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Predictive Modeling of Harmful Algal Blooms in Presque Isle Bay

Blooms of so-called ‘harmful algae’ have occurred in Presque Isle Bay, Erie, PA, and around the Great Lakes, causing concern by environmental, public health, and water supply officials. Due to the threat posed by harmful algal blooms in the Bay, a study funded by the Pennsylvania Sea Grant Program was conducted to determine the factors that may contribute to such blooms. Presque Isle Bay forms the harbor at Erie, PA, on the southern shore of Lake Erie, and supports numerous commercial, ecological, and recreation services. It was proposed to simulate the aquatic ecosystem of the Bay using the EPA’s computer model AQUATOX. The model was developed for the purpose of simulating aquatic systems such as streams, lakes, and reservoirs for the purpose of understanding the threat posed by toxic substances and other pollutants including excessive nutrients.

The field work for the project was conducted over a two year period beginning in late May, 2016. Bay sampling and measurements were conducted every two weeks during the summer of 2016 and 2017. Monitoring stations were established on the two major streams flowing into the Bay in order to estimate the discharge of water and nutrients to the Bay from its watershed. Coincidentally, a data-gathering buoy was deployed in the Bay by the Pennsylvania Department of Environmental Protection during those same summers.

Data, once analyzed and processed into meaningful information, was used as input to the model. Calibration of the model was accomplished using the field data of summer 2016, and verification of the model’s output was accomplished by comparison with data for summer 2017.

It was found that the occurrence of algal blooms was greatly affected by temperature. With the threat of global climate change, it is possible that the summer water temperature will increase. It was predicted by the model that a one degree increase in water temperature would result in a 163% increase in the maximum concentration of Microcystis, the cyanobacterium of greatest concern for toxin production.

The threat posed by increased water temperatures might be mitigated by various control strategies. The PA DEP has documented the status of Presque Isle Bay as “eutrophic”, which means that the water of the Bay is enriched with nutrients. Enrichment of the water contributes to blooms of toxin-producing cyanobacteria and other undesirable water quality problems. Therefore, reduction in nutrients to the Bay would seem a logical objective. The model was used to explore such a strategy, and it was found that the in-Bay phosphorus concentration and the abundance of Microcystis could be reduced by a combination of measures to reduce the dissolved phosphorus and the sediment (detritus) discharged from the watershed, but it would require significant reductions over several years to achieve this.

The model as configured in the course of this project appears to reasonably simulate the phytoplankton community of Presque Isle Bay. Thus, a competent model operator could explore many scenarios in addition to those conducted and reported on here. Data collection on trophic levels above zooplankton and improved estimates of biokinetic values for specific taxa could be used to refine the model and improve its accuracy. Agencies and other groups seeking to sponsor projects in the Bay and its watershed could use the model to evaluate the potential benefits of a given proposal. Local governments might find it helpful to use the model to evaluate projects aimed at improving erosion and pollution control, and thus contribute to improved conditions in the Bay, which is widely agreed to be the defining feature of the greater Erie community.

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