An Ecosystem Model of the Sacramento-San Joaquin Delta and Suisun Bay, California USA

ABSTRACT\ud AN ECOSYSTEM MODEL OF THE SACRAMENTO-SAN JOAQUIN\ud DELTA AND SUISUN BAY, CALIFORNIA USA\ud by\ud Marissa Bauer\ud Master of Science in Environmental Science:\ud Professional Science Master Option\ud California State University, Chico\ud Summer 2010\ud The San Francisco Estuary is a highly exploited system that has been drastically changed from its natural state through urban development, agriculture, and water management. Ecopath with Ecosim (EwE 6) modeling software was used to explore direct and indirect drivers of ecosystem dynamics of the Sacramento-San Joaquin Delta and Suisun Bay, with emphasis on the Pelagic Organism Decline species (delta smelt (Hypomesus transpacificus), threadfin shad (Dorosoma petenense), longfin smelt (Spirinchus thaleicthys), and striped bass (Morone saxatilis)). An Ecopath model was built to simulate the Delta and Suisun Bay ecosystem for the year 1982 based on best available data. The Ecopath modeling approach incorporates trophic interactions and community food web structure to help understand the dynamic interactions between trophic groups. The model uses parameters such as biomass, production, and mortality\ud estimates of 40 functional groups, including birds, fish, benthic invertebrates, zooplankton, phytoplankton, and detritus. Model parameters were calculated from published and unpublished data taken from the literature or estimated by local experts when appropriate.\ud The balanced Delta Ecopath model gave some surprising results about the food-web within the Delta. Of the total system biomass (270.5 t km-2) 37 % is from trophic level 3, which consists primarily of the fish species. Trophic level 1 including both detritus and phytoplankton composes 55 % of total system biomass. The final 8 % of total system biomass is composed of second and fourth trophic levels. Trophic transfer efficiencies for the Delta average 8.3%, with the highest trophic transfer efficiency at trophic level 1 (17.6%) and the lowest at trophic level 4 (3.7%).These results are consistent with the general ecological principle of a decreasing trend in transfer efficiency with increasing trophic level. The Delta Ecopath model also brings attention to the fact that the microbial and detrital loops may play an important role in the Delta food web and that further investigation into these pathways may prove useful. It was also apparent that the current Delta model was attempting to describe both the pelagic and littoral food webs of the Delta system and that while these two food webs share some similar species composition, further development of the model is needed to better understand and address this issue.\ud The development of an ecosystem-wide Ecopath model of Suisun Bay and the Sacramento-San Joaquin Delta is a major step in exploratory ecosystem modeling of this highly altered system. The basic input parameters are a solid foundation for the\ud development of the more in-depth Ecosim model, the time-dynamic simulation model driven by the mass-balanced Ecopath model. The EwE modeling approach can be used to address multiple areas of management concern. For example, it can be used to evaluate ecosystem effects of fishing mortality, explore management policy options, predict movement and accumulation of contaminants, and model effects of environmental changes.CSU, Chic