Isotopic and Chemical Analysis of Nitrate Source and Cycling in the San Joaquin River, California

The sources and cycling of nitrate was investigated during a pilot study at four sites along the San Joaquin River using carbon and nitrogen isotopes of total dissolved and particulate organic matter along with hydrological measurements and various concentration data including chlorophyll-a. The nitrate source, its relationship to phytoplankton, and the effect of the nitrate source and cycling on the isotopic composition of dissolved and particulate organic matter were the primary concerns of the study. Samples were collected between July and October 2000 at (1) Crow's Landing, (2) Laird Park, (3) Vernalis, and (4) upstream of the Merced River. Particulate organic matter samples (POM) were collected on pre-combusted glass fiber filters. Combined dissolved organic and inorganic samples were prepared by roto-evaporating filtered waters (RV samples). Both the RV and the POM samples were acidified to eliminate inorganic carbon. Stable carbon and nitrogen isotopes and C:N ratios of POM in addition to chlorophyll-a concentrations were consistent with POM derived primarily from plankton at all sites and sampling times except in late October during a dam release event. The late October samples showed a shift toward isotopically heavier carbon and lighter nitrogen isotopes and higher C:N ratios reflecting a significant input from non-planktonic (probably terrestrial) sources. About 90 percent of the nitrogen in the RV samples was inorganic, 97 percent of which was in the form of nitrate. Assuming that the nitrogen isotopic composition of the minor organic fraction fell within the range of common organic samples (0 to 25 per mil), the delta 15N value of the RV samples was a close representation of the nitrogen isotopic composition of the nitrate. The POM and RV samples therefore appear to be reasonable proxies for the nitrogen isotopic compositions of plankton and nitrate, respectively. By comparison with other dissolved species, most of the variation in nitrate concentrations appears to be a function of dilution/evaporation, as river stage varied. For the minority of samples that fell below the dilution/evaporation trend (i.e. those with nitrate concentrations lower than predicted), delta 15N values indicated assimilation rather than denitrification as the cause of the lower concentration. With one notable exception, the pattern of delta 15N of the POM samples over time generally mirrored that of the RV samples and showed no correlation with nitrate or chlorophyll-a concentrations. This relationship indicates (1) that nitrate was an important and probably the primary nutrient for the plankton and (2) that nitrate concentrations remained too high for the delta 15N to be appreciably affected by phytoplankton growth. The delta 15N values of the RV samples averaged +14.4 per mil. The fairly high nitrate concentrations and lack of evidence for denitrification in the river suggests that the delta 15N values are a source signature. Although synthetic fertilizers are a likely source of nitrate in the area, the relatively high delta 15N values are consistent with a significant fraction originating from animal waste or sewage.