Persistent blooms of filamentous cyanobacteria in a cormorant-affected aquatic ecosystem: Ecological indicators and consequences

Abstract

Globally, the establishment of protected reserves contributes positively to species protection but also presents a real threat of unexpected changes. A few reports showed that progressively increasing the number of roosting and breeding cormorants across Europe was associated with food-rich water bodies. Our studies concerned detailed and novel research on annual cyanobacterial growth against other ecological factors, that is, biotic (ciliates, rotifers, cladocerans, and copepods) and abiotic (temperature, nutrients) factors, in a lake where the piscivorous great cormorants evidently affects the water quality. Mass development of cyanobacteria lasted throughout the whole of the ice-free months (March–December). However, in the under-ice period they also formed a relatively high biomass, suggesting an annual persistence. The potentially toxic filamentous Pseudanabaena, Aphanizomenon, Planktothrix, and Limnothrix were constant dominants in the cyanobacteria-dominated phytoplankton almost year round. Water temperature played a major role in stimulating the growth of filamentous cyanobacteria. However, nitrogen, phosphorus, and even some zooplankters were significantly related to cyanobacteria. A very low trophic efficiency suggested a reduced energy flow at a lower food-web level starting from dominant cyanobacteria. Our results also suggest that numerous breeding colonies of great cormorant contributed to bottom-up (nutrients) and top-down (predation on fish) controls and consequently to the formation of a site-specific refuge for filamentous cyanobacteria. This affects the functioning of the water ecosystem by disturbing the energy flow in the classical food webs, which is important in water management and warming-induced global problems with cyanobacterial dominance and water quality.