Research Technical Report

Nutrient Inputs to Elkhorn Slough, CA: Interpreting an 8 Year Record of Volunteer Water Quality Monitoring

Caffrey, J.M, S. Shaw, M. Silberstein, A. De Vogelaere, M. White, and K. Thomasberg (1998)

Report submitted to Monterey Bay National Marine Sanctuary

ABSTRACT

Elkhorn Slough National Estuarine Research Reserve (ESNERR) and the Elkhorn Slough Foundation (ESF) have been supporting a volunteer monitoring program since 1988. Twenty four stations are sampled monthly for temperature, salinity, dissolved oxygen, pH, turbidity, nitrate, ammonium, and dissolved inorganic phosphate. This program represents a partnership among ESNERR, ESF, Monterey County Water Resources Agency, and highly dedicated volunteers. One significant result is an appreciation of agricultural inputs of nutrients, particularly nitrate. Intensive fertilization of row crops such as strawberries and artichokes can lead to extremely high nutrient concentrations in the freshwater reaches of the Slough seasonally. At the head of Elkhorn Slough, nitrate concentrations can exceed 500 µM during the winter rainy season, although average concentrations are usually about 16 µM. Nitrate concentrations are excessive in the lower Salinas River and old Salinas River Channel, which drains into the lower reaches of the slough. Concentrations usually exceed 1,000 M and are highest during the summer months. Nitrate concentrations have increased significantly since the 1970s with peak concentrations in the 1990s an order of magnitude higher than 1970s measurements. In contrast, peak concentrations of ammonium and dissolved inorganic phosphate have doubled over the 20 yr period, although ammonium and dissolved inorganic phosphate concentrations average 7 and 4 M, respectively, at all stations.