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EXECUTIVE SUMMARY
Bottom (benthic) organisms have many characteristics that make them useful as indicators of environmental stress. For this reason, they have been used worldwide to assess the effects of municipal wastewater outfalls, disposal of dredged materials, and other anthropogenic activities. Monitoring programs in southern California have provided useful information for evaluating local impacts. However, there is also a need for regional data that can be used to scale the severity of individual impacts and make regional assessments. Recognizing this need, 12 agencies joined in the Southern California Bight Pilot Project (SCBPP) to assess the environmental status of soft-bottom habitats on the mainland shelf of southern California.
In July-August 1994, benthic infauna were collected from 251 stations at depths of 10-200 m between Point Conception and the U.S.-Mexico international border. Stations were chosen using a stratified random design with geography (Santa Monica Bay), and proximity to input sources (wastewater outfalls and river mouths) as the primary strata. Samples were taken with a modified 0.1m2 Van Veen grab and sieved through a 0.1 mm mesh screen. Samples were then sorted and identified according to protocols established in a laboratory manual developed for the project.
A primary objective of the survey was to characterize benthic communities. This objective was accomplished by calculating community characteristics, such as the number of taxa and individuals, and determining the effects of latitude and depth on community characteristics and species distributions. An average of 85 taxa/sample, 3850 individuals and 58 grams wet-weight biomass/m2 were collected on the mainland shelf of the Bight. Individuals were relatively evenly distributed among the taxa; no one taxon was dominant. Approximately 50% of the organisms were annelids; 19, 13, and 10% were arthropods, ophiuroids, and mollusks, respectively.
Most community characteristics were not correlated with latitude. Even where statistically significant relationships were found, correlations were relatively weak. Most community characteristics were correlated with depth, but, with one exception, the regressions did not explain more than 7% of the variance.
Sixteen taxa averaged 40 or more individuals/m2 and occurred in at least 30% of the samples. All but five of these taxa were annelids. No indication of a latitudinal gradient was found for most taxa; however, distributions did vary with depth. The majority of taxa were most abundant in 40-80 m of water.
A second objective of the survey was to identify groups of stations with similar species composition. Cluster analysis defined four groups of stations. Each group occupied a different habitat, characterized by differences in depth and sediment grain size. Groups 1 and 2 were found in greater than 115 m of water in coarse and fine sediment, respectively. Group 4 was found in less than 30 m of water or in 30-45 m in coarse sediment. Group 3 was found in intermediate depths.
A third objective of the survey was to estimate the areal extent of alterations to benthic communities in the Bight and to compare the amount of altered area in Santa Monica Bay, in the vicinity of discharges from municipal wastewater outfalls (POTW?s) and near rivers and stormdrains. The assessment of infaunal condition was based on analysis of: 1) species composition, 2) community parameters (e.g., number of species) and 3) the Benthic Response Index (BRI). The BRI is the abundance-weighted average pollution tolerance of species in a sample. Pollution tolerance was measured by determining the position of a species on a gradient between the most and least affected stations in an ordination space. If most of the species in a sample are reference species, the index score for the station is low. If most of the species are pollution tolerant, the index value for the station is high. For this assessment, the percent of area exceeding the reference threshold of the BRI was determined for four levels of biological response: I) marginal deviation, a change in relative abundance of species; II) loss of biodiversity, the exclusion of sensitive species that often causes a change in species composition of the assemblage; III) loss of community function, the exclusion of groups of species, particularly arthropods and ophiuroids; and IV) defaunation, the exclusion of most species.
While the BRI measures changes in benthic communities caused by a disturbance, the BRI cannot be used to determine the source of the disturbance. Species respond in a similar manner to both natural and anthropogenic disturbances. For example, pollution tolerant species may colonize an area near the head of a submarine canyon as well as an area that is affected by a discharge from an outfall. For this reason, benthic communities that are determined to be altered should not be assumed to be anthropogenically impacted.
Most of the Bight had healthy benthic communities. Over 90% of the Bight was classified as reference by the BRI. Alterations, where found, were limited in magnitude. Eight percent of the mainland shelf of the Bight was classified in Response Level I, marginal deviation from reference. Less than two percent was classified in Response Level II, loss in biodiversity. None of the mainland shelf was classified in Response Level III or IV. Most stations classified in Response Levels I and II were located in the Santa Barbara Channel, near the mouth of the Santa Clara and Ventura Rivers, in central and northern Santa Monica Bay or on the Palos Verdes shelf
The condition of benthic assemblages was marginally poorer in Santa Monica Bay than in other areas of the Bight. Benthic assemblages were classified as reference by the BRI in 87% of Santa Monica Bay, compared to 92% of areas outside of Santa Monica Bay. The number of taxa/sample and total abundance of organisms were lower in Santa Monica Bay. Proportionally more mollusks and fewer annelids were found in Santa Monica Bay than in other areas of the Bight.
The condition of benthic communities in POTW areas was similar to other areas of the Bight. Benthic assemblages were classified as reference by the BRI in 89% of POTW areas, compared to 92% of non-POTW areas. While the number of taxa was similar, dominance was higher and diversity lower in mid-depth POTW than in non-POTW areas. Proportionately more annelids and fewer arthropods were found in POTW than in non-POTW areas.
More area was altered in stormwater areas than in the Bight as a whole; however, changes in benthic assemblages were limited in magnitude. Benthic assemblages in 60% of stormwater discharge areas were classified as reference by the BRI, compared to 87% of shallow non-stormwater areas; 23 and 17% of the area was categorized in Response Levels I and II, respectively. Diversity, abundance, and other characteristics of the populations in stormwater and non-stormwater areas were similar. The proportion of biomass contributed by mollusks was lower in stormwater discharge areas; however, the difference was small. The causes of alterations in stormwater discharge areas are not known, but could include natural and/or anthropogenic disturbances such as seasonal changes in salinity or sediment movement caused by waves.
Even though altered benthic communities were found in areas within the influence of municipal wastewater outfalls and stormwater runoff, little relationship was found between the level of biological response as measured by the BRI and concentration of chemicals (e.g., chlorinated hydrocarbons and trace metals) in the sediment. The reasons for the lack of correspondence between sediment chemistry and alterations in benthic communities are not known. The fact that normal benthic communities were found in areas with high concentrations of chemicals could be related to the sequestering effect of binding factors, such as organic carbon, in the sediment. It is also possible that organisms in southern California have become adapted to high concentrations of chemicals in the sediment. In areas with altered benthic communities and low concentrations of chemicals in the sediment, it is possible that whatever is causing the disturbance occurs intermittently and/or is not captured in the measured sediment chemistry. The alterations may be caused by natural events.