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Sediment, Season, and Salinity Effects on Phosphorus Concentrations in an Estuary¹

ABSTRACT

A 4-yr study was made to better understand nonpoint-source inputs of P into the Wye River—a shallow, well-mixed estuary of the Chesapeake Bay. Water and sediment samples were taken periodically from along the length of the river and from the main freshwater tributary. Phosphorus forms and concentrations in solution were measured and P sorption isotherms were determined from sediment samples.

Molybdate-reactive P concentrations were highest during the summer in the uppermost reaches of the river under tidal influence and decreased with increasing salinity downstream. Molybdate-reactive P concentrations in the river thus appeared to depend on the season and to indicate that increasing temperatures promote P release from sediment and P mineralization by benthic and other marine organisms. In addition, molybdate-reactive P in the river was released from sediment when fresh water from a tributary diluted the divalent cation concentration of the brackish water. Best-management farm practices to reduce the loss of P by runoff would not be expected to markedly lower the concentration of molybdate-reactive P in the river.

Key Words: calcium • magnesium • molybdate-reactive phosphorus • hydrolyzable phosphorus • organic phosphorus • phosphorus sorption isotherms

¹ Contribution from the USDA-ARS Soil Nitrogen and Environ. Chemistry Lab., Beltsville, MD 20705.

² Soil scientist, and research chemist, USDA-ARS, Beltsville, MD.

Received for publication July 30, 1984.