Impact of Environmental Variables and Food Availability on Rotifer Assemblage in the Karstic Barrage Lake Visovac (Krka River, Croatia)

Abstract

Impact of Environmental Variables and Food Availability on RotiferAssemblage in the Karstic Barrage Lake Visovac (Krka River, Croatia)key words: karstic riverine lake, rotifer density and diversity, macrofilter-feeder rotifer, microfilter-feeder rotifer AbstractWe evaluated the impact of 18 environmental variables (physico-chemical, nutrients, food resources) on rotifer assemblage in the sub-Mediterranean karstic barrage Lake Visovac. In terms of the spacetime distribution the highest density of rotifers was noted in the summer period (average 386 ind/l, relative annual abundance 62%), and in the epilimnetic layer (average 309 ind/l, relative annual abundance 58%). A total of 41 rotifer taxa belonging to 22 genera and 14 families were identified. Three rotifer species: Gastropus stylifer, Synchaeta tremula and Trichocerca birostris were dominant and perennial during the annual investigation. Pearson's product-moment correlations and canonical correlation analysis suggest that temperature, pH values, alkalinity, chemical oxygen demand and chlorophyll a concentration significantly influenced rotifer density. According to rotifer food collection and selection, in Visovac Lake macrofilter-feeders predominated, and microfilter-feeders were in a minority. IntroductionMany recent studies have established the relationship between the plankton community density and diversity and limnological characteristics of different water body types, for instance: floodplains (DE AZEVEDO and BONECKER, 2003), backwaters (ARORA and MEHRA, 2003), riverine lakes (ECKERT and WALZ, 1998;WALZ and WELKER, 1998;FERNÁNDEZ-ROSADO and LUCENA, 2001), karstic lakes (HABDIJA et al., 1993;MIRACLE and ALFONSO, 1993;RODRIGO et al., 2000). Lake Visovac is a lentic dilatation of the Krka River and is bounded by two barriers with impressive waterfalls, Roški slap to the north and Skradinski buk to the south. With respect to the origin of the lake basin, Visovac Lake belongs to a group of karstic barrage lakes (HUTCHINSON, 1957). Water flowing through the karstic area precipitates calcite (limestone), which creates different sedimentary forms, one of them being barriers.According to field investigations and experimental laboratory studies provided by many authors, rotifer distribution in lakes is related to both abiotic factors, such as temperature (BÇRZINš and PEJLER, 1989;WATANABE, 1992), oxygen content (MIRACLE and ARMENGOL-DÍAZ, 1995), pH values (B - ERZINš and PEJLER, 1987), light intensity (SAUNDERS-DAVIES, 1989), and to biotic factors, such as food availability (POURRIOT, 1977; GILBERT and BOG-DAN, 1981;BOGDAN and GILBERT, 1987;ARNDT, 1993;HABDIJA et al., 1993) (GILBERT, 1985;KIRK, 1991), interference competition (GILBERT and STEM- BERGER, 1985;BURNS and GILBERT, 1986) and predation . According to KIRK (2002), rotifers show a gleaner-opportunistic trade-off. With such characteristics, they are able to adapt quickly to changing and fluctuating physico-chemical and hydrological conditions, which exist in barrage lakes. Thus, in freshwater communities ...

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