PISCO: Partnership for Interdisciplinary Studies of Coastal Oceans

photo by Mark Carr The great diversity and productivity of algae, invertebrates, and fishes that inhabit rocky intertidal and shallow subtidal reefs have fascinated ecologists since the time of Darwin. Research on species interactions (i.e., competition and predation) and physical and ecological processes that determine the abundance of species in these communities has spawned many of the leading theories of how ecological communities are organized and how they persist through time. But richness of the natural environment, rapid growth of coastal populations, and increasing number of human activities focused on these habitats frequently exposes them to human threats.

Critical to our ecological understanding and protection of these communities is our ability to distinguish their natural dynamics from changes caused by local or global human influences. However, our ability to do so and to predict how these communities will change in the face of natural or human caused environmental variation is poor. To rectify this shortfall in our understanding of and ability to protect these ecological systems, PISCO has implemented a large-scale, long-term study of the patterns of community structure in rocky intertidal and shallow subtidal habitats and the physical and ecological processes responsible for structuring these communities. Ultimately, our goal is to understand and predict how environmental change (including human impacts) influences these physical and ecological processes and how communities respond to such changes.

Research Questions

What are the spatial patterns of rocky intertidal and subtidal community structure?
How do these spatial patterns relate to habitat features and coastal oceanographic conditions?
How does the structure of these communities vary over time?
How do these changes over time relate to temporal changes in coastal oceanographic conditions?
What are the specific environmental and ecological processes responsible for the observed patterns of (and changes in) community structure?
Do spatial and temporal patterns of community structure differ inside and outside of marine reserves and if so, how?

Approach

We are describing and monitoring the structure (i.e., species composition and relative abundance) of representative rocky intertidal and subtidal communities across an unprecedented range extending from northern Washington to southern California.The surveys started in 1999, and the annual sampling of the abundance of targeted organisms is currently conducted at about 100 intertidal and 20-30 subtidal rocky-reef and kelp-forest sites distributed across well-known oceanographic regimes and habitat types. Photo by Spencer Wood Initial sampling of each of these sites includes quantitative description of habitat features (e.g., substratum type and relief) that allows us to examine relationships between community structure and habitat features across an immense geographic range. Initial sampling of each of these sites includes quantitative description of habitat features (e.g., substratum type and relief) that allows us to examine relationships between community structure and habitat features across an immense geographic range.

The distribution of sampling sites across oceanographic regimes, in conjunction with measures of oceanographic conditions (e.g., nutrients, chlorophyll, temperature, and currents) at core sites, will allow us to examine the relationships between conditions in the nearshore ocean and community structure, and how those relationships vary over time. In addition, this design allows us to examine whether oceanographic conditions influence communities in different habitats similarly or differently.

We also examine the linkages between oceanographic events, recruitment of key species, and changes in community structure by relating rates of larval supply and recruitment to changes in community structure measured at core sites.

Annual surveys of community structure at sites within and outside marine reserves (e.g., Hopkins Marine Life Refuge, Pt. Lobos State Reserve, and Big Creek Ecological Reserve) provide an opportunity to determine how community dynamics vary with and without the protection offered by marine reserve designation.

Research Findings

community structure graph

Intertidal Community Structure

Scientists generally believe that species diversity varies inversely with latitude -- i.e., lower diversity at higher latitudes. By measuring patterns of diversity along the entire U.S. West Coast, PISCO has documented an important exception to this general rule. On rocky shores, we observe a greater number of species in the north than in the south. However, this pattern is not evident at all sites, indicating that local factors also affect species diversity. Efforts are currently underway to define what these factors are and how they influence species diversity.

Collaborative intertidal surveys have documented extraordinary and apparently natural shifts in distributions of some intertidal species. This finding is contrary to mostly anecdotal reports that described intertidal communities as being fairly stable. Instead we have found that through ecological processes such as succession and variable larval input, communities can change dramatically over periods as brief as 5 years, which has tremendous implications not only for an understanding of the basic functioning of marine communities but also for estimation of human-induced impacts. Simply put, there is no way to identify anthropogenic impacts unless we can first account for natural variation.

Subtidal Community Structure

kelp macro photo Unlike intertidal communities, latitudinal variation is not the sole indicator of diversity within subtidal communities. Rather, several factors including substratum type and relief, wave exposure, and ocean circulation influence local patterns of community structure. For example, giant kelp, Macrocystis pyrifera, are abundant in northern and southern California sites, but not around Point Conception. In contrast, understory algae are most abundant at sites near Point Conception. The low abundance of kelp at Point Conception may be due to a combination of high wave exposure coupled with the soft, friable substratum.

Geographic patterns of fish community structure reflect both latitudinal gradients and habitat type. For example, sites in southern California had a higher proportion of subtropical species (wrasses, damselfishes, groupers) than northern sites, where temperature rockfishes dominate. Habitat type modifies these trends locally, emphasizing the influence of geologic characteristics on the structure of reef fish communities.

Relating Climatic Events to Community Structure

PISCO scientists observed changes in the dynamics of giant kelp, which contributes significantly to both primary productivity and habitat structure, during recent climatic events. The 1999 La Niña had positive effects on giant kelp, especially at sites with soft substratum. We hypothesize that these positive effects were due to high nutrient concentrations during La Niña that aided recovery from the previous year's El Niño. Positive effects did not continue in the following years (2000 and 2001) when neither event occurred, possibly because dense kelp cover led to a resumption of biotic regulation of kelp growth via shading. Through long-term monitoring, we will continue to explore the roles of large-scale climatic changes on populations of this key species.

Temporal changes in kelp cover, coupled with climatic regimes have complex, synergistic effects on fishes. Rockfish species with short larval durations require understory macroalgae as settlement habitat. Understory macroalgae inversely correlate with giant kelp cover due to shading. Following La Nia conditions, these fishes experienced both low recruitment and poor settlement habitat, which may have long-term negative effects on their populations. By relating year-to-year variation in recruitment and kelp dynamics, we will be able to tease apart the relative contribution of these factors to population change.