Effects of experimentally induced cyanobacterial blooms on crustacean zooplankton communities

1. Large in situ enclosures were used to study the effects of experimentally induced cyanobacterial blooms on zooplankton communities. A combination of N and P was added to shallow (2 m) and deep enclosures (5 m) with the goal of reducing the TN : TP ratio to a low level (similar to5 : 1) to promote cyanobacterial growth. After nutrient additions, high biomass of cyanobacteria developed rapidly in shallow enclosures reaching levels only observed during bloom events in eutrophic lakes. 2. In the shallow enclosures, particulate phosphorus (PP) was on average 35% higher in comparison with deep enclosures, suggesting that depth plays a key role in P uptake by algae. Phytoplankton communities in both deep and shallow enclosures were dominated by three cyanobacteria species -Aphanizomenon flos-aquae , Anabaena flos-aquae and Microcystis aeruginosa - which accounted for up to 70% of total phytoplankton biomass. However, the absolute biomass of the three species was much higher in shallow enclosures, especially Aphanizomenon flos-aquae . The three cyanobacteria species responded in contrasting ways to nutrient manipulation because of their different physiology. 3. Standardised concentrations of the hepatotoxic microcystin-LR increased as a result of nutrient manipulations by a factor of four in the treated enclosures. Increased biomass of inedible and toxin producing cyanobacteria was associated with a decline in Daphnia pulicaria biomass caused by a reduction in the number of individuals with a body length of >1 mm. Zooplankton biomass did not decline at moderate cyanobacteria biomass, but when cyanobacteria reached high biomass large cladocerans were reduced. 4. Our results demonstrate that zooplankton communities can be negatively affected by cyanobacterial blooms and therefore the potential to use herbivory to reduce algal blooms in such eutrophic lakes appears limited.