Understanding transport and mixing processes to mitigate detrimental effects of algal blooms in a large shallow estuary

High biomass algal blooms tend to occur when transport and mixing time scales exceed phytoplankton growth time scale, resulting in many detrimental effects to the ecosystem and possibly to humans. In order to implement appropriate management strategies to overcome the nuisance of such blooms, it is essential to study the hydrodynamic mechanisms influencing horizontal transport and mixing rates that in turn determine locations conducive to excessive phytoplankton growth. An integrated approach consisting of a theoretical method, a field experiment, historical data and three-dimensional hydrodynamic and ecological modeling was used to understand the physical processes affecting high biomass blooms of the diatom Aulacoseira sp. in the shallow upper Río de la Plata estuary (Argentina-Uruguay). Two high biomass blooms events impacted the drinking water treatment plants of the city of Buenos Aires (Argentina) causing a disruption to normal operation of the plants and risked the drinking water supply to more than 10 million people. Firstly, it was shown that the shallow nature of the estuary tends to direct the flow towards the deeper shipping channels, creating stagnation zones near the Argentinean boundary. Energy levels available for mixing were higher near the Uruguayan boundary and center of the estuary than in the stagnant region along the Argentinean margin. High discharge regimes were shown to also intensify mixing, as opposed to periods of low inflow rates. Secondly, it was demonstrated that these stagnation zones, combined with the transport of algae cells from contributing inflows, and low concentrations of suspended solids in the water column were the main drivers for the occurrence of two high biomass bloom events. Finally, three mitigation strategies were explored to decrease the effects of high biomass algae blooms on the drinking water intakes. An automated real-time and forecast management system was successfully implemented in the Río de la Plata estuary to monitor and predict the occurrence of Aulacoseira sp. near the drinking water intakes. Two engineering solutions were proposed and the results demonstrated that extending the drinking water intakes further offshore or placing groynes near the drinking water intakes would decrease the concentration of Aulacoseira sp. by up to 50 % and 40 % respectively.