Epiphytic metazoans on emergent macrophytes in oxbow lakes of the Krapina River, Croatia: differences related to plant species and limnological conditions

This study investigated the structure of the epiphytic metazoans on emerged macrophytes in the littoral zone of two oxbow lakes with different trophic levels. Differences in the diversity and density of the epiphytic metazoans were analyzed in relation to plant architecture (simple or complex stems), food resources (algae and detritus) and water characteristics (transparency and derived trophic state index). Asignificant negative correlation was found between detritus on plants as food resource, and diversity and density of epiphytic metazoans, indicating grazing of microphagous species. Rotifers dominated in diversity and density in the epiphyton on all habitats. Total density of metazoans, rotifers and copepods in epiphyton were significantly higher on Mentha in mesotrophic lake than on Iris in a eutrophic lake.We presume that macrophyte belt width and trophic state governed biotic interactions and consequently epiphytic assemblages more strongly than macrophyte architecture. However, a Mentha habitat showed a slightly higher density and diversity of epiphytic metazoans in relation to Iris at the same site, but these differences were not significant

Spatial and temporal variation in microcystin concentrations during perennial bloom of Planktothrix agardhii in a hypertrophic lake

Temporal and spatial variation in the concentrations of intra- and extra-cellular microcystins were studied in a hypertrophic lake with bloom of Planktothrix agardhii (Gomont) Anagnostidis et Komarek. Concomitantly with increase in water temperature (from 2 to 20 °C) abundance of P. agardhii increased from 1.9 x 105 to 4.3 x 107 trichomes L-1. In autumn, in spite of temperature lower (14°C) than in summer it was still very high. Mass development of P. agardhii (to 6 x 106 L-1 and higher) caused a severe decrease in water transparency (to 0.5 - 0.2 m in summer/autumn). The cyanobacterium density was relatively uniform within water column; only in summer (July) it was significantly higher (by about 30%) in surface than in bottom layer. From spring to autumn microcystins (MCs) were mainly biomass-bound (up to 90 μg MC-LR equiv. L-1), whereas the level of extra-cellular toxins was much lower (up to 2 μg L-1) and relatively stable. Only in winter, high amounts of MCs (11.3 μg L-1) were released from decaying biomass into water. The increasing concentrations of biomass-bound microcystins in the lake water positively correlated (R2 = 0.9863; y = -0.1285x2 + 7.14x ) with the abundance of P. agardhii and the highest concentrations of the intracellular MC fraction were found during the exponential phase of P. agardhii growth. In addition, the surface-sampled biomass of P. agardhii contained in autumn 2-fold more MCs (2.75 μg MC-LR equiv. per 106 P. agardhii trichomes) than the bottom-sampled one (1.41 μg MC-LR equiv. per 106 trichomes). This is the first report showing that despite the homogenous distribution of P. agardhii in water column of a shallow lake, various seasonal and spatial distributions of both extra-cellular and intracellular fractions of microcystins occur