LEAF-ASSOCIATED PERIPHYTON IN HETEROTROPHIC STREAMS: EFFECT ON MACROINVERTEBRATE ASSEMBLAGES AND GROWTH

Abstract

Temperate headwater streams are often shaded, limiting autochthonous production, and therefore energetically supported by allochthonous material, e.g., leaves, via fungal and bacterial decomposition. Macroinvertebrate shredders feed on this leaf matrix, providing food for other organisms. Recent work indicates that periphyton (e.g., diatoms, green algae, cyanobacteria; hereafter, algae) interacts with microbial decomposers and provides higher quality food. Little work has, however, examined these interactions in natural settings. I investigated leaf-associated algae’s impact on macroinvertebrate leaf colonization in the field, followed by measuring growth and food preferences in the lab based on field results. First, I manipulated leaf light availability in high- and low-nutrient streams in winter and spring. Leaf-associated algal and fungal biomass were positively correlated in winter. Leaf C:N negatively correlated to algae in winter and fungi in spring, while N:P and C:P negatively correlated to fungi in winter and algae in spring. These factors predicted functional feeding guild biomass and abundance, e.g., predator biomass by algal and fungal biomass and spring shredder biomass by leaf stoichiometry. Algal biomass elicited differential taxon responses; e.g., Ephemerella (Ephemeroptera:Ephemerellidae) and Stenonema (Ephemeroptera:Heptageniidae) responded positively while Tipula (Diptera:Tipulidae) responded negatively. Second, I fed light- and dark-conditioned leaves to Ephemerella invaria and Caecidotea communis (Isopoda:Asellidae), which both consumed leaves and algae. C. communis experienced greater growth on light-conditioned leaves, indicating a high-quality resource, while E. invaria had no growth differences between treatments. Third, light- and dark-conditioned leaves were offered to five taxa, Amphinemura (Plecoptera:Nemouridae), Tipula, Stenonema, Lepidostoma (Trichoptera:Lepidostomatidae), and Caecidotea communis. Tipula alone demonstrated a preference which was for dark-conditioned leaves. These results indicate that leaf-associated algae are a food resource and attractant for some macroinvertebrates and a deterrent to others. Natural headwater streams are heterogeneous with leaves exposed to varying light levels, altering leaf-associated algae and providing differential food resources. Anthropogenic impacts often homogenize these streams. Although restoration seeks to restore heterogeneity, headwater stream algae are largely ignored. This work demonstrates the important role algae play in macroinvertebrate interactions with senescent leaves, highlighting the need to incorporate allochthonous and autochthonous resources into stream restoration and management efforts to support biodiversity.