Background/Question/Methods Estuaries often represent steep stress gradients for organisms, with abiotic stress due to temperature and salinity typically increasing with distance into estuary. Invertebrate communities and their predators are strongly influenced by these stress gradients. However, how the effects of predation on invertebrate community composition vary across a range of environmental conditions is unknown. The Environmental Stress Hypothesis predicts that importance of predation in structuring communities decreases with increasing environmental stress. Therefore, we predict that predation on sessile invertebrate communities will decrease along the estuarine gradient from ocean to river. We investigated this in Tomales Bay, CA using settlement plates distributed at sites along this gradient in the summer of 2019. Results/Conclusions Our data show that community composition differed across sites, and predation had a significant effect on community composition when averaged across all sites. However, the effect of predation changed across sites, with mid-bay sites experiencing the greatest effects of predators. This was likely due to patterns in functional group abundance, as certain functional groups, such as solitary ascidians, were more susceptible to predation than others. Overall, predation didn’t follow the predictions of the Environmental Stress Hypothesis, but rather followed the distribution of particular functional groups, whose distribution could be mediated by environmental stress gradients. These communities include many non-native species, so these results also highlight factors that influence the distribution on invaders in this system.
