Effects of extreme freshwater disturbance during the 2016-17 wet winter on San Francisco Bay mudflat infaunal macroinvertebrates

Massive precipitation experienced in California during the 2016-17 winter brought the salinity of surface waters in San Francisco Bay Estuary (SFBE) down to values not seen in years. Bay-dwelling organisms such as intertidal invertebrates are at risk of mortality in salinity conditions that exceed their tolerance limits. While some fluctuation in salinity and other environmental conditions is a staple feature of estuarine life, extreme salinity conditions during a wet winter may present an exceptional stress. Observations from field and laboratory investigations provide insight in the role of salinity on the composition of mudflat infauna. A field study conducted at six sites along the estuarine gradient of northern SFBE revealed a trend of decreasing species richness and abundance with increasing distance upstream during the wet winter. Low diversity during the wet winter and differential succession of communities at upstream sites relative to downstream sites suggests that massive freshwater years substantially change intertidal mudflat communities depending on their location in the estuarine gradient. Notably, non-native species dominated the intertidal mudflat communities during and after the wet winter across all sites in this study. The lack of a numerically dominant non-native clam, Gemma gemma, in the least saline region was reflected by the higher mortality observed laboratory treatments where salinity was less than ten. Reductions in freshwater availability to SFBE due to drought and diversions may increase likelihood of G. gemma to spread upstream. Yet this spread may be counteracted by occasional extremely wet winters like that of 2016-2017.