San Francisco Estuary and Watershed Science

Abstracts are not presented with Editorials. --SFEWS Editors

Aquatic trophic interactions in the upper San Francisco Estuary are synthesized here as a conceptual food web model, using over 35 years of scientific research, and highlighting key uncertainties for restoration. The food web was created as part of the Delta Regional Ecosystem Restoration Implementation Program to evaluate the benefits of restoration actions. Historic changes to the hydrology and geomorphology of the region have decreased ecosystem resiliency. More recently, pressures from water export, alien species introductions, and nutrient loading have disrupted the food web and increased the vulnerability of pelagic and juvenile fishes. One of the key features of the contemporary food web is a decoupling of pelagic and the detrital pathways. Low production and high mortality of phytoplankton since the 1980s have led to declines of pelagic organisms, including zooplankton, mysids, and planktivorous fish. In contrast, detrital pathways support abundant epibenthic invertebrates, such as amphipods and crayfish, which have become a dominant food source for adult demersal and piscivorous fish. Fishes that are obligate to the pelagic web will likely continue to decline, although fishes able to use the detrital pathway may be more robust. Fishes with pelagic larvae may be vulnerable to recruitment failures if they are unable to obtain planktonic food during the critical period of their ontological development. Options for increasing pelagic production at large scales are limited, but may include management of clams, nutrient ratios, and off-channel habitat subsidies. Restorations at small to intermediate scales may produce pelagic food, but volumetric constraints will limit the extent of subsidies. Creating spatial opportunities where pelagic and detrital food webs can re-integrate may offer some opportunities for local recruitment, and species able to use localized detritally-based webs will benefit strongly from such activities.

Using stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) and mixing models, we investigated the trophic levels and carbon sources of invertebrates and fishes of a large tidal marsh in the San Francisco Estuary. Our goal was to better understand an estuarine food web comprised of native and alien species. We found the following: (1) the food web was based largely on carbon from phytoplankton and emergent-aquatic and terrestrial vegetation, but carbon from submerged aquatic vegetation and phytobenthos was also used; (2) alien species increased the complexity of the food web by altering carbon-flow pathways and by occupying trophic positions different from native species; and (3) most consumers were dietary generalists.

Freshwater is a scarce and precious resource in California; its overall value is being made clear by the current severe drought. The Sacramento–San Joaquin Delta is a critical node in a complex water supply system that extends throughout much of the western U.S. wherein demand is exceeding supply. The Delta also underpins a major component of the U.S. economy, helps feed a substantial part of the country, is a unique and valuable ecological resource, and is a place with a rich cultural heritage. Sustaining the Delta is a problem that manifests itself in many dimensions including the physical structure of the Delta, the conflicting demands for water, changing water quality, rapidly evolving ecological character, and high institutional complexity. The problems of the California Delta are increasingly complex, sometimes chaotic, and always contentious. There is general agreement that current management will sustain neither the Delta ecosystem nor high-quality water exports, as required under the Delta Reform Act, so there is a renewed urgency to address all dimensions of the problem aggressively. Sustainable management of the Delta ecosystem and California’s highly variable water supply, in the face of global climate change, will require bold political decisions that include adjustments to the infrastructure but give equal emphasis to chronic overuse and misuse of water, promote enhanced efficiency of water use, and facilitate new initiatives for ecosystem recovery. This new approach will need to be underpinned by collaborative science that supports ongoing evaluation and re-adjustment of actions. Problems like the Delta are formally “wicked" problems that cannot be “solved” in the traditional sense, but they can be managed with appropriate knowledge and flexible institutions. Where possible, it is advisable to approach major actions incrementally, with an eye toward avoiding catastrophic unexpected outcomes. Collaborative analyses of risks and benefits that consider all dimensions of the problem are essential. Difficult as the problems are, California has the tools and the intellectual resources to manage the Delta problem and achieve the twin goals of a reliable water supply and an ecologically diverse Delta ecosystem.