In order to find the trophic state of the lake the trophic state index (TSI) method developed by Carlson (1977) – based on the total phosphorus level in water column – and Secchi depth (SD) were used. While calculation TSI values the following formulas were used: TSI (SD) = 60 – 14.41 ln (SD) and TSI (TP) = 14.42 ln (TP) + 4.15 [SD (Secchi disk, m), TP (Total phosphorus, µg/L)].
Sládeček (1983) has suggested an index Q for the determination of the trophic level of a given lake. This index is shown by the formula: QB/T = number of species of Brachionus/number of species of Trichocerca. He reported that if the ratio is Q = 1.0 the lake is oligotrophic, if it is Q = 1.0 – 2.0 it is mesotrophic, and if Q > 2 the lake is eutrophic. This index was used for Lake Īznik in the present study.