Abstract
The aim of our study was to assess whether cyanotoxins (microcystins) can affect the composition of the zooplankton community, leading to domination of microzooplankton forms (protozoans and rotifers). Temporal variations in concentrations of microcystins and zooplankton biomass were analyzed in three eutrophic reservoirs in the semi-arid northeast region of Brazil. The concentration of microcystins in water proved to be correlated with the cyanobacterial biovolume, indicating the contributions from colonial forms such as Microcystis in the production of cyanotoxins. At the community level, the total biomass of zooplankton was not correlated with the concentration of microcystin (r2 = 0.00; P > 0.001), but in a population-level analysis, the biomass of rotifers and cladocerans showed a weak positive correlation. Cyclopoid copepods, which are considered to be relatively inefficient in ingesting cyanobacteria, were negatively correlated (r2 = - 0.01; P > 0.01) with the concentration of cyanotoxins. Surprisingly, the biomass of calanoid copepods was positively correlated with the microcystin concentration (r2 = 0.44; P > 0.001). The results indicate that allelopathic control mechanisms (negative effects of microcystin on zooplankton biomass) do not seem to substantially affect the composition of mesozooplankton, which showed a constant and high biomass compared to the microzooplankton (rotifers). These results may be important to better understand the trophic interactions between zooplankton and cyanobacteria and the potential effects of allelopathic compounds on zooplankton.
Highlights
Cyanobacteria are a group of prokaryotic microorganisms that reach high levels of biomass and density when they bloom in eutrophic environments (Paerl and Huisman, 2008)
To investigate if the presence of cyanotoxins has some negative effect on the mesozooplankton biomass, we evaluated the seasonal concentration of microcystins and the zooplankton biomass in three eutrophic reservoirs in the Brazilian semi-arid northeast region
The studies that describe the trophic interactions between cyanobacteria and zooplankton suggest that herbivores such as Daphnia and calanoid copepods are nutritionally disadvantaged when cyanobacterial blooms occurs, as a result of high density of algae cells, size, nutritional quality and toxin production (Porter and Orcutt, 1980; Lürling, 2003; Sarnelle et al, 2010; O’Farrell et al, 2012; Chislock et al, 2013)
Summary
Cyanobacteria are a group of prokaryotic microorganisms that reach high levels of biomass and density when they bloom in eutrophic environments (Paerl and Huisman, 2008). One of the reasons suggested to explain this dominance is that morphological and chemical defenses can negatively affect zooplanktonic herbivores (Carmichael, 1992; Chan et al, 2004; Camacho and Thacker, 2006). One of the most common cyanotoxins found in eutrophic systems is microcystin, a hepatotoxin produced by certain genera of cyanobacteria such as Microcystis, Anabaena, Oscillatoria and Nostoc. The majority of studies on the effects of cyanobacteria on the structure of the zooplankton community have yielded conflicting results, but in general, they suggest that in systems dominated by cyanobacteria, the zooplankton community is dominated by small cladocerans and rotifers, in response to the toxic effects of the cyanotoxins, difficulties in ingesting cells, and nutritional deficiency of the food source (Ferrão‐Filho et al, 2000; Hansson et al, 2007)
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