Effects of Grazing and Nitrogen Enrichment on the Taxonomic Structure of Periphyton Assemblages in Lotic Ecosystems

The taxonomic composition of benthic algal assemblages, which form the basis of the food web in many streams, is determined in part by herbivory and nutrient concentrations. The effects of grazing and nitrogen enrichment on the taxonomic structure of periphyton were investigated in two experiments, one in laboratory streams and the other in a natural stream.

Effects of grazing by the pleurocerid snail Juga silicula (Gould) and larvae of the limnephilid caddisfly Dicosmoecus gilvipes (Hagen) were investigated in a 50-day laboratory experiment. Grazing by Dicosmoecus at high densities (50 m-2) produced assemblages with a relatively high biomass and dominated by the long, linear diatom Synedra ulna, and a group of small, prostrate diatoms (Achnanthes minutissima, Navicula arvensis, Navicula minima, and Nitzschia fonticola). Algal assemblages in streams grazed by Juga at high densities (500 m-2) were characterized by low biomass and by high abundances of the cyanobacterium Phormidium tenue and basal cells of the heterotrichous chlorophyte Stigeoclonium tenue. Effects of concurrent grazing by the dissimilar herbivores were not additive, and produced algal assemblages intermediate in biomass and of distinct taxonomic composition. Algal assemblages in mixed-grazer streams were characterized by intermediate abundances of basal cells of S. tenue. Assemblages in high density (50 caddisflies m2 and 500 snails In-2) mixed-grazer streams were dominated to agreater degree by prostrate diatoms than in intermediate density (25 caddisflies In-2 and 250 snails In-2) mixed-grazer streams. Cobble-sized (22 x 22 x 4 cm) channel-spanning substrate blocks in intermediate density mixed-grazer streams increased substrate heterogeneity and influenced the effects of the herbivores on periphyton succession. The top substrates in these streams were characterized by high algal biomass, and algalassemblages were dominated by large, linear diatoms and by filaments of S. tenue and Ulothrix. The recessed substrates also possessed high biomass but had greater abundances of the prostrate diatoms, the filamentous cyanobacterium Oscillatoria agardhii, and basal cells of S. tenue.

Effects of nitrogen enrichment on periphyton were investigated by adding ammonium sulfate to Lookout Creek (Oregon) in the summers of 1991 and 1992. A maximum concentration of 90 lig NH4+-N was achieved in the enriched section, whereas 1\11-14+-N concentrations in the non-enriched section were low (1-6 lig 1-'). The taxonomic composition of periphyton in the enriched section was conspicuously different from the composition of algal assemblages in the non-enriched section. Assemblages in the non-enriched section in late summer were dominated by the large diatom Epithemia hyndmanii, which contains nitrogen-fixing cyanobacterial endosymbionts, and by nitrogen-fixing heterocystous cyanobacteria (Calothrix jUsca, Calothrix sp. 2, and Nostoc sp. 1). Assemblages in the enriched section in late summer were characterized by the diatoms Rhoicosphenia curvata, Gomphonema rhombicum, Gomphonema dichotonnim, and Arnzschia oregona, and the filamentous cyanobacterium, Phormidium tenue, which are often found in nitrogen-rich aquatic ecosystems.

The results of the two experiments described here demonstrate that both grazing and nutrient enrichment result in significant changes in the taxonomic composition of algal assemblages which may in turn affect the abundance and distribution of consumer organisms at all trophic levels.