Drift benthos relationships in the seasonal colonization dynamics of alpine streams

Abstract

We examined seasonal patterns in macroinvertebrate benthic abundance and drift in relation to their colonization dynamics in five contrasting streams. The streams, within an alpine glacial flood plain, differed over an annual cycle in flow permanence and surface hydrologic connectivity with the main floodplain channel. Study streams included an upper and lower main channel, a side-channel seasonally connected upstream to the main channel, and an intermittent side-slope tributary and permanent springbrook having no upstream surface connection to the main channel. Flow in the main and side-channels was influenced seasonally by upstream glaciers, whereas the tributary and springbrook had no direct glacial input. Macroinvertebrate assemblages differed markedly among streams. Density, biomass, and taxonomic richness typically were lowest in the upper main channel and highest in the springbrook. Seasonal changes in assemblage composition were more pronounced in the main and side-channels than in the tributary and springbrook. In the upper main channel, larvae colonizing cages filled with natural substrata reflected benthic assemblage structure regardless of season and colonizing taxa exhibited a high propensity to drift. In contrast, larval colonists differed significantly from benthic assemblages in the tributary and springbrook, and colonists were mostly from drift. The lower main and side-channels displayed high seasonality in colonization dynamics, being related to their hydrologic connectivity to other streams that may have provided larval colonists. Benthos to drift abundance ratios suggested that the upper main channel represented a relatively closed system with little interaction among other streams in the flood plain, whereas the springbrook and tributary seasonally subsidized larval colonists to downstream channels.