Across the salinity transition: Changes in N and P biogeochemistry along an estuarine salinity gradient.

Collaborative Research: Principal Investigator: Thomas E. Jordan
Smithsonian Environmental Research Center

Co-Principal Investigators: Jeffrey C. Cornwell and Walter R. Boynton
University of Maryland Center for Environmental Sciences
Horn Point Laboratory and Chesapeake Biological Laboratory
Funding from the National Science Foundation

Project Summary

Eutrophication is mainly caused by large-scale anthropogenic alterations of the cycles of nitrogen (N) and phosphorus (P). Generally, P enrichment has the greatest impact in freshwater while N enrichment has the greatest impact in seawater. In estuaries, where freshwater and seawater mix, there are spatial and temporal changes in the relative abundance of N and P, which present difficulties for prioritizing nutrient management. This project will compare the interactions of N, P, sulfur, and iron cycles along the estuarine transition from freshwater to saltwater. The research will focus on the interface of fresh and salt waters in the Patuxent River estuary where there is a well-defined salinity gradient and a wealth of background information from previous and ongoing research and monitoring.
      Along the salinity gradients of estuaries, biologically available N can be consumed by denitrification while available P may be enriched by the dissolution of inorganic particulate P. The objectives of this study are to assess the importance of terrigenous inorganic particulate P as a source of available P, to determine the dominant mechanisms responsible for increases in available P at the upstream edge of the salinity gradient, to determine the effects of salinity on denitrification and on releases of available N from sediments, and to compare net burial of N and P in sediment along the salinity gradient.
      The study will use a combination of observations and experiments to investigate the changes in N and P biogeochemistry from seasonally fresh to mesohaline waters. Mass balances of forms of N and P will be used to synthesize the results and assess the importance of different biogeochemical processes. The study will measure watershed inputs of nutrients (including different forms of particulate P) and the transport and release of P from suspended particles moving down the estuary. Denitrification and the releases of various forms of N and P from sediments will be measured by incubations of intact cores taken from along the salinity gradient. To investigate changes in the biogeochemistry and net burial of N and P, profiles of N, P, iron, sulfur, 210Pb, and 7Be in sediments will be measured along the salinity gradient. To investigate mechanisms accounting for changes rates of denitrification and release of available P along the salinity gradient, cores will be incubated with experimental manipulations of concentrations of seawater, sea salts, sulfate, and nitrate.
      Intellectual merit--The analysis of changes in N and P biogeochemistry along the transition from fresh to mesohaline water will provide insights into the factors accounting for the usual switch from P limitation in freshwater to N limitation in estuarine waters. The study will also provide the most detailed look ever at the origins and fates of different particulate P fractions in an estuary. Linking the results with data from other studies will form a more complete picture of nutrient dynamics in the Patuxent estuary, especially in the zone of transition from fresh to saline water. The study will employ the newest and most accurate method for measuring denitrification to observe changes across the salinity front. The experimental manipulations will reveal mechanisms responsible for the changes in rates of denitrification and benthic dissolved inorganic P release in response to changes in salinity.
      Broader impacts--The study has relevance to managing the widespread problems of coastal eutrophication. To reduce the impacts of nutrient loading, it is essential to understand the factors that determine which nutrient will limit primary production. It is also important to know how nutrient loading may be mitigated as potential sinks for N and P change along the estuarine salinity gradient. The study will address both these issues. The research will also assess the potential effects of watershed discharges of different forms of particulate P. Although most P enters estuaries in particulate forms, little is known about how much of this particulate P is converted to biologically available P in estuaries. The research will provide information about this, which may be applied to setting limits (such as TMDLs) on P loads. The project will support two Ph.D. students. In addition, 6 undergraduate interns (2 per year) will participate in the research with support from other funds.

Read the abstract of a paper presented at the 8th International Estuarine Biogeochemistry Symposium, May 2004.