Abstract
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.
Highlights
Restoration of aquatic habitat in the upper San Francisco Estuary is being planned in hopes of reversing historic declines of native fish (Herbold et al 2014)
Because scientific monitoring of the system did not begin until the 1960s, the initial and irreversible loss of ecosystem resilience is difficult to separate from subsequent stressors that have led to sometimes puzzling and precipitous declines in native and introduced species alike
The following qualitative conceptual model of the aquatic food web of the upper estuary, including Suisun Bay and the Sacramento–San Joaquin Delta, was produced to assist conservation and management decisions (Gilbert 1980; Mills et al 1993; Polis et al 1997). It synthesizes over 35 years of ecosystem research into the Driver–Linkage–Outcome format used in the Delta Regional Ecosystem Restoration Implementation Program (DRERIP) (DiGennaro et al 2012)
Summary
Errors in the value reported on page 13 of this paper were discovered after publication of this paper in Volume 13, Issue 3 of the online journal San Francisco Estuary and Watershed Science. These errors are corrected in this errata published in November 2016.
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