Abstract
Quantitative and qualitative responses of subtropical benthic freshwater nematodes to changing water levels (low versus high) have not been considered before. Three distinct locations with different depths in two freshwater channels (Rowaina and Shalma) of the northern Nile delta had been chosen for this purpose. Low and High-water levels (LWL & HWL) were in January and July 2019 respectively. Nematode abundance and diversity were significantly different between water levels, locations, and their interactions. They were categorized into six feeding groups in both water levels, locations and channels (bacterivores, plant ectoparasites, fungivores, Nematovorous, omnivorous, and predators). However, bacterivorous and omnivorous disappeared during HWL in Rowaina channel. The relative dominance of the feeding groups differed between water levels, localities, and channels. Dominancy was generally higher in Shalma. Diversity pattern in Rowaina contrasted that of Shalma. Environmental variables differently influenced nematode community. It could be concluded that water level fluctuation was not the only factor determining benthic nematode abundance, diversity, and trophic categorization. Specific water body conditions affect benthic nematode community structure. Nematode communities are environmentally sensitive agent. Furthermore, the current findings might improve our understanding of aquaculture, as water level fluctuations affect benthic nematodes, which directly or indirectly affect the reared organisms.

Key words: abundance, benthic, diversity, freshwater, nematode, water level.