Can tropical grazers control toxic blooms after all?

Posted on September 24, 2012

Blooms of cyanobacteria (blue-green algae) can have harmful effects on water quality and often develop in water bodies where nutrients are in over-supply.  Top-down control of cyanobacteria by grazing zooplankton is thought to be weak, partly because cyanobacterial filaments or colonies tend to be too large to be ingested by most zooplankters.   It’s been suggested that top-down control of cyanobacteria is especially limited in tropical waters, where large species of planktonic crustaceans are relatively scarce.  However, this conclusion has been challenged by recent findings.  In a Senegalese study, local species of cyanobacteria and zooplankton (copepods, cladocerans and rotifers) were mixed in jars to set up a series of 78 grazing experiments.  Other jars containing cyanobacteria but not grazers were used as controls.  Experiments lasted for 24 hours, after which time the concentration of phytoplankton-sized particles in the jars was measured and differences in particle concentration between control and experimental flasks were used to estimate grazing rates.  The cladocerans consumed the smaller cyanobacterial filaments, while the copepods and rotifers took a wider size range of particle sizes.  And “sloppy feeding” by copepods had the effect of cutting and shortening the filaments of the larger cyanobacteria.  In the wild, these shorter filaments would be available to small cladocerans and other grazers.  Feeding rates of the zooplankton were high, with the small cladocerans and rotifers taking the equivalent of 50-700% of their body carbon per day.  Such rates were similar to those recorded in other plankton feeding experiments with non-filamentous algae.  So contrary to expectations, the results indicated that zooplankton in tropical lakes may have the ability to consume and control cyanobacterial populations.  Calculations suggested that the zooplankton community in the Senegal River system could account for  a significant fraction ( 13-49%) of the biomass of focal cyanobacterial species per day.

 Reference:  Ka, S. et al.  2012. Can tropical freshwater zooplankton graze efficiently on cyanobacteria?  Hydrobiologia 679, 119–138.