Human impact on large rivers: the influence of groynes of the River Oder on larval assemblages of caddisflies (Trichoptera)

The influence of groynes in large rivers on caddisflies has been poorly studied in the literature. Therefore, we carried out an investigation on the 420-km stretch of the River Oder equipped with groynes. At 29 stations, we caught caddisflies in four habitats: current sites, groyne fields, riverine control sites without groynes and in the river’s oxbows. We found that groyne construction increased species richness, diversity, evenness, and altered the structure of functional groups into more diversified and sustainable ones compared to the control sites. The groyne field fauna is similar to that of natural lentic habitats, but its composition is largely governed by the presence of potential colonists in the nearby oxbows. We distinguished three of the river’s caddisfly assemblages. The distribution of Trichoptera was governed inter alia by the plant cover and the amount of detritus, and consequently, the food resources. Oxygen, nitrates, phosphates and electrolytic conductivity were important as well. Groynes have had positive effects for caddisflies—not only those in the river itself, but also those in its valley. They can therefore be of significance in river restoration (although originally they served other purposes), especially with respect to the radically transformed ecosystems of large rivers.