Secondary production of Gammarus pulex Linnaeus in small temperate streams that differ in riparian canopy cover

Variation in the amount of riparian canopy cover affects bioenergetic processes in streams. Effects of canopy opening on environmental parameters including water temperature and the quality and quantity of food resources (leaf litter and/or associated biofilm) are likely to influence detritivore production. The objective of this study was to quantify population dynamics of the common shredder Gammarus pulex in three temperate lowland stream reaches that differ in riparian canopy cover. Benthic samples were taken monthly from October 2001 until October 2002, and secondary production was estimated using the size-frequency method. Populations differed in size/age structure, length-weight ratio, mean and maximum amphipod size, which were related to the amount of riparian canopy cover. Population density and secondary production were less clearly associated with canopy cover, which was probably related to a difference in fish predation between sites. Canopy opening was associated with differences in the range of variation in annual thermal conditions and most likely in the nutritional quality (microflora) of the food source, whereas the autumnal pulse of leaf fall lead to oxygen depletion at the heavily shaded woodland reach. Our study demonstrated that moderate differences in riparian canopy cover affect shredder-detritivore population dynamics and suggests that even detrital-based and shredder-dominated food webs can be influenced by the indirect effects of light. The temporal differences in temperature appear biologically less important unlike the effect of increased algal production. Whereas the array of interacting factors (temperature, radiation, leaf processing and microbial colonization) emphasize the complexity of effects associated with canopy opening.