Rocky subtidal communities at three sites in Northern Massachusetts have now been under continuous study for up to 20 years. Methods include photography of marked quadrats, transects of predator abundance, and measurement of selected aspects of the physical environment. Since 1978, there has been a dramatic increase in sea urchin (Strongylocentrotus droebachiensis) population density and a decrease in most fish populations. Dr. Sebens and collaboartors have also recorded the appearance and effects of several non-indigenous species; the predatory nudibranch Tritonia plebeia and the colonial ascidian Botrylloides sp. have been the most successful, and have had measurable effects on indigenous assemblages. Other non-indigenous ascidians (Botryllus schlosseri, Styela clava, Diplosoma sp.) appeared only rarely at these sites, and have not yet developed stable local populations. Tritonia had a severe impact on populations of the octocoral Alcyonium siderium, which still have not recovered. The removal of Alcyonium by Tritonia and the doubling of sea urchin population density (both during 1984-1986) caused a major decline in most other invertebrates on vertical rock surfaces, and an increase in the extent of surfaces dominated by crustose coralline algae. Horizontal surfaces dominated by kelp and foliose red algae during 1978--1988 were reduced to urchin barrens (coralline-dominated) when urchins expanded into new areas in 1989. Botrylloides sp. established colonies on rock walls and horizontal surfaces, even where urchin grazing is intense, and thus may have defenses that deter urchins. The synergistic effects of urchin population increase, and the appearance of several nonindigenous species within one decade, has caused a major shift in invertebrate and algal assemblages at these sites. Continued high urchin densities could favor urchin-resistant spatial dominants, which include coralline algae, sea anemones (Metridium senile), sponges (Isodictya spp.), and nonindigenous species such as Botrylloides sp. These studies of subtidal rock surfaces (1978-1997) in northern Massachusetts have produced and tested hypotheses concerning the long-term stability and short term regulation of community structure on rocky substrata. The previous research focused on the interactions among species on rock walls, on population biology of several of the most common species, and on the effects of water movement on individual, population, and community processes. The present project is an extension of the long-term research: an experimental and correlative approach will be used to address the interactions among predation, physical effects, larval dispersal, recruitment, and the resulting impact on community structure. The goals of the study are to: 1) identify important chronic and transient processes structuring the community (physical, biological), 2) determine which processes have strong direct and indirect effects (e.g. sea urchin predation) on community structure, 3) conduct quantitative studies of recruitment and its effects on the community, and 4) incorporate these findings into a model of species coexistence and community structure on isolated sub-units of a landscape, using multiple dispersal scales. Once complete, the long-term data set will be examined statistically (autocorrelation, cross-correlation, other time-series analyses) to determine which physical processes and which predator populations are most strongly correlated with changes in percent cover of the common space occupiers. This project continue long-term studies during 4-6 trips annually, using photographic, video, and direct (SCUBA) sampling (quadrat, transect) at five field sites along 8 km of the Massachusetts coast.
