SEASONAL CHANGES IN DISTRIBUTION AND ABUNDANCE OF THE CLADOCERAN SPECIES IN RELATION TO ENVIRONMENTAL FACTORS IN GOKOVA BAY (MUGLA, AEGEAN SEA, TURKEY)

Abstract

Seasonal distribution and abundance of zooplankton species belonging to Cladocera were investigated and some physical and chemical characteristics of Gokova Bay were determined between March 2007 and February 2008. During this period, zooplankton samples were collected monthly by vertical hauls at 15 stations and by horizontal hauls at 3 stations by WP2 standard plankton net. Annual average temperature value was 21.03 degrees C at surface and 20.98 degrees C at 10 m depth. Also, surface water average salinity value was determined as 35.90 parts per thousand and 38.56 parts per thousand at 10 m depth. From March 2007 to February 2008, average dissolved oxygen value was found as 7.37 mg/L at surface and 7.41 mg/L at 10 m. Average concentration of the chlorophyll-a was measured as 0.165 mg/L during study period. 5 Cladoceran species were determined; Penilia avirostris, Evadne spinifera, Pseudevadne tergestina, Pleopis polyphemoides and Podon intermedius. P.avirostris was the most abundant species, followed by E.spinifera. Abundance of P. avirostris reached maximum value (2315.94 ind./m(3)) in September. Abundance of E.spinifera showed maximum value (1704.75 ind./m(3)) in August and minimum in November. P.tergestina was showed maximum abundance value in August. P. polyphemoides was found in a few numbers in July, October and November whereas P. intermedius was determined only in April. Cladocera species were showed maximum abundance in autumn months and formed 7% of total zooplankton. Temperature and dissolved oxygen of the surface and 10 m depth seem to be the main factors affecting distribution of the cladoceran species.