Shaping of macroinvertebrate structures in a small fishless lowland stream exposed to anthropopressure, including the environmental conditions

In studies of abiotic and biotic factors influencing macroinvertebrate assemblages, there is always the problem of which factor – fish predation or environmental conditions – has the strongest impact on the invertebrates and whether the impact is positive or negative. The aim of our study was to determine the impact on the structures of macrozoobenthos in a small field watercourse exerted by abiotic conditions, with the concurrent lack of predators and varied intensity of anthropopressure. During the entire study period, the presence of 49 taxa of macroinvertebrates was recorded. The highest number of taxa and value of biodiversity was observed in the upper part of the watercourse, and subsequently decreased down the stream, reaching the lowest value at the sites located near the outlet. The tributaries significantly differed between each other in the number of taxa. In the tributary carrying water from wetland, a much higher number of taxa was noted than in the tributary carrying municipal water where the density achieved a significantly higher value of individuals than the remaining sites. The most limiting factors for the abundance of the investigated taxa were the oxygen concentration, nutrients and ammonia