Truncated foodweb effects of omnivorous minnows in a recovering acidified lake

Cyprinids (Margariscus margarita, Phoxinus spp., Pimephales promelas) have resumed reproduction in a boreal headwater lake (Lake 302S, Experimental La kes Area, northwestern Ontario) that is recovering from experimental acidif ication. Concomitant changes to the littoral food web suggested that these omnivorous minnows suppressed the development of green algal mats, termed m etaphyton. We tested this hypothesis by conducting an experiment using minn ow enclosures, minnow exclosures, and open control plots in the shallow lit toral zone of Lake 302S. Minnows significantly suppressed zooplankton bioma ss, and altered community composition by disproportionally reducing large d aphnids and chydorids. Epiphytic chronomids were also significantly less ab undant in the presence of minnows. Minnows had a significant time-dependent , negative effect on benthic invertebrate biomass and community composition because chironomids and anisopterans were suppressed during the second hal f of the 6-wk experiment. However, minnows did not reduce the abundance of the dominant primary producer, namely metaphyton. Stable isotope analyses r evealed that minnows did not suppress metaphyton because these algae were n ot the primary C source for the food web. Instead, our findings suggest tha t the littoral food web depended mainly on sedimentary C, which resulted in the foodweb effect of minnows being truncated at the level of invertebrate s. Therefore, metaphyton appears to be regulated primarily by abiotic facto rs (e.g., availability of dissolved inorganic C) and not herbivory in recov ering acidified lakes.