WaterSubject

The influence of Daphnia galeata×hyalina grazing and of infochemicals released by the daphnids on the colony size and growth rate of the colonial gelatinous green alga Sphaerocystis schroeteri (Chlorococcales) was investigated in laboratory batch experiments run for 96 h. High zooplankton grazing pressure was exerted by a final concentration of 100 daphnids L^?1 in the Daphnia treatments. Infochemicals were obtained by filtration (0.2 ?m) of water from D. galeata×hyalina cultures (200 ind. L^?1 exposed for 24 h). This filtrate was added to the S. schroeteri cultures in two concentrations corresponding to 7 and 50 daphnids L^?1, respectively. The growth rate of S. schroeteri was neither affected significantly by direct Daphnia grazing nor by the presence of Daphnia infochemicals, in comparison to the control. However, the portion of inedible S. schroeteri colonies (diameter>50 ?m) increased under direct grazing pressure, whereas the Daphnia infochemicals did not influence the colony size significantly. We conclude that the shift in colony size by direct zooplankton grazing denotes an effective defence mechanism against size selective feeding for colonial gelatinous green algae. This effective defence in combination with unchanged growth rates of the larger colonies (under non-limiting nutrient and light conditions) falsifies the assumption of a trade-off between minimising grazing losses and maximising growth by optimising the colony size.Zooplankton grazing is an important loss factor for such types of phytoplankton which do not develop effective defence mechanisms against grazing. In addition to the bizarre or filamentous shape of cells or colonies and the formation of chemical repellents, an increase in size by the formation of colonies is a common and efficient defence mechanism. The latter mechanism is particularly efficient against size selective filter feeders which feed mainly on particles between 2 and 30 ?m. Higher sinking rates, the decreasing efficiency of nutrient uptake and utilisation of light cause a decline of net growth rates of algal populations with increasing size. Thus, a trade-off for algal size exists between efficient grazing resistance and high growth rates. Gelatinous green algae are a common example of the colonial organisation of phytoplankton. divided these algae into two functional groups: The first comprises the immotile genera Sphaerocystis (synonym Pseudosphaerocystis), Coenochloris and Oocystis, which occur mostly during the early summer period in mesotrophic lakes. The second group contains the genera Eudorina, Pandorina and Volvox, which are motile species and also occur during the summer period but in eutrophic lakes. The functional difference between these two groups results from the motility which allows species from the second group to regulate their position in relation to the light field. Species of the first group react sensitively to phosphorus and nitrogen enrichment because of their lower competitive ability for light and carbon. Gelatinous green algae have been reported to benefit from high zooplankton biomass overall and, because of zooplankton induced nutrient recycling, at unsaturated nutrient levels in particular. The functional role of the gelatinous sheath has been discussed and is controversial. The secretion of a gelatinous sheath has long been postulated as a mechanism for reducing sinking rates. Furthermore, the increase in size using gelatine can prevent ingestion by filter-feeding zooplankton with concomitant improved intercellular diffusion in comparison to non-gelatinous colonial forms. Otherwise, the production of gelatine is energy-consuming and also leads to an increase of sinking velocity with increasing size (Stoke’s law). reported the viable gut passage of gelatinous green algae in Daphnia with an additional nutrient uptake and enhanced algal growth after the passage. She concluded that the gelatinous sheath constitutes an effective protection against digestion and zooplankton grazing enhances the growth of gelatinous green algae. In contrast, found neither significant increase in mortality nor decrease in the number of moults and clutches of Daphnia feeding on monocultures of Sphaerocystis or Pandorina. However, the number of offspring per clutch was significantly reduced. In comparison to Diaptomus, Daphnia utilised the gelatinous green algae more effectively in study.

A grazer-mediated unicell-to-colony transformation and formation of spines, connected with an improved protection against grazing, has been reported in some strains of the non-gelatinous green alga Scenedesmus. After exposure to an infochemical released by Daphnia the formerly unicellular algae formed colonies. Grazer-induced colony formation has also been reported in the green alga Chlorella vulgaris. As a result of this phenotypic plasticity, these algae have a fundamental advantage in the phytoplankton community due to their high competitive ability in the presence as well as in the absence of grazers.

In the present work we investigate the influence of a filter feeder (Daphnia galeata×hyalina) on the growth rate and colony size of an immotile gelatinous green alga (Sphaerocystis schroeteri). The following three hypotheses derived from the literature cited above were tested: (i) the presence of Daphnia increases the colony size of S. schroeteri, (ii) the shift in colony size is mediated by an infochemical released by Daphnia, and (iii) the growth rate of S. schroeteri is not significantly reduced in Daphnia-induced large colonies when light and nutrients are available at saturation levels.