Simulating Algal Interactions and Eutrophication Dynamics in Development of the Clear Lake Nutrient TMDL, California

Clear Lake, the largest natural lake entirely within the borders of California, suffers from significant nutrient enrichment, causing frequent blooms of blue-green algae that impact municipal water supply, agriculture (irrigation and stock watering), recreation (contact and noncontact), warm freshwater habitat, warm spawning, and wildlife habitat (CVRWQCB, 1998). The major external nutrient source is from watershed runoff, while Past studies also suggest that internal loading from benthic release and nitrogen fixation represent significant contributions to the nutrient budget. Wastewater and groundwater contribute, but not significantly, to nutrient loading in Clear Lake. In order to accurately represent the complex chemical and biological interactions exhibited by Clear Lake, and blue-green algae blooms in particular, a detailed hydrodynamic and water quality model was developed based on an enhanced 3-D Environmental Fluid Dynamics Code (EFDC) modeling framework. The boundaries of the EFDC model were forced by a Loading Simulation Program- C++ (LSPC) watershed loading model. Based on this linkage, the hydrodynamic model was able to simulate water balance, water circulation, and temperature distribution in the lake. The receiving water EFDC model was configured to receive loads from LSPC and subsequently represent the limiting effects of nutrients, the interactions between blue-green and other algal species, nutrient fate and transport within the water column, and dissolved oxygen dynamics. In addition to water column processes, sediment diagenesis was also represented and linked to the water column model to account for internal sources and sinks of nutrients and dissolved oxygen. This capability is particularly useful during a TMDL analysis, where loading scenarios should (and in reality, would) have a direct impact on sediment nutrient contributions to the water column. It was observed that the model is able to simulate both the inter-year and seasonal succession of diatom populations, blue-green algae and green algae, and establish significant relationships between algal biomass, nutrients, and dissolved oxygen. This was demonstrated through successful representation of the worst observed blue-green algae bloom on record (summer of 1990) and the recovery that followed. This capability provides a valuable tool to investigate the relative impacts of sources and sinks in the Clear Lake system, and thus conditions that are critical in maintaining the beneficial uses in the waterbody. Once the Clear Lake modeling system was calibrated and validated to observed physical and water quality data, it was utilized to estimate an appropriate nutrient target for Clear Lake. The nutrient criteria for Clear Lake is narrative, providing an opportunity to utilize the calibrated model to estimate an appropriate, site-specific TMDL target for Clear Lake. Based on the analysis of historical water quality and algal bloom records, a TMDL target and subsequent allocations were developed for Clear Lake.