Detecting the environmental impacts of human activities on natural communities is a central problem in applied ecology. It is a difficult problem because one must separate human perturbations from the considerable natural temporal variability displayed by most populations. In addition, most human perturbations are generally unique and thus unreplicated. This raises the problem of deciding whether observed local effects are due to human intervention or to the natural differences in temporal patterns that often occur among different sites. These problems can be successfully addressed with the Before-After/Control-Impact (BACI) sampling design, in which Impact and Control sites are sampled contemporaneously and repeatedly in periods Before and After the human perturbation of interest. In the present case, we use this design to examine the ecological effects of the cooling water discharge from a coastal nuclear power plant in southern California. The results suggest some general lessons about the process of impact assessments that are applicable in many ecological contexts. In systems where plants and animals are long-lived and recruit sporadically, the rates of change in density are often so low that sampling more than a few times per year will introduce serial correlations in the data. As a result, for studies of few years duration, few samples will be taken. A small sample size means that the tests of the assumptions underlying the statistical analyses, e.g., independence and additivity, will have low power. This injects uncertainty into the conclusions. Small sample size also means that the power to detect any but very large effects will be low. In our study, sampling periods of 2- yr both Before and After the impact were not long enough to detect a halving or doubling of populations at the impact site. We concluded that there were significant environmental impacts because: (1) the effect size was generally very large (°-75%); (2) there was a consistent pattern among species; (3) there were two Impact sites, and effects were larger at the site nearest the discharge; (4) the observed effects accorded with physical changes that could be linked with the source of impact; and (5) a number of alternative mechanisms, unrelated to the source of impact, were examined and rejected. Relative to control populations, there were statistically significant reductions in density of snails, sea urchins, and sea stars, all of which occurred primarily on rocky substrates. All of the reductions were larger at the Impact station about 0.4 km from the discharge than at a second Impact station 1.4 km away. The most plausible mechanisms for the declines seem to be linked to the turbidity plume created by the power plant and the resultant increase in suspended inorganic and organic materials (+46% at the Impact site nearest the discharge). Any associated flux of fine particles on rocks would have deleterious effects on many of the hard benthos. Populations of two filter-feeding species, a gorgonian coral and a sponge, showed relative inc...

We assessed impacts of the 'Exxon Valdez' oil spill on bentluc communities within and adjacent to eelgrass beds in Prince William Sound, Alaska, USA. The concentration of total polynuclear aromatic hydrocarbons (TPAHs),'benthic community composition. diversity, biomass, and abundance were compared between matched pairs of oiled and reference sites in 1990 (approx. 16 mo after the spill), andin 1991, 1993, and 1995. TPAHs in sediments were high (up to 15300 ng g.') at sites adjacent to oiled shorelines in 1990, but declined sharply thereafter. Some reference sites in 1990-91 also had elevated TPAHs in sediments and signatures matching Exxon Valdez oii, but concentrations were significantly lower than at oiled sites. Based on classification and ordination analyses, cornrnunities of infauna and small epifauna at some oiled sites in 1990 differed from communities at reference sites, and from the Same sites in subsequent years. Percent sand and mud aild concentration of total chrysenes (PAH analytes indicative of crude oil) explained significant proportions of the temporal and spatial variation in benthic community structure. Total abundance and biomass of epifauna were generaliy higher at oiled sites, primarily because of higher densities of epifaunal bivalves. Otherwise, there were few consistent cornrnunity-wide responses to oiling in diversity. richness, total abundance. total biomass. or the abundances of major taxonomic groups (e.g. polychaetes or bivalves). We attnbute the lack of a stronger community-wide response to the varying sensitivities of constituent taxa to oil and organic enrichment. Over half of the dominant famiiies differed with respect to abundance at oiled versus reference sites Most, including 9 families of polychaetes, were more abundant at oiled sites. Most of these were stress-tolerant or opportunistic. and their increase was likely due to organic enrichment. Negative impacts were most evident in oil-sensitive amphipods, especially the families Isaeidae and Phoxocephalidae. There were consistently more of these amphipods at reference sites, and abundances at oiled sites were likely reduced as a result of oil toxicity. Most of these differences between oiled and reference sites persisted through 1995,6 yr after the spill. We suspect that these differences are a result of the spill, but we rely on post-spill coinparisons to infer impacts, and our conclusions rely on the untestable assumption of equality between oiled and reference sites in the absence of a spill. Future assessments of the impacts of oil spiiis or other accidental environmental disturbances could benefit from pre-iinpact studies that provide objective critena for selection of matched pairs of sites, thereby supporting the assumption of equality in the absence of the disturbance.

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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe ecological effects of acute oil releases into marine systems, such as occurs in major oil spill incidents, have been well documented. Less information has been collected together on the effects of chronic oil inputs, such as produced formation water discharges or municipal/industrial run-off into oceans, which are of greater frequency and total quantity. A review of existing field and laboratory knowledge has been conducted and the results analysed with respect to oil type and source, habitats at risk and species impacted.The results are discussed under key themes including habitats/ecosystems and the basis for assessing ecological risk. Some gaps in knowledge have been identified and suggestions made for future work to address scientific and regulatory needs. Fig. 3− Vulnerability of marine communities to chronic and press hydrocarbon contamination.