MEPS 233:39-53 (2002)  -  doi:10.3354/meps233039

Biogeochemical processes in a small California estuary. 1. Benthic fluxes and pore water constituents reflect high nutrient freshwater inputs

Jane M. Caffrey1,*, Neil Harrington2, Bess Ward3

1Center for Environmental Diagnostics and Bioremediation, University of West Florida, 11000 University Parkway, Pensacola, Florida 32514-5751, USA
2Department of Ocean Sciences, University of California Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA
3Department of Geosciences, Guyot Hall, Princeton University, Princeton, 110 West College, Box 430, New Jersey 08544, USA

ABSTRACT: Elkhorn Slough, a small estuary in central California, receives nutrient inputs from agricultural and other non-point source runoff. To evaluate the effect of nutrient loading on ecosystem processes, rates of sediment oxygen consumption, benthic nutrient fluxes, pore water ammonium (NH4+), dissolved inorganic phosphate (DIP) and sulfide (S2-) concentrations were measured at 5 sites several times each year between 1998 and 1999. Two sites near the head of the slough receive direct runoff from agricultural fields, while the sites in the middle reaches and mouth receive direct runoff from grasslands and woodlands. Sites receiving agricultural runoff usually had high water column concentrations of dissolved inorganic nitrogen and chlorophyll a (50 µM and 30 µg l-1, respectively). High precipitation in the winter of 1998 caused low salinities in the slough at that time. In general, salinity and nutrient concentrations were inversely correlated. Sites receiving agricultural runoff had high pore water NH4+, DIP and dissolved S2- concentrations, particularly in the summer. Oxygen penetration into sediments was the greatest at the sites adjacent to grasslands and woodlands, and least at the sites receiving high nutrient runoff. Sediment oxygen consumption was 4 times greater at sites near agricultural fields than the site near the mouth, while NH4+ flux out of sediments was between 4 and 20 times greater at agricultural sites than at non-agricultural sites. This study suggests that high nutrient runoff has primarily a local rather than estuary-wide effect on sediment biogeochemical processes in Elkhorn Slough, perhaps due to longer water residence time at the head of the slough compared to the mouth.

KEY WORDS: Estuary · Eutrophication · Nutrients · Oxygen · Elkhorn Slough

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