Cyanobacteria blooms pose significant challenges to freshwater ecosystems, impacting water quality, biodiversity and community health. Nutrient dynamics in water bodies during cyanobacterial blooms involving nitrogen-fixing species like Dolichospermum is particularly complicated results from nitrogen fixing and akinete release. This research examines the nutrient dynamics during Dolichospermum blooms by laboratory studies, field data analyses, 1-D and 3-D modeling in Woods Lake, a cool temperate, highland lake in Tasmania valued for its recreational and ecological significance.
Laboratory molecular analyses confirm that the bloom-forming species belongs to the genus Dolichospermum and lacks detectable toxin genes. Laboratory experiments demonstrate that this isolate forms heterocysts and can fix atmospheric nitrogen, with growth primarily limited by phosphorus availability and suppressed under low light and cooler temperatures.
Long-term monitoring data between 2022 – 2024 including sensor water level, temperature, oxygen phycocyanin, chlorophyll-a concentrations and regular nutrient, phytoplankton surveys reveals that the first bloom occurred in May 2023, which coincided with an anecdote macrophyte bed collapse. Following the macrophyte bed collapse and growth of Dolichospermum, nitrogen fixation of the species led to the highest value of heterocyst concentrations within the study period. Two months after the bloom, the concentrations of dissolved inorganic nitrogen (DIN) and ammonium (NH4) peaked, followed by the nitrate and nitrite concentration peaks four months after the bloom. In early 2024, another Dolichospermum bloom occurred caused by nitrogen fixation, indicated by the coincidence between high Dolichospernum/phycocyanin concentrations and hyterorcyst concentrations.
To investigate the mechanisms driving bloom formation and to inform management strategies, one-dimensional General Lake Model (GLM) and the three-dimensional TUFLOW model, each coupled with the Aquatic EcoDynamics (AED) library. These models simulate thermal stratification, nutrient cycling, and phytoplankton dynamics, including the nitrogen-fixing function by Dolichospermum. The GLM model simulates lake hydrodynamic accurately and the GLM-AED simulates the Dolichospermum bloom reasonably well, with the two spikes aligning with field data trends.
The findings underscore the critical roles of macrophyte loss and nitrogen fixation as potential triggers of Dolichospermum blooms and highlight the necessity for targeted nutrient management strategies to mitigate bloom development. The current research integrates high-frequency field datasets, laboratory experiments, and coupled hydrodynamic, water-quality modeling to understand Dolichospermum bloom dynamics in temperate lakes, providing potential prediction and controlling methods for water managers and government stakeholders.