Littoral Zoobenthic Communities: Resource Partitioning in a Heterogeneous Resource Environment

Background/Question/Methods Benthic macroinvertebrates are important food sources for higher trophic levels in lake food webs. These assemblages are dependant on benthic algae (periphyton), phytoplankton, and pelagic- or terrestrial-derived detritus. Diverse functional groups of invertebrates are found in close proximity in the littoral zone and may compete for food resources. The littoral zone is an extremely heterogeneous habitat. Resource partitioning may reduce competition among invertebrate species and affect invertebrate distributions in the littoral zone. We used stable isotopes of carbon (13C) and nitrogen (15N) to assess macroinvertebrate reliance on periphyton versus pelagic/detrital food resources. We sampled periphyton and macroinvertebrates along depth transects in 4 north temperate lakes and analyzed them for 13C and 15N. Both periphyton and zoobenthic δ13C signatures varied greatly with depth and among lakes. Variation of δ13C of sediments with depth was closely correlated with benthic algal primary productivity (R2 =0.84). This suggests that a most, if not all, of the variation in δ13C signal is due to periphyton. Invertebrates were categorized as grazers, deposit feeders, collectors, predators and filter feeders. The δ13C for each functional group was compared to the δ13C of sediments at that depth. Results/Conclusions Invertebrates relied heavily on benthic algae as a food source, and exhibited depth-specific variation in δ13C that paralleled patterns in benthic algae. For example, snail δ13C signals that matched or where more positive then periphyton δ13C signatures indicating that snails consume the newest and most productive periphyton. In contrast, collectors and deposit feeders show a less enriched signal that may signify utilization of less productive periphyton that is found deeper in sediments. These results indicate that resource partitioning allows diverse assemblages of macroinvertebrates to coexist in the littoral zone. Resource partitioning may lead to ecosystem wide increase of reliance on littorally derived carbon and increase pelagic-littoral linkages