Influence of periphyton biomass dynamics on biologi-cal colmation processes in the hyporheic zone of a gravel bed river (River Lahn, Germany)

摘要

Riverbed permeability is a key factor controlling important ecological functions taking place in the hyporheic zone in river ecosystems. However, no direct long-term and continuous measurements of colmation in rivers exist. Hence, we studied dynamic colmation patterns in the River Lahn (Germa-ny) in order to quantify the biological colmation under eutrophic conditions. Daily temperature curves were measured in vertical profiles in the riverbed, which permitted the estimation of vertical infiltration velocities. These were related to the seasonal dynamics of discharge, water temperature, and of peri-phyton biomass. In addition, a hyporheic mesocosm experiment was conducted in order to reveal diffe-rences between internal and external colmation processes, including short-term dynamic patterns of gravel permeability. The results showed that the vertical infiltration rates across riffle structures were reduced from 0.20 m/h to 0.08 m/h during low discharge conditions. A negative exponential relation-ship between infiltration rates and periphyton biomass explained between 37% and 95 % of the variabi-lity of riverbed infiltration, with the higher value detected at low levels of vertical infiltration velocities. We conclude that in eutrophic rivers external colmation by periphyton is an important process control-ling hyporheic exchange patterns.