Effects of Macroalgae on a Stream Invertebrate Community

Abstract

The effects of macroalgae on stream invertebrates were studied in riffle zones in a coastal southern California stream. Dense growths of macroalgae (Cladophora glomerata, Nostoc sp.) were experimentally removed at different times of the year and in different years, and the insect communities which developed were compared with those in unmanipulated controls. Marl precipitated by algae was also removed in one experiment. The presence of macroalgae was associated with greater total densities and taxon richness of invertebrates, and nearly all taxa responded significantly to algal removal on at least some dates. Insects formed most of the community and were classified according to three categories of macroalgal effects on benthic densities: 1. Negatively affected by macroalgae (and marl) due to competition for space--e.g., Blepharicera (strong response to both algal taxa); large Simulium (strong with Cladophora, weak with Nostoc). 2. Positively affected due to structural habitats created by algae--e.g., Micrasema, Rhyacophila and Hydropsyche (all strong); Tinodes (weak-Nostoc); Rheotanytarsus (strong-Nostoc). 3. Positively affected by both macroalgal structure and associated food resources (macroalgae or epiphyton)--e.g., Baetis and Chironomidae (strong-Cladophora, weak-Nostoc); Hydroptila, Ochrotrichia, and Euparyphus (strong-Cladophora); and endosymbiotic Cricotopus (strong-Nostoc). Natural disturbances will indirectly affect invertebrate distributions and abundances by affecting the distributions and abundances of macroalgae.