Mounting evidence indicates significant human-driven alterations to the hydrological cycle of the western United States over the last half of the twentieth century, including increasing variability and extreme fluctuations in flow. Most work to date has documented these hydrological changes, and management policy has focused on ensuring water supply for human and agricultural uses and freshwater ecosystems. However, little is known about the ecological impacts of either extreme climactic fluctuations or human alterations to the hydrological cycle in the downstream, estuarine systems. Here we present evidence of significant ecological impacts to estuarine and marine communities in the San Francisco Bay-Delta driven by salinity levels, which are directly affected by freshwater flows. Using observational and experimental approaches, we assessed the effect of changes in water column conditions on epifaunal community diversity over five years, including one of the wettest and several of the 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 temperature and salinity levels to directly assess their effects on survival.

Results/Conclusions

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. These wide variations in the hydrologic cycle, now exacerbated by human impacts, thus cause the benthic epifaunal community to vary widely within the span of a few years.