Structure of arthropod communities in some saline lakes of central British Columbia

Aquatic arthropods communities were examined with respect to factors determining species distributions and community structure in a series of eight lakes on the Chilcotin Plateau of British Columbia. Climate, altitude, physical location, water temperature and basin shape were similar for all lakes, and although size differed, no evidence was found for the influence of basin morphology on community structure. Salinity and vegetation characteristics differed widely among lakes, so three major processes were investigated: the association of (1) salinity with faunal communities, (2) salinity with floral communities, and (3) faunal with floral communities. These relationships were examined in light of diversity-stability hypotheses. The severity component of environmental stability was represented by salinity, and mean surface water conductivity ranged from 56 to 13115 μS cm-¹ at 25 °C. Salinity variations among lakes were determined primarily by the ions Na, HCO₃, CO₃, C1 and K. Two classification schemes (taxonomic and ecological) and several analytical techniques (community parameters and cluster analysis) indicated that the distribution and structure of faunal and floral communities were related to salinity. In total, 84 arthropod taxa and 26 macrophyte species were found and divided into three groups: those characteristic of high salinities (>5000 μS), of moderate or low salinities (<5000 μS), or tolerant of all salinities. Faunal assemblages in all lakes were dominated by filter feeders, and predators were more abundant in saline lakes. Shredders, collectors and predators were found in all the lakes, but saline lakes had fewer size groups. Floating leaved macrophytes occurred only in freshwater lakes, submerged forms were rare in highly saline lakes, and emergent forms were found in all lakes, although they were less abundant at high salinities. Generally, this study supports the hypothesis that saline habitats have less diverse communities than freshwater ones. In all floral and faunal sample sets, increased salinity was accompanied by a decrease in species richness. Virtually all measures of macrophyte community diversity and productivity were inversely correlated with salinity. Faunal subgroups must be examined separately when measures of community structure incorporate relative abundances. Patterns of association observed in the entire faunal community were dictated by the numerically dominant entomostracan subcommunity, and patterns in other subgroups were masked. Zooplankton trophic level diversity increased with decreasing salinity and changes in community composition were analogous to those of eutrophication. In both coleopteran and hemipteran communities, diversity decreased and density increased with increasing salinity. Possible causal mechanisms structuring each community are hypothesized. Faunal distributions corresponded to their known habitat preferences in terms of macrophyte communities. It was difficult to distinguish between the influence of salinity or macrophyte communities on animal communities as animal communities were often associated with both.

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