Temporal and spatial macroinvertebrate variance compared: crucial role of CPOM in a headwater stream

The aim of the present study was to estimate the extent to which macroinvertebrate assemblages in a headwater stream are determined by environmental conditions and temporal dynamics. Six mesohabitats defined by substratum were sampled monthly throughout one year, environmental conditions at each sampling point being precisely described. Environmental variables could be truncated into two main gradients related to (a) the availability of food and space resources (CPOM) and (b) hydraulic conditions. The response of the macroinvertebrate assemblage to the environmental gradients and temporal dynamics were analyzed using GAM and (p)RDA. Twice as high portion of variance in the faunal data was attributed to environmental gradients compared to temporal dynamics. Total abundance, as well as the abundance of almost all feeding groups, was dependent on the availability of food and space resources (CPOM), while their proportions were determined by the quality of food resources driven by hydraulic conditions. Temporal dynamics was of lower importance. Our results suggest the role of CPOM be crucial in woodland headwater streams, as it greatly enhances habitat quality, serving both as a food source to dominant shredders and a space source to most macroinvertebrates.