Nutrients

Nutrients and the effects of nutrients on water quality in the Sacramento-San Joaquin Delta is a priority focus area for the Delta Regional Monitoring Program (Delta RMP). The Program’s first assessment question regarding nutrients is: “How do concentrations of nutrients (and nutrient-associated parameters) vary spatially and temporally?” In this analysis, we confirmed previously reported declining trends in the San Joaquin River for nutrient concentrations at Vernalis and chlorophyll-a concentrations at Buckley Cove and Disappointment Slough. A slight increasing trend for dissolved oxygen at Buckley Cove was also detected which could be confirmation that management actions for the San Joaquin River Dissolved Control Program are having the desired effect. Finally, at stations in Suisun Bay, the Confluence region, and Franks Tract, chlorophyll-a showed modest increasing trends, which were not evident in previous analyses. The new analyses presented in this report and the findings from earlier reports constitute encouraging early progress toward answering the Delta RMP’s assessment questions. Specifically, due to the existence of long-term data sets and synthesis efforts, spatial and temporal trends in the concentrations of nutrients and nutrient-related parameters are reasonably well understood and so are the magnitudes of the most important sources of nutrients from outside the Delta. However, additional synthesis work could be done to understand the factors behind these trends. Large knowledge gaps remain about nutrient sinks, sources, and processes within the Delta. The mechanistic, water quality-hydrodynamic models being developed for the Delta may be able to address these questions in the future.

Nutrient loads are an important water quality management issue in the Sacramento-San Joaquin Delta (Delta) and there is consensus that the current monitoring activities do not collect all the information needed to answer important management questions. The purpose of this report is to use hydrodynamic model outputs to refine recommendations for monitoring nutrients and related conditions in the Delta. Two types of modeling approaches were applied: 1) volumetric water source analysis to evaluate the mix of source waters within each subregion; and 2) particle tracking simulations.The analysis revealed that each Delta subregion has a unique “fingerprint” in terms of how much of its water comes from different sources. Three major recommendations for a future monitoring design were derived from this analysis:

Recommendation #1: The subregions proposed for status and trends monitoring in a previous report should be redrawn to better reflect the mixtures of source waters.

Recommendation #2: Long-term water quality stations are needed in the North Delta, Eastside, and South Delta subregions.

Recommendation #3: Areas with a long-residence time and where mixing of different water sources occurs are potential for nutrient transformation hotspots. High-frequency water quality mapping of these areas has the