, characterized by high DO during the day from photosynthesis and low DO at night from respiration by microbes, plants and animals.  Occurrence and duration of diel hypoxia can be influenced by the stability of the water column (i.e., no vertical mixing), cloud cover, water temperature, freshwater inflow (e.g., rain), tidal mixing, wind events, and time of day.  Although the underlying cause of diel-cycling hypoxia is biological activity (usually fueled by high nutrients or eutrophication), the rates of this activity are influenced by the physical conditions listed above.

Scientists still have little understanding of diel-cycling hypoxia causes and impacts
Although research scientists have known about diel-cycling hypoxia for several decades, there is still relatively little understanding of the factors that determine why and how much hypoxia happens in shallow diel-cycling systems.  The many factors influencing DO has made it difficult to understand exactly what causes diel-cycling hypoxia.  In addition, although hypoxia is detrimental to most organisms, linkages between deeper water hypoxia and reduced commercial fisheries landings are not always evident and can be paradoxical (e.g., high fisheries landings adjacent to “dead zones’).  Why is this?  One hypothesis claims that increased shallow water productivity offsets the corresponding productivity losses in deeper, oxygen-poor waters.  So what would happen if hypoxia occurred more frequently in adjacent shallow water refuges? Would fisheries productivity in these areas decline with associated system-wide productivity declines?  Could shallow water hypoxia reduce the potential for shallow water production to compensate for deep water hypoxic habitat loss?

Our hypoxia research studies diel-cycling hypoxia, pH, and the productivity paradox
NCCOS has awarded diel-cycling hypoxia funding to two research teams, each studying a different aspect of the low DO problem.  One project has delineated the myriad factors that cause and influence DO in shallow estuarine waters, specifically the Delaware Inland Bays, and additionally its impact on several fish species that live in those shallow waters.  The second project, just getting started in 2010, will explore the impact of hypoxia and associated pH on commercially and ecologically important finfish and oysters living in the shallow waters of Chesapeake Bay.  Both of these awards are part of the NCCOS Coastal Hypoxia Research Program (CHRP).

The first study, led by the University of Delaware with participation by the Virginia Institute of Marine Sciences (College of William and Mary), was undertaken in the coastal Delaware Inland Bays, specifically tributaries of the Indian River Bay. The project compiled an extensive database from years of observations of DO, water temperature, meteorological (i.e., air temperature, solar insolation, precipitation, wind), tide, streamflow and daytime surface to bottom profiles of DO, temperature and salinity.  This project is the first to examine the spatial extent of hypoxia in shallow diel-cycling systems.  The project found that most of the variation and duration in the spatial extent of diel-cycling hypoxia (minutes to hours) within Delaware Inland Bays can be explained by water temperature, previous day’s solar insolation, percentage of morning ebb tide, and daily streamflow, in that order.  The longest periods of severe hypoxia were associated with the combination of high water temperature, low solar insolation (i.e. cloudy weather), and high streamflow following a heavy rain event.  The findings of this project have application to other shallow estuaries with similar characteristics and contribute greatly to our understanding of this widespread and growing phenomenon.

The project also looked at the effects of diel-cycling hypoxia on the feeding habits of fish in a tributary of Indian River Bay and the changes in benthic (bottom) community structure that occurred in response to hypoxia.  They found that some species of fish that are physiologically capable of tolerating exposure to short-term hypoxia (e.g., weakfish and summer flounder) may take advantage of hypoxia-stressed prey associated with those productive benthic habitats.  These results suggest that the ability of fish to respond behaviorally and take advantage of increased prey resources in hypoxia-stressed habitats is species dependent.  For more details on this project see the “Linking Water Quality Models with Individual-Based Models to Investigate Impacts of Diel-Cycling Hypoxia on Nursery Habitat Quality for Estuarine Dependent Fishes and a recent project research publication “Temporal and Spatial  Dynamics of Diel-Cycling Hypoxia in Estuarine Tributaries”.

The second project will investigate diel-cycling hypoxia in the shallow waters of Chesapeake Bay.  Unlike the first project this study is investigating the hypothesis that shallow-water diel-cycling hypoxia and associated changes in pH may reduce the system-wide ability to compensate for deep-water hypoxia fishery production loss.  The project will investigate effects of diel-cycling hypoxia and associated day-night swings in pH as factors that may tip the relationship between nutrient loads and system-wide production of upper trophic level biota from positive to negative.  The Smithsonian Environmental Research Center is leading a multi-disciplinary team that includes the University of Delaware, Louisiana State University, Maryland Department of Natural Resources, U.S. EPA Chesapeake Bay Program and the NOAA Chesapeake Bay Office.  Results of this study will help Chesapeake Bay area management officials pinpoint key areas for habitat and fisheries restoration and better protect shallow water habitat that serves a critical nursery function.

Impacts of the research
The findings of these NCCOS research projects will have application to other shallow estuaries with similar characteristics and contribute greatly to our understanding of this widespread and growing phenomenon.  In addition these studies underscore the importance of implementing pollution management strategies aimed at reducing eutrophication.

1pH or “potential of hydrogen” is a scale expressing the acidity or alkalinity of a solution.  The pH decreases (i.e., becomes more acidic) with excess hydrogen ions.  Respiration produces carbon dioxide (CO2) that reacts with water to produce carbonic acid and thus more hydrogen ions.  Accompanying diel-cycling pH may exacerbate impacts of hypoxia, and the combination of these factors may lead to more severe effects than predicted by lab experiments that examine effects of low oxygen alone.

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