Seasonal patterns in macroinvertebrate drift and seston transport in streams of an alpine glacial flood plain

Abstract

1. We measured drift of macroinvertebrates and organic matter in five different channel types (upper and lower main channel, intermittent side‐channel, side‐slope tributary, and a groundwater channel) draining a glacial flood plain in the Swiss Alps. Samples were collected seasonally over 3 years (May 1997 to November 1999) to encompass major periods of floodplain expansion and contraction.2. Total wetted channel length ranged from ≤ 5 km during maximum floodplain contraction to ≥ 24 km under system expansion. Principal components analysis (PCA) of key physical and chemical measures collected over an annual cycle showed that the main and side channel sites differed spatially and seasonally from the tributary and groundwater sites, reflecting the glacial influence on the former.3. Two‐way ANOVA indicated a significant Site × Date interaction for all measured variables. Transported organic matter, caused by blooms of Hydrurus foetidus, was typically five to 10 times higher in the lower main channel and side‐channel than the other channel types, and was least variable among dates in the groundwater and tributary channels.4. The number of drifting macroinvertebrates was usually highest in the groundwater channel, although highly variable among years. Other channel types had highest drift rates in November and lowest rates in May, reflecting seasonal changes in the life cycles of particular taxa, such as Baetis alpinus and the Simuliidae. The lower main channel exhibited the greatest, and the groundwater the lowest, seasonality in drift compared with the other channels.5. The Chironomidae displayed the highest drift rates in most seasons, especially in summer when other taxa were relatively low in the drift. The number of taxa in the drift also varied among channel types and season, ranging from less than two in early spring to four to six in other seasons.6. The observed differences in drift rate and transported organic matter among channel types and seasons have important implications for ecosystem dynamics, through its effect, for instance, on dispersal among channel types or on organic matter exchange.