Santa Barbara Coastal Long Term Ecological Research (LTER): Nutrient Concentrations in Coastal Streams and Variations with Land Use in the Carpinteria Valley, California

Along the southern California coast, near Santa Barbara, we are (1) measuring nutrient loading to the nearshore environment from representative watersheds, and (2) developing a model to predict export from changes in land use. The area is characterized by a Mediterranean climate and short steep catchments producing flashy runoff; a majority of the annual nutrient export occurs within a few days each year. Six land use classes within the drainages of the Carpinteria Valley are being sampled to develop a nutrient export coefficient model within the context of a geographic information system (natural/undisturbed chaparral vegetation, avocado orchards, greenhouse agriculture, open-field nursery agriculture, residential development, and commercial and light industrial development). The sites chosen represent relatively homogeneous areas for each of the land use classes and are large enough to have defined drainages. Stream water samples were collected either manually, just below the water surface in the thalweg, or by auto-samplers. Water samples were taken every two weeks during the dry season, approximately May through October, once a week during the rainy season, and every one to four hours during storms. At most sites, stage was measured with pressure transducers at 5-minute intervals, and staff gauges have been installed to visually observe stage during sampling. Nitrate concentrations during baseflow varied over three orders of magnitude, from a few micromoles per liter (μmol/L) in undeveloped catchments, to hundreds of μmol/L in agricultural and urban watersheds, to thousands of μmol/L where intensive greenhouse agriculture dominates. Nitrate loading ranged from a few moles per hectare per storm at undeveloped and residential sites to hundreds of moles per hectare per storm at the greenhouse site. Phosphate concentrations had a similar, but smaller, variation from 1 to 100 μmol/L. Stormflow concentrations fluctuated with the storm hydrograph: phosphate increased with flow, while nitrate typically decreased due to dilution from impervious surface runoff. Future research will entail implementing nutrient export coefficient modeling techniques to enable a regional analysis of nutrient loading to the ocean.