The frequency and local abundance of Ruppia occidentalis in relation to sediment texture and lake salinity

Abstract

Ruppia occidentalis S. Wats. is a submerged macrophyte often found in saline and subsaline (> 3000 mg ∙ L−1 and 500–3000 mg ∙ L−1 total dissolved solids (TDS), respectively) lakes but rarely in freshwater (< 500 mg ∙ L−1, TDS) in Alberta, Canada. To determine the effect of water salinity on its distribution, we investigated the frequency and local abundance of R. occidentalis in relation to sediment texture within three lakes, having 2165, 1557, and 275 mg ∙ L−1 total dissolved solids. Sediment texture has the largest effect in freshwater and differences among lakes are greatest on the sediment with greatest local abundance. Ruppia occidentalis occurred in 82, 79, and 42%, respectively, of the sites examined within these lakes. This reduction in frequency was associated with a decrease in the diversity of sediment textures in which R. occidentalis occurred. Within the saline lakes, the frequency of R. occidentalis was not dependent on sediment texture. In freshwater, however, R. occidentalis is found primarily on coarse-textured sites. The absence of R. occidentalis from fine-textured sediments, when in freshwater, is not correlated with the abundance of other macrophytes. Local environments associated with coarse sediments, such as low organic content or reduced biotic interactions, are necessary for R. occidentalis to occur in freshwater. However, contrary to the patterns of frequency among sites, local abundance in relation to sediment texture was similar among lakes and least on coarse-textured sediments. Moreover, local abundance was not significantly correlated with lake salinity, except on sandy sites. Therefore, the effects of water salinity on the colonization of new sites, rather than the performance within sites, may be most important in determining the distributional limits of R. occidentalis in central Alberta.