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

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 (S^2-) 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 1^-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 S^2- 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.