Estimation of production and sedimentation of cyanobacterial toxins (microcystin) based on nutrient budgets in the reservoir of Isahaya Bay, Japan

The increasing eutrophication of freshwater and brackish habitats globally has led to a corresponding increase in the occurrence of harmful cyanobacterial blooms. Cyanobacteria can produce highly toxic substances such as microcystins (MCs) that affect the health of livestock, wildlife, and humans. The present study broaden the understanding of cyanobacteria ecology and MC dynamics in the field, focusing on the estimation of the production and sedimentation rates of MCs in a natural habitat. The nutrient concentrations of the reservoir water and sediment pore water were monitored at 3-h intervals for 24 h during the summer cyanobacterial bloom. The DIN uptake rate of Microcystis in the Isahaya reservoir was estimated and the large-scale blooms in the reservoir were largely controlled by the interactions between rainfall and nutrient levels in the warm season. By using calculations based on the nitrogen budgets and tracking changes of the MC concentrations in the water column, the total MC production and sedimentation rates were estimated to be 52.2 kg MCs d−1 and 21.5 kg MCs d−1, respectively. Although MCs could be degraded in the environment, the MC sedimentation still comprised 41% of the in-water production. •Survey for nutrient budgets and MCs determination were conducted in a reservoir.•Uptake rate of DIN by Microcystis and the production rate of the MCs were estimated.•Microcystis blooms could be mainly controlled by temporal rainfall in worm season.•Sedimentation rate of the MCs on the bottom sediment was calculated.•Contribution of the sedimentation to the production in the water was high (41%).