Abstract
BackgroundMass occurrences of cyanobacteria frequently cause detrimental effects to the functioning of aquatic ecosystems. Consequently, attempts haven been made to control cyanobacterial blooms through naturally co-occurring herbivores. Control of cyanobacteria through herbivores often appears to be constrained by their low dietary quality, rather than by the possession of toxins, as also non-toxic cyanobacteria are hardly consumed by many herbivores. It was thus hypothesized that the consumption of non-toxic cyanobacteria may be improved when complemented with other high quality prey. We conducted a laboratory experiment in which we fed the herbivorous freshwater gastropod Lymnaea stagnalis single non-toxic cyanobacterial and unialgal diets or a mixed diet to test if diet-mixing may enable these herbivores to control non-toxic cyanobacterial mass abundances.ResultsThe treatments where L. stagnalis were fed non-toxic cyanobacteria and a mixed diet provided a significantly higher shell and soft-body growth rate than the average of all single algal, but not the non-toxic cyanobacterial diets. However, the increase in growth provided by the non-toxic cyanobacteria diets could not be related to typical determinants of dietary quality such as toxicity, nutrient stoichiometry or essential fatty acid content.ConclusionsThese results strongly contradict previous research which describes non-toxic cyanobacteria as a low quality food resource for freshwater herbivores in general. Our findings thus have strong implications to gastropod-cyanobacteria relationships and suggest that freshwater gastropods may be able to control mass occurrences of benthic non-toxic cyanobacteria, frequently observed in eutrophied water bodies worldwide.
Highlights
Mass occurrences of cyanobacteria frequently cause detrimental effects to the functioning of aquatic ecosystems
The somatic growth rate of L. stagnalis was significantly higher in the treatments where L. halophila and the mixed treatment was offered compared to the treatments in which L. stagnalis was provided with diatoms (Fig. 1b)
L. stagnalis feeding upon L. halophila had a significantly lower C:N ratio compared to all other treatments (Fig. 2b), except for A. repens and N. communis
Summary
Mass occurrences of cyanobacteria frequently cause detrimental effects to the functioning of aquatic ecosystems. Control of cyanobacteria through herbivores often appears to be constrained by their low dietary quality, rather than by the possession of toxins, as non-toxic cyanobacteria are hardly consumed by many herbivores. Due to the multiple threats that cyanobacteria may pose to ecosystems, various attempts have been made to control cyanobacterial mass abundances (‘blooms’) through herbivory [13, 14] This requires that the herbivores are able to efficiently utilize cyanobacteria as a food resource. Herbivores may compensate for a low quality diet through compensatory feeding [24,25,26] This is a strategy in which herbivores increase their consumption rate as dietary nutrient concentrations decrease in order to maintain a sufficient uptake of the limiting nutrient(s). When consuming food in higher quantities, the dosage of potential toxins in the diet can increase [32]
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