California Sea Grant College Program

Estuarine and coastal ecosystems face both large scale physical environmental stresses from climate change and local scale community changes, including non-native species introductions. In northern California, USA, climate change projections indicate increasingly severe droughts in the future punctuated by more frequent, intense winter storms, significantly affecting freshwater outflow to estuaries like San Francisco Bay. Using long-term physical and biological data sets coupled with experiments, I found drastic changes in epifaunal communities in the Bay between years with extreme high and low freshwater outflow (Chapter 1). In dry years (low outflow), competition determines community composition, but in wet years (high outflow), low salinity stress shifts the community to being recruitment limited, with community membership determined by the availability of larvae to recruit.

Shifts in outflow extremes drive changes in the dominant species, resulting in altered community performance (Chapter 2). I experimentally tested the effect of functional group diversity on community performance in different outflow regimes. Communities containing more functional groups generally occupied more space, but the strength of this effect varied from year to year. In the absence of normally dominant species during a wet year, resident community functional group diversity had no effect on subsequent invasion by non-resident species. These results suggest that outcomes of short-term, unreplicated studies of biodiversity-ecosystem function relationships may thus be contingent on prevailing conditions.