Spatial Subsidies, Trophic State, and Community Structure: Examining the Effects of Leaf Litter Input on Ponds
In aquatic systems, light and subsidy input often co-vary along a canopy cover gradient. This creates systems where subsidies are more prevalent in areas with low primary productivity and less prevalent in areas with high primary productivity. We expect ecosystem processes and community structure to respond to these changes in resources. We examined the effects of light and subsidy input (leaf litter) on ponds by placing pond mesocosms along a canopy gradient and manipulating litter input. We then sampled mesocosms for ecosystem and community parameters for 2 years during April (pre-leaf out), July (full canopy), and November (start of leaf fall). Neither canopy cover nor litter input tended to dominate effects overall, highlighting the importance of the light-subsidy gradient combination that is frequently encountered in temperate aquatic systems. Ponds shifted from an autotrophy/heterotrophy balance to net heterotrophy with increasing canopy cover in concordance with our predictions. Although litter input affected dissolved oxygen, we did not detect an effect of litter on trophic state, primary production, or community respiration. We additionally found effects of both canopy cover and litter input on community composition, but very differently than that found in streams. In general, the grazer and shredder macroinvertebrate functional feeding groups were rare in the pond mesocosms, and thus did not respond to treatments. The collector proportion of the community, mainly chironomids, increased with canopy cover and was higher in mesocosms with litter input than those without. Contrary to predictions, there were few differences between subsidy input type (leaves or grass) despite differences in litter quality. This study, along with many others, highlights the importance of canopy gradients in determining ecosystem function and community composition.