Temperate headwater streams have traditionally been considered energetically supported by allochthonous organic matter decomposition, e.g., leaves. Leaves entering streams are quickly colonized by fungal and bacterial decomposers, and macroinvertebrate shredders feed upon this matrix. Recent evidence, however, points to interactions between microbial decomposers, macroinvertebrates, and autochthonous algal primary producers. To better understand these interactions, we manipulated leaf light availability in six streams across two seasons and measured algae, fungi, and macroinvertebrates, and we conditioned leaves under light and dark treatments in the lab prior to offering them to macroinvertebrates in feeding and preference studies. Field experiments indicated that changes in light altered algal biomass, with resultant changes in macroinvertebrate assemblages, driven by responses of common taxa. Ephemerella and Stenonema mayflies were positively related to algal biomass, while Tipula crane flies were negatively related. Within the lab, Ephemerellainvaria equally consumed leaves conditioned in light and dark conditions, and stable isotopes indicated assimilation of both leaf and algal material. In contrast, Caecidotea communis isopod shredders consumed and grew more on light-conditioned leaves, corresponding to a different microbial community than dark-conditioned leaves. In preference tests, C. communis, Amphinemura sp., Lepidostoma sp., and Stenonema sp. equally consumed both light- and dark-conditioned leaves. Tipula sp., however, fed more on dark-conditioned leaves. The field and lab studies provide a more complete picture of how leaf-associated algae interact with different macroinvertebrate taxa. Natural heterogeneity in streams, such as light availability, supports diverse leaf conditions, including microbially, which in turn may support diverse macroinvertebrate assemblages.
