Dynamics of toxic cyanobacteria in lakes and artificial water reservoirs

Abstract

Cyanobacteria are photosynthetic prokaryotes occurring worldwide in almost every ecosystem. They are most common in marine and freshwater habitats where they are an integral part of the phytoplankton community, and account for a large share of the global primary production. Under certain environmental conditions, cyanobacteria can form dense agglomerations in the water body, so-called cyanobacterial blooms, which are characterized by sudden growth and high cell densities. Cyanobacterial bloom incidents have been recognized worldwide with a significant increase over the last decades. Scientists agree that anthropogenic eutrophication of global surface waters is the primary cause for the proliferation of cyanobacteria, but also that climatic changes may play a significant role. Cyanobacteria can produce secondary metabolites known as cyanotoxins, which are toxic to humans and wildlife. Cyanobacterial blooms, especially if formed by cyanotoxin-producing species, can have harmful effects on human, wildlife and ecosystem health and can cause tremendous financial costs. The present work deals with several aspects regarding the temporal and spatial dynamics of cyanobacterial blooms in lakes and artificial water reservoirs. The few studies providing quantitative evidence for the historical increase of cyanobacterial blooms in lakes employ single sediment cores as natural bloom archive. The present work verified the sampling procedure of these paleolimnological studies by investigating spatial and historical distribution of bloom proxies in sediments of Lake Rotorua, a small eutrophic lake in New Zealand. The results from bacterial community (16S rRNA metabarcoding) sequencing, cyanotoxin analysis and radionuclides measurements showed that potentially toxic blooms increased since the 1950’s. Furthermore, the single core from a central site captured dominant microbial communities of the whole lake. Nutrient load reduction is the most promising bloom mitigation strategy. While the efficiency of phosphorus (P) load reduction is widely accepted, there is an uncertainty regarding the efficiency of nitrogen (N) reduction. Some cyanobacteria are diazotroph, meaning they can fix atmospheric nitrogen (N2). Therefore, N load reduction is suspected to cause a shift from green algae to diazotrophic cyanobacteria. Competitive growth experiments were performed, using the N2-fixing cyanobacterium Dolichospermum sp. and the green alga Chlorella, under N-limiting and N-saturating condition. Results indicated that Dolichospermum can fully compensate N limitation by N2 fixation. It was also shown that Dolichospermum competitively excluded Chlorella under N-limiting condition. N load reduction should therefore be considered carefully on a case-by-case basis and be combined with simultaneous P load reduction. Cyanotoxin concentrations of blooms can differ tremendously within days or even hours. Understanding these dynamics and their causes is of great importance for the cyanotoxin risk assessment of recreational and drinking waters. This study monitored cyanotoxin and species dynamics of a natural Dolichospermum bloom and its abiotic and abiotic triggers. Results indicated that concentrations of the toxins anabaenopeptins and microcystins were correlated to varying abundances of different Dolichospermum phylotypes, rather than environmental parameters. Furthermore, selective fungal parasite pressure was identified as a potential factor steering Dolichospermum phenotype and toxin dynamics. In summary, the presented work helps to understand and reverse environmental changes promoting cyanobacterial blooms; it improves future bloom monitoring and risk assessment in bathing waters and drinking water supplies; and it provides an information tool for water managements, which helps to decide upon appropriate bloom mitigation strategies.