Using Comparative Mass-Balanced Trophic Models to Understand Food-Web Changes in Relation to the Decline of Pelagic Organisms in the Sacramento-San Joaquin Delta

The San Francisco Estuary, including the Sacramento-San Joaquin Delta has been changed from its natural state by urban development, agriculture, and water management. Ecopath with Ecosim modeling software was used to develop a model for exploring direct and indirect drivers of ecosystem dynamics of the Sacramento-San Joaquin Delta and Suisun Bay. The analysis was focused on the Pelagic Organism Decline species—delta smelt Hypomesus transpacificus, threadfin shad Dorosoma petenense, longfin smelt Spirinchus thaleicthys, and striped bass Morone saxatilis. Two Ecopath models (1980s and 2000s) were built and then compared to explore changes in ecosystem structure of the Delta and Suisun Bay ecosystems. In the 1980s, prior to the decline in pelagic organisms, pelagic fish populations were more abundant than in the 2000s, and introduced centrarchids and submerged aquatic vegetation were much less abundant. In the 2000s, the system was very different, with significantly lower levels of pelagic fishes and higher levels of introduced centrarchids and submerged aquatic vegetation. Invasive populations of zooplankton and clams have also become abundant, further changing ecosystem dynamics. Comparison of these two ecosystem time periods will help explore a shift from a pelagic dominated system to a littoral dominated system in the context of food-web drivers and tropic cascades. This research may help support management actions intended to improve food-web support for the Delta and Suisun Bay.