Cause and Management of Infrequent Widespread Blue Green Algal Bloom in Temperate Waters
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Blue-green algal (BGA) blooms have been reported more often in recent years as a threat to human health and natural resources. In order to understand widespread BGA blooms that occur only occasionally in many smaller water bodies, we used Sodus Bay, which experienced a widespread bloom in 2010, as an example to demonstrate a systematic approach to investigate the foremost factors influencing bloom formation using a state-of-the-art linked hydrodynamic-ecological model and field data. First, the hydrodynamic and ecological models were set up and calibrated to field data from 2013. Then the calibrated models were applied to explain the impact of hydrodynamic factors and to evaluate main nutrient sources. Last, model simulations and field data between 2010 and 2013 were integrated to develop an understanding of the cause of the widespread bloom and to suggest viable management options. As a result, exchange flow between Lake Ontario and Sodus Bay was revealed to be the most important hydrodynamic factor, causing colder water to enter the bay as an underflow during lake upwelling periods. It also determined three mixing zones with different residence times in the bay, and played a critical role in the total phosphorus budget. After ruling out three popular hypotheses as the primary cause of the bloom, we suggest that a set of special bloom-forming weather conditions (i.e., five days of low solar radiation followed by days of calm wind) was the trigger of the widespread 2010 bloom in Sodus Bay. To protect the public from harmful BGA blooms, continuous monitoring is essential and management strategies need to set at least two lines of defense - prevention and mitigation, with prevention strategy aiming at nutrient reduction and water temperature decrease, and mitigation strategy aiming at bloom prediction and public education. At the end, the customized hydrodynamic-ecological model opens up wide opportunities for future hypothesis development and what-if scenario testing, and this study can be adopted to support development of a well informed management plan for any water body in a temperate climate that experiences infrequent widespread BGA blooms.