Benthic Assemblage Variability in the Upper San Francisco Estuary: A 27-Year Retrospective

Heather Peterson,Marc Vayssieres

doi:10.15447/sfews.2010v8iss1art2

Abstract

We used multivariate methods to explore changes in benthic assemblage structure over 27 years (1977–2003) at four monitoring stations located along a salinity gradient in the upper San Francisco Estuary. Changes in benthic assemblage composition were assessed relative to hydrologic variability and to the presence of the high-impact invader Corbula amurensis in the estuary. We also explored the composition of benthic assemblages during a recent collapse of several pelagic populations in the upper estuary. Our results show that the Corbula invasion had both direct and indirect effects on the benthos in the estuary, causing significant changes in assemblage structure. We found no unprecedented patterns of benthic assemblage composition during the period of the Pelagic Organism Decline (2000–2003) in the upper estuary. Hydrologic variability was associated with significant changes in benthic assemblage composition at all locations. Benthic assemblage composition was more sensitive to mean annual salinity than other local physical conditions. That is, benthic assemblages were not geographically static, but shifted with salinity, moving down-estuary in years with high delta outflow, and up-estuary during years with low delta outflow, without strong fidelity to physical habitat attributes such as substrate composition or location in embayment vs. channel habitat. Organism abundance and species richness showed a bi-modal distribution along the salinity gradient, with lowest abundance and richness in the 5 to 8 psu range. We conclude that the continuity of benthic assemblages and community metrics along the salinity gradient is a powerful and necessary context for understanding historical variability in assemblage composition at geographically static monitoring stations.

Highlights

A substantial portion of the water used by California’s agricultural and urban consumers is drawn from the Sacramento–San Joaquin Delta, in the upper San Francisco Estuary
The highest salinities were found at San Pablo Bay (SPB), while Old River (OLR) remained “fresh” throughout the study period (Figure 2)
While the decline in overall benthic organism density at Lower Sacramento River (LSR) concurrent with the Corbula invasion supports the hypothesis that suppression of phytoplankton biomass by the clam has affected populations that rely on pelagic supplies of organic carbon, as many previous studies have shown in detail (Alpine and Cloern 1992; Mueller Solger and others 2002; Jassby and others 2002; Jassby 2006), we found no evidence from the benthic abundance data that the influence of benthic grazing underwent a significant change coincident with the Pelagic Organism Decline (POD)

Summary

Introduction

A substantial portion of the water used by California’s agricultural and urban consumers is drawn from the Sacramento–San Joaquin Delta, in the upper San Francisco Estuary. As demand for this water outpaces the supply, many options for managing freshwater resources and environmental quality are being considered for the upper estuary (Lund and others 2008). Recent investigations of pelagic populations (Kimmerer 2002a; Sommer and others 2007) showed precipitous declines in abundance in several pelagic fish populations during recent years with high freshwater outflow when population increases were expected These declines in pelagic organism populations have encouraged a series of assessments of biotic processes and pollutant effects in the estuary. It is timely to re-assess benthic invertebrate assemblage patterns and their relationship to potential forcings

Methods

Results

Discussion

Conclusion