Rapid community shifts in an estuarine ecosystem linked to climatic extremes and water management

Background/Question/Methods

Human alteration of the hydrological cycle in the western United States is resulting in greater variability and extreme interannual fluctuations in freshwater flow. The ecological impacts of extreme climatic fluctuations and human alterations to the hydrological cycle are less well known in downstream, estuarine systems. We present results from a thirteen-year study of sessile marine invertebrate communities in San Francisco Bay demonstrating clear links between salinity levels, which are controlled by high and low estuarine outflow extremes, and the mechanisms driving community composition and abundance. Using observational and experimental approaches, we assessed the effect of changes in water column conditions on epifaunal community diversity over thirteen years, including some of the wettest and dryest years in the past half century. We examined correlations between sessile invertebrate recruitment and survival patterns and seasonal variation in salinity and temperature in San Francisco Bay. We then experimentally manipulated: (1) temperature and salinity levels to directly assess their effects on survival and community assembly processes, and (2) community composition to examine community assembly processes in the presence and absence of dominant species from wet and dry extremes. 

Results/Conclusions

We show that even modest, decadal extremes in wet and dry climatic conditions in the San Francisco Estuary over a recent 13-year period (2001-2013) drive dramatic, yet predictable shifts in sessile marine invertebrate community composition and abundance. Results indicate that during high flow (wet) years, low salinity levels cause mass mortality in epifaunal communities, significantly altering community composition and function throughout the estuary. In contrast, during dry years, epifaunal communities are dominated by non-native species, and sessile epifaunal species move upstream into areas of the estuary traditionally inhabited by freshwater or brackish communities. Adult organisms' high salinity tolerances allow persistence and continued impacts following recruitment to areas more often inhabited by freshwater organisms. Our study shows consistent, predictable community changes driven by environmental changes tightly linked to both climatic change and management regimes.