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Annual variations in biochemical composition of seston and zooplankton community in Mersin bay-Northeastern Mediterranean

Zenginer Yılmaz, Arife
In this study, annual variations in biochemical composition of seston and zooplankton community were investigated to characterize the nutritional environment of zooplankton in the Mersin Bay, NE Mediterranean Sea. For this goal, seawater and zooplankton samples were collected at monthly intervals from two stations; one representing coastal and other representing open waters characteristics from November 2004 to January 2006. Seawater samples were collected with Niskin bottles from the sea surface. Zooplankton samples were collected both in the horizontal and vertical plane by towing a Nansen net (70 cm mouth diameter with 112 m mesh). Surface seston chl-a, lipid, protein and carbohydrate concentrations were measured by fractionating seawater into three different size groups, 0.7-2.7, 2.7-18 and >18 m representing pico, nano and micro particulates in the seston. Zooplankton biomass and abundance were determined at four size fractions: 112-200, 200-500, 500-1000 and >1000 m; dry and organic weights were measured by gravimetric method and major taxonomic groups of zooplankton was identified under stereo-microscope. The nearshore station was always more productive than the offshore station in terms of chl-a, particulate organic matter (POM: protein+lipid+carbohydrate), zooplankton abundance and biomass. Chl-a maxima occured in spring and autumn at both stations. Very low chl-a concentrations at the offshore station (0.02-0.35 g L-1) confirmed oligotrophic character of the Northeastern Mediterranean. The highest chl-a concentration (2.4 g L-1) was observed in March 2005 at the nearshore station due to the input of Lamas River nearby. POM varied from 42.1 g L-1 (in January 2006) to 1082 g L-1 (in March 2005) and 53.7 g L-1 (in January 2006) to 246 g L-1 (in May 2005) at the nearshore and offshore stations, respectively. The oligotrophy of this system was indicated by the extremely low particulate lipid, protein and carbohydrate concentrations (1-3 times lower than in more productive systems). The most evident characteristic of this oligotrophic environment was the dominance of pico-POM throughout the study period, accounting for 3165 % of the total carbohydrates, proteins, lipids and chl-a. The prt:cho ratio was generally lower than 1 (low in organic nitrogen). Carbohydrate was the dominant biochemical component at both stations. Zooplankton varied during the sampling period, and they showed two peak abundances, in spring and autumn, with small increase in summer. The higher biomasses of zooplankton were observed in summer and autumn in the entire water column, but in spring and autumn periods in the surface water. Zooplankton data showed that 200-500 and 112-200 m size fractions were dominant in abundance at both stations. However, 200-500 m size fraction was dominant in zooplankton biomass at nearshore, whereas >1000 m size fraction was at offshore station. Copepods were the most abundant zooplankton group and dominated the distribution of total zooplankton, followed by crustace nauplii, appendicularia, cladocera and pteropoda.