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The biological determinants of the structure of harpacticoid copepod communities on an estuarine intertidal flat (Fraser River Delta, B.C.) Harrison, Brenda Jane

Abstract

The abundance and microspatial distribution of harpacticoid copepods were studied from Jan 1977 to Jan 1980 at two intertidal sites on the Fraser River Delta, B.C.: one (Sand) on the crest of a sand wave and the other (Mud) on the edge of its muddy trough. Eight species formed the communities at the two sites. The composition of the communities was studied by biweekly survey in 1978. Longer term changes in the organization of the sand community were followed by annual mid-winter surveys over four years. Factor analysis was used to group species with similar patterns of seasonal occurrence and macrohabitat distributions. Microspatial patterns in the two communities were compared by heterogeneity chi-square analysis from random core samples collected monthly. The distribution of species relative to small-scale hydraulic bedforms (sand ripples) was studied at the sand site. Distinct patterns of community organization were seen at the two sites. The mud community was characterized by high temporal overlap of species; the sand species showed a distinctive pattern of temporal segregation. Although species in both communities were segregated at small spatial scales, microspatial partitioning was stronger and seasonally more variable in the mud. Spatial patterns in the sand were related to the rippled bedforms. Biogenic structuring by macrobenthos and disturbance by predators were implicated as the causes of microspatial patterns observed in the mud community in summer. Although the two communities showed distinct features of organization they were not completely isolated. Two mud species, Scottolana canadensis and Tachidius triangularis, 'migrated' to the sand in summer. The role of biological interactions in structuring the communities was investigated. Predation by small tidepool fish, Clevelandia ios and Leptocottus armatus, was studied by gut analysis. Predation was species- and size-selective, with two epipelic species, S. canadensis and T. triangularis, occurring most frequently in fish gut samples. A morphometric basis for ontogenetic variation in predation by C. ios was proposed. Evidence to support the hypothesis that competitive interactions help to structure harpacticoid copepod communities was found in changing patterns of abundance, changing patterns of microhabitat use and morphometric character displacement over four years. Competitive interactions were highly dynamic and appeared to be part of the cause of both the temporal and microspatial segregation observed in the sand community in 1979. By 1980, the eight species present in the January sand community formed a displacement series in body sizes. Although each harpacticoid community is unique, numerous parallels exist between the structure and function of the Iona North assemblage and those from shallow water habitats, worldwide. It is clear, therefore, that the conclusions from this research have more than local significance.

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