Cobitis elongata and Sabanejewia balcanica from the family Cobitidae were collected monthly from September 2004 to October 2005 from the Petrinjčica River in the Danube basin of Croatia. On each sampling date, samples of the macrozoobenthos were taken from three different substrates. The density of the macrozoobenthos was calculated and compared with the diet of the loaches. During all months the most dominant group in the macrozoobenthos and in the guts of the analysed loaches was insect larvae of the family Chironomidae. Ivlev’s index was used as a measure of selectivity for various macroinvertebrate taxa in the fish rations and showed changing selectivity that indicate opportunistic feeding strategies and wide diet overlaps between the species.
This research was focused on the relationship between river discharge and organism drift. It was carried out for three years in a small heavily modified river in Saxony (Germany). The amount and species composition of drifting invertebrates were observed, depending on discharge and flow velocity. A station was installed where the flow velocity was continually measured and drifting organisms were caught with nets. An inventory of the aquatic community (benthic invertebrates) was taken to determine the species living in the river at the research station. The highest drift density measured was 578 organisms per m3 at a flow velocity of 0.90 m s-1 , the mainly drifting organisms were Chironomidae. Different organisms groups started drifting at different flow velocities. Heavy impacts, such as dredging the river and flood waves, affected the aquatic ecosystems and severely changed the aquatic community regarding the number and the diversity. Some of the aquatic invertebrates such as the Anthothecata completely disappeared after dredging. It was found that many different terrestrial organisms were part of the drift. The typical family of soil biota Collembola represented the largest share.