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RESPONSE OF GULF COAST ESTUARIES TO NUTRIENT LOAD: DISSOLVED OXYGEN DEPLETION

Contact

email: CEMM@epa.gov

Impact/Purpose:

The objectives of this project are to define and quantify relationships between nutrient loading and dissolved oxygen levels in coastal receiving waters and provide tools useful for developing nutrient criteria protective of aquatic life and designated uses in the Gulf of Mexico region. Nutrients are one of the most often cited stressors reported by States as the reason for listing waters as impaired. The process by which nutrient enrichment causes hypoxia is complex and indirect, but a typical scenario is as follows: excess nutrients stimulate production of algae in the receiving water; some of this algae sinks to bottom waters where decomposition by oxygen-consuming organisms depletes oxygen more rapidly than it can be resupplied via atmospheric exchange. As a result, dissolved oxygen decreases to a level below which animal life can survive. In most cases, a necessary prerequisite for hypoxia is water column stratification, which restricts atmospheric exchange with bottom waters. Estuaries along the Gulf coast experience episodes of hypoxia naturally, but excess nutrients can exacerbate the spatial extent and duration of hypoxic areas. Long-term increases in nutrient loading have been related to increased prevalence of hypoxia in some well-studied systems, and it is likely that reducing nutrient loads will decrease the incidence of hypoxia.

Description:

GED has developed a process-based approach to hypoxia research on Pensacola Bay as a model Gulf of Mexico estuary. We selected Pensacola Bay because, like many Gulf coast estuaries, it is shallow, microtidal, and experiences seasonal hypoxia. We also have an historical database from EMAP-related activities and other research programs resulting from the close proximity of GED to this system. In examining the historical data, we identified data gaps that we are addressing in this project. Existing data did not provide critical information needed to model the factors that contribute to hypoxia development. Examples include seasonal dynamics, physical processes, resolution of major water quality gradients, and measurements of biological rates. To resolve these data gaps, we initiated a limited field program in 1999 and expanded the effort in 2002. The sampling design includes 16 stations, sampled monthly, located along major salinity gradients in the Pensacola Bay system. Hydrographic variables are profiled at each station. Surface and bottom water samples are collected and analyzed for: chlorophyll (<5 m and whole water), particulate organic carbon and nitrogen, total suspended solids, dissolved nutrients (nitrogen, phosphorus, silica), total nitrogen and phosphorus, dissolved organic carbon. The resulting dataset has thus far supported development of models describing physical transport processes using a salt- and water-balance approach and will support further model development. Important biological rates have also been measured on a more limited basis. Recognizing the importance of the benthos in shallow Gulf systems, we have measured benthic oxygen and nutrient exchanges, planktonic primary production and respiration. These process studies will describe turnover times of nutrients, carbon and oxygen in the water and sediments, and will help describe the factors that control these rates. The results of process-based research in Pensacola Bay will guide our approach to predicting susceptibility of other estuaries to hypoxia based on less data, and will help guide the design of efficient and effective field efforts to characterize other Gulf systems for their susceptibility to hypoxia.

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Record Details:

Record Type:PROJECT
Start Date:10/01/1997
Projected Completion Date:09/30/2011
OMB Category:Other
Record ID: 83347