Benthic mats offer a potential subsidy to pelagic consumers in tundra pond food webs

We quantified the potential nutritional contribution of benthic mats to pelagic consumers in tundra ponds using three approaches. (1) We compared benthic and pelagic habitats based on their algal biomass and community composition and assessed the dietary quality in terms of fatty acid (FA) concentrations in both habitats. The algal community compositions differed significantly between habitats. Both benthic and pelagic habitats contained saturated and unsaturated FAs, but on average there were more FAs relative to carbon in the pelagic habitat (44 +/- 29 mu g mg C-1) than the benthic habitat (23 +/- 18 mu g mg C-1) across all studied ponds. (2) We quantified the contribution of benthic mats to the whole-lake FA pool and found that benthic mats dominate (on average > 90%) the basal FA resource within tundra ponds. (3) Using a series of feeding experiments, we found that Daphnia survival and FA concentrations increased when a benthic mat slurry supplemented low concentrations of phytoplankton. However, Daphnia could not survive solely on benthic mats, and nor did benthic mats increase adult Daphnia survival when mats were offered as coherent pieces. The extent to which pelagic consumers could be supported by benthic production challenges the view that benthic habitats are functionally separate from the pelagic food webs within northern lake ecosystems.