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

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 (NH 4 + ), 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 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 NH 4 + , 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 NH 4 + 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 SloughResale or republication not permitted without written consent of the publisher

Signatures of Restoration and Management Changes in the Water Quality of a Central California Estuary

“…4). Caffrey et al measured a benthic flux in APS as high as 8.6±2.9 mmol NH 3 m −2 day −1 , with concentration peaks at least one order of magnitude smaller than the concentration spikes seen in our data (Caffrey et al 2002b). Similar microcosm studies for nitrate have shown net negative benthic fluxes and indicate that the sediments in Elkhorn Slough can act as a sink for nitrate rather than a source.…”

“…The magnitude of this sink is on the order of −12 to −2 mmol nitrate m −2 day −1 depending on the sedimentary environment (K. Johnson, personal communication). A similar flux of −4.43±2.67 mmol NO 3 m −2 day −1 was measured in APS (Caffrey et al 2002b). While this sedimentary sink for nitrate could be contributing slightly to the decrease in concentrations seen at APC and APN, the magnitude is small and is not enough to be the primary mechanism accounting for the overall decrease seen postrestoration.…”

“…While this sedimentary sink for nitrate could be contributing slightly to the decrease in concentrations seen at APC and APN, the magnitude is small and is not enough to be the primary mechanism accounting for the overall decrease seen postrestoration. For dissolved inorganic phosphate (DIP), maximum benthic fluxes measured in APS were 1.08± 0.52 mmol DIP m −2 day −1 and were observed to be negative fluxes in other parts of the slough (Caffrey et al 2002b). Together these studies indicate that typical benthic fluxes for these three nutrients are too small to account for the decreases in nutrient concentrations seen at the Azevedo Ponds postrestoration and that these concentration changes therefore are most likely coupled to land processes associated with changes in agricultural runoff.…”

Temporal variability in nitrification rates and related biogeochemical factors in Monterey Bay, California, USA

“…1). Those results have been published separately in conjunction with nutrient and nitrification rate measurements from that site by Caffrey et al (2002Caffrey et al ( , 2003, who described sequence-analysis methods and results and their findings are related to the current study in 'Discussion' below.…”

“…In companion studies carried out over roughly the same time period as the Monterey Bay study reported here (Caffrey et al 2002, potential nitrification rates (measured in sediment slurries) and supporting measurements were made at monthly or slightly longer intervals at 5 sites in the slough. Although the slough sites varied amongst themselves in terms of their temporal variability, they all showed strong seasonal trends in temperature (Caffrey et al 2002) and increasing salinity (from nearly fresh in the spring up to 40 or 70, depending on site) in latesummer. Nitrification rates varied among sites and over time; maximum potential nitrification rates varied by up to 500-fold within a single site during the course of a year .…”