Seasonal variations in the length of zooplankton related to certain physicochemical variables in two freshwater reservoirs

Seasonal variations in the body length of zooplankton were studied in relation to water temperature, nitrate (NO3), soluble reactive phosphorus (SRP), total chlorophyll, Secchi disk depth, pH, conductivity, and oxidation-reduction potential (ORP) in a mesotrophic (Ikizcetepeler) and a eutrophic (Caygoren) reservoir from February 2007 to March 2008. During the study, the body lengths of a total of 7590 zooplankton specimens (1110 rotifers, 3270 cladocerans, and 3210 copepods) were measured. The length of the majority of the species was significantly smaller in summer than in winter, fall, and spring, including that of the most dominant species, Asplanchna priodonta, Daphnia galeata, Daphnia longispina, Diaphanosoma brachyurum, Bosmina longirostris, Leptodora kindtii, Ceriodaphnia pulchella, Cyclops vicinus, Metacyclops gracilis, and Acanthocyclops robustus (F > 5, p < 0 . 05 ). Correspondence analysis (CA) showed that the body length of the zooplankton studied was inversely related to water temperature, whereas it was positively related to ORP and pH. The results of our study suggest that, although nutrients (NO3 and SRP) apparently have an effect on zooplankton body length only in the mesotrophic reservoir, temperature influences the body length in both the mesotrophic and the eutrophic reservoir.