Effects of water quality changes on the Ostracoda (Crustacea) species diversity and seasonal occurrence patterns in Lake Egirdir (Isparta, Turkey)

Changes in water qualities of the natural aquatic bodies cause habitat and species diversity lost. To understand effects of seasonal water quality changes on the ostracod species diversity and occurrences in Lake Egirdir (Isparta, Turkey), a total of 23 ostracod taxa (10 live species, 13 taxa) were collected from ten stations in the lake during three years of seasonal samplings from 2016 to 2018. Six taxa (Candona candida, C. cf. weltneri, Fabaeformiscandona breuili, Heterocypris incongruens, Ilyocypris decipiens, Limnocythere inopinata) are new reports for the lake. There was no significant difference between the numbers of species found from littoral and pelagic zones. The number of ostracods were higher (4 live, 14 taxa) in winter seasons than the spring (5 live, 10 taxa), summer (4 live, 9 taxa) and fall (8 live, 5 taxa) seasons. Ostracod Watch Model illustrated that Physocypria kraepelini, Neglecandona neglecta and Darwinula stevensoni showed similar seasonal occurrences in almost all seasons during the study. Of these, P. kraepelini was the most abundant species while the others were relatively rare and/or very scarce in their occurrence and abundance values. The mean value of the dissolved oxygen was significantly different between pelagic and littoral zones (p < 0.05). The CCA diagram displayed 84.4% of the correlation between eight species and five ecological variables (water temperature, ammonium, pH, dissolved oxygen, depth). Two species (P. kraepelini and F. breuili) and D. stevensoni showed significantly negative correlation with NH4 and pH, respectively. Species with similar optimum values seemed to have similar ecological and/or habitat preferences along with similar seasonal occurrence patterns. Results point to a reverse relationship between ecological tolerance and optimum estimates of the species. Thus, ostracods with a narrow tolerance range can be considered good indicator species for the water quality estimates. Accordingly, our measurements exhibited that Lake Egirdir is mainly dominated by NO3-N and NH4-N, suggesting a critical reduction in water quality values. Compared with the fossil ostracods, finding six live of 12 fossil species during the present study provides supportive evidence that aquatic conditions of the lake have been declining since the Holocene age. Indeed, dominance of cosmoecious species and absence of several rare species can also be other supportive evidence of the lake water quality decline.