Trophic status of Zayandeh-rood dam water (Isfahan) during autumn and winter of 2010-2011

Trophic status of Zayandeh-rood dam water and changes in that were investigated in autumn and winter. Sampling was performed in October, November, January, and February of 2010-11 with 45-d intervals. Nine different sampling points was defined in three different stations, including first three points in lake input, another three points was close to Chadegan village area and the last three points was close to dam crest. Water sampling was done from 30 cm depth of water column in each sampling point by using of Nansen bottle. Water transparency was determined by Secchi disk (1.0-4.30 m), total phosphorus were 0.01-11.57 mg/l, total nitrogen were 0.2-2.20 mg/l and chlorophyll-a were obtained 0-8.84 mg/l in this study. The trophic state index (TSI) was determined for phosphorus 25.22, 33.37, 29.80 and 31.02 at four above-mentioned times. The TSI for chlorophyll a were 44.82, 31.28, 32.37 and 35.14 respectively while this value for Secchi depth was 40.83, 42.42, 50.34 and 48.29, respectively at four sampling time. The obtained appropriate relationship between chlorophyll-a and total phosphorus revealed the important role of this element in algae growth rate while Secchi depth is mainly related to silt particle, washed from ambient land following the raining in these seasons. The TSI mean in all sampling times was 36.5-38.8, showing that the lake can be oligotrophic in autumn and winter. In addition, this lake was an oligotrophic lake based on measured amount of chlorophyll a and total phosphorus in water though the Secchi depth categorized this lake as a mesotrophic.