Why do cladocerans fail to control algal blooms?

Field studies show that even at high nutrient loads phytoplankton may be kept at low levels by filter-feeding zooplankton for a period of weeks (spring clear water phase in lakes) or months (low-stocked fish-ponds). In the absence of planktivorous fish, large-bodied cladocerans effectively control the abundance of algae of a broad size spectrum. Laboratory experiments show that, although difficult to handle and of poor nutritional value, filamentous algae can also be utilized by large-bodiedDaphnia and prevented from population increase, exactly as the principles of the biomanipulation approach would predict. This is not always the case, however. Even when released from predation, large cladocerans often cannot grow and reproduce fast enough to prevent bloom formation. Sometimes, they disappear when the bloom becomes dense, and the biomanipulation approach is not applicable any more. Recent experimental data on four differently-sizedDaphnia species are used in an attempt to (1) explain why cladocerans fail to control filamentous cyanobacteria when filament density is high, and (2) determine the critical filament density at whichDaphnia becomes ineffective. At this critical concentration,Daphnia growth and reproduction is halted, and no positive numerical response to growing phytoplankton standing crop should be expected fromDaphnia population. Bloom formation thus becomes irreversible. The question of what can be done to overcome this bottleneck of the biomanipulation approach may become one of the most challenging questions in plankton ecology in the nearest future.