Elevational changes in community structure of rocky intertidal invertebrates and macroalgae due to climate change

Hannah N. Mittelstaedt, Laura A. Braun, Brian J. Olsen and Amanda J. Klemmer, University of Maine, Orono, ME, Adam Kozlowski and Aaron S. Weed, Northeast Temperate Inventory and Monitoring Network, National Park Service, Woodstock, VT, Kathryn Miller, Northeast Temperate Network, National Park Service, Bar Harbor, ME

Background/Question/Methods
The rocky intertidal is dominated by sessile invertebrates and macroalgae, the community structure of which is determined by the interaction of abiotic and biotic environmental factors. Environmental and anthropogenic pressures (e.g. climate change, invasive species) can have profound effects on intertidal communities, yet these effects may vary across the environmental gradient that characterized the rocky intertidal. The objective of this study is to describe the geographic and temporal sources of variation in intertidal communities in two parks located in the Gulf of Maine: Acadia National Park (ACAD) and Boston Harbor Islands National Recreation Area (BOHA). To assess the ecological stability of intertidal communities, the Northeast Temperate Network of the National Park Service has collected annual measurements of community-based parameters (e.g. density, percent cover, and elevational range) of invertebrate and macroalgal species at nine study sites in ACAD and BOHA for over seven years. These metrics were collected at permanently marked plots within distinct community zones and along vertical transects across the intertidal. PERMANOVA was used to quantify differences in community composition due to spatial factors (e.g. park, site) and temporal factors, as well as under different abiotic conditions.
Results/Conclusions
We found that across the entire intertidal, differences in invertebrate community composition were most closely associated with spatial factors, such as site and park, with temporal factors playing a smaller but still significant role. Within community zones significant changes in composition were also associated with site and park, but only communities in the upper and lower intertidal zones changed significantly between years. The degree to which changes in communities were driven by static site characteristics and dynamic abiotic variables varied across zones, indicating that both site level characteristics and changing environmental conditions differentially impact communities across the tidal gradient. These results elucidate how the impacts of changing conditions in costal habitats vary with spatial factors and across environmental gradients.