Benthic Turfs vs Floating Mats of Algae in River Food Webs

In the Eel River of northern California, as in many other sunlit rivers worldwide, large growths of filamentous green algae accumulate during summer low flow periods. Algae initially grow as turfs, attached to the river bed, but over time detach to form floating mats. Floating mats differ from attached turfs as habitats for invertebrates in a number of respects. In paired trials, larval chironomids were 15-16 times more likely to be consumed by fishes when exposed in algal turfs on the river bed than when exposed in floating algal mats on the river surface. On average, 2.7 and 6.1 times more adult insects emerged from floating mats than from benthic algal turfs in shallow (< 30 cm deep) and deep (30-60 cm deep) habitats, respectively. Differences between algal mats and turfs as cover and habitat for resident insects lead to the hypothesis that the timing, abundance, and duration of floating mat habitats strongly influences the routing of insect production in river food webs, to aquatic consumers or to the land.