Seasonal variation of macrobenthic organisms in relation to the substrate type in Zayandeh Rud river (from Isfahan to Varzaneh)

Between Birds Garden and Varzaneh town, a distance of approximately 140 km, eight stations were selected to sample and identify the macrobenthos of Zayandeh Rud, for a full year commencing fall 2002. Sampling was carried out twice at each season, using a Surber, an Ekman sampler and a PVC tube. The specimens were sorted out, counted and identified to the nearest genus, and some to a family level, using the appropriate identification keys. Identified specimens were classified into 19 genera, 17 families, 13 orders, and 5 classes. Amongst the 13 identified orders, Ephemeroptera and Trichoptera were the most diverse groups and were mostly distributed in Isfahan region and upstream. Oligochaeta with 3 orders and 4 families, and Diptera with 3 families and 2 genera were mostly distributed in east of Isfahan. At the same time, Lumbriculidae, Lumbricidae, and Tubificidae and the genus Chironomus were present in all the stations and throughout the year, though they showed a wide range of differences in density. In contrary, Glossiphoniidae and Baetis, Piscidium, Hydropsyche, Agrion, Valvata were present only in some stations and seasons. Statistical analysis of the data indicated that Shannon, Simpson, Margalef richness index wear significantly and negatively correlate with EC and substrate content of organic matter. The Shannon index was lower in mudy-sloughy stations, while Margalef richness index positively correlated with water pH. The differences in benthic population structure could be attributed to physical changes in the river substrate, chemical properties of the water and/or their life cycle or the interactions between them.