Consumer pressure in estuarine rocky shores varies with ecological context

Background/Question/Methods

Consumer pressure plays a major role in structuring many biological communities. However, the relative importance of consumers is context dependent, shifting with environmental factors (e.g. species identity, system productivity) and anthropogenic impacts. We used a manipulative consumer-exclusion experiment to examine the combined influence of environmental context and human-induced changes in nearshore primary productivity (due to nutrient pollution) on the intertidal mussel Mytilus edulis. We hypothesized that in areas with elevated levels of nearshore primary productivity, intertidal community dynamics would be qualitatively distinct from those in less impacted areas.

We conducted the experiment in three estuaries from Maine to New York: Casco Bay (CB), Narragansett Bay (NB), and Long Island Sound (LI). Two sites within each estuary were classified a priori as high or low productivity based on previous data (n=12 sites total). The difference between exclusions and controls after 18 months was used to quantify consumer pressure. We also quantified Mytlilusrecruitment and abundance, as part of a broader effort to monitor spatial and temporal variation in rocky shore communities in this region.

Results/Conclusions

We observed marked geographic variation in consumer pressure on Mytilus. In LI and NB, consumer exclusions led to moderate increases in abundance (up to 65% and 77% on average, respectively) while CB showed very little (up to 2%). Contrary to our predictions, consumer pressure was not greater at high productivity sites. We observed the inverse pattern with Mytilus recruitment: overall, LI and NB sites had very low recruitment in comparison to CB. High productivity sites had greater recruitment in CB and LI, but not in NB. Finally, based on surveys conducted in 2010 and 2011, adult Mytilusabundances were positively associated with site-level productivity in LI and CB, but not in NB.

These results suggest that anthropogenically-driven variation in nearshore productivity can influence Mytilus populations significantly, although in context-dependent ways. This has important implications for the management of anthropogenic nutrient inputs (e.g. waste-water treatment and storm-water runoff) as well as Mytilus populations, which contribute to fisheries, habitat, and other estuarine ecosystem services. As the ecosystem health of urbanized estuaries shifts with climate change and other more local-scale human impacts, it will be imperative to consider these implications.