Cyclical changes of biomass and taxonomic composition of phytoplankton are normally observed in eutrophic lakes of temperate zone. Resistant to grazing by herbivores, colonial and filamentous cyanobacteria and algae become dominant in summer and often form water blooms. Cyanobacteria blooms cause unwanted by humans and unfavorable for most of the aquatic plants and animals changes in abiotic conditions in the reservoir. Changes in phytoplankton community result in changes in species composition and size structure of herbivorous zooplankton. Colonial and filamentous cyanobacteria presence often causes increased abundance of small-bodied cladocerans, rotifers and copepods and decreased numbers of large-bodied cladocerans e.g. of the genus Daphnia. Taxa which can use cyanobacteria as food, and thus are potentially able to limit the growth of cyanobacteria, fall out from the community of herbivorous zooplankton. This decrease in grazing pressure is possible due to series of direct and indirect mechanisms developed during coevolution in the phytoplankton-zooplankton exploitation system (Fig. 1). There are three different means by which cyanobacteria presence affects herbivorous zooplankton. The morphology of cyanobacteria is the first of them. Cyanobacteria forming colonies or long filaments (i) are to large for small-bodied filter feeders so they are not grazed by them and (ii) mechanically interfere with filtration process of large-bodied Daphnia, causing dramatic decrease of effectiveness and/or rate of food collection which, in consequence, leads to reduced growth rate of the animals and decreases their abundance in zooplankton community. Second, cyanobacteria low nutritional value and their indigestibility, limit growth of the animals. Thick cell wall or different gelatinous surrounding enable cyanobacteria to survive the passage through the gut of the animals. Also low assimilation rate of the nutrients from cyanobacteria cell and lack of some essential compounds lead to reduced growth and/or fecundity of the animals. Finally, toxicity of cyanobacterial secondary metabolites reduces the growth of zooplankton and thereby limits grazing pressure. Intracellular toxins are effective protections against selective grazers such as copepods and cladocerans from genus Bosmina. This toxicity is, however, insufficient for nonselective grazers, because it kills the animals only when the cyanobacteria are digested or when the cells are damaged e.g through breaking the filaments during process of cleaning of the filtering apparatus. Extracellular toxin, instead, can kill not only all potential consumers of cyanobacteria, but also their competitors e.g. eukariotic algae. Toxin synthesis can be therefore highly adaptive for cyanobacteria, because it allows to release from grazers and competitors pressure. Vulnerability of different planktonic species to direct effects of cyanobacteria presence is strongly dependent on the mode of feeding (nonselective filtration vs. selective food collection) and the size of the animals. Low food conditions favor the large-bodied cladocerans, e.g. Daphnia, which are the most effective filtrators and require lower threshold food concentrations to sustain positive growth rate than small-bodied species do. However, low food quality, i.e. cyanobacteria presence, causes effects similar to fish predation, because it creates favorable conditions for dominance of copepods and small bodied cladocerans and eliminates large-bodied cladocerans of the genus Daphnia. Cyanobacteria presence can indirectly reduce the growth of herbivorous zooplankton through (i) allelopathic suppression of growth of algae, which are high nutritious food source for zooplankton, (ii) forcing the animals to stay in deeper (colder and poor in food) water layer, (iii) disturb adaptive responses of the animals to predation and (iv) changing the abiotic conditions in the lake. Unlike direct effects of cyanobacteria presence, that concern mostly the large-bodied Daphnia, indirect effects reduce growth of all zooplankton taxa. Cyanobacteria presence can induce changes of behaviour, morphology and life history of the animals which are exposed to them (Table I). Cyanobacteria can cause changes in parameters essential for fitness like: growth rate, age and size at first reproduction, number, size and sex of the offspring, and lifespan. In the lake dominated by cyanobacteria, their presence can be a strong selection factor which favors zooplankton clones less vulnerable to one or all ways of their negative influence.
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