A number of factors affect algal growth including climatic conditions and nutrient availability. Increasing temperatures in recent years are creating longer growing seasons for algae and contributing to increased algal growth. More frequent high intensity spring storms are resulting in nutrients being washed off of urban and agricultural lands, as well as causing bypasses and overflows from sewage treatment facilities, which also contain nutrients. Spring nutrient loads are an important factor affecting algal growth.
Overall, monitoring has shown that despite significant year-to-year variation in loads, the total annual amount of phosphorus entering the Lake has been relatively stable over the past 15 years. However, there has been a significant increase in the proportion of the phosphorus load to Lake Erie that is in dissolved, as opposed to particulate, form. Dissolved phosphorus is more easily taken up by algae and contributes to increased algal growth. The reason for the increase in the proportion of phosphorus in dissolved form is not fully understood.
Zebra and Quagga mussels, both aquatic invasive species, have increased water clarity and changed nutrient cycling in Lake Erie, resulting in higher levels of algal growth, particularly close to the shore.So why is Lake Erie experiencing more algal growth than the other Great Lakes? The answer is that, as the smallest of the Great Lakes by volume, and the shallowest, Lake Erie waters are the warmest and most biologically productive. Drainage basin characteristics also play a role. All of the Great Lakes experience issues associated with algae. The governments of Canada and the U.S. have committed to the review of phosphorus targets for all Great Lakes through the 2012 GLWQA. The experience garnered in reviewing Lake Erie targets will inform these reviews. Importantly, experts agree that, while many factors contribute to algal growth, controlling phosphorus concentrations and loads is the best way to reduce algal growth in the Great Lakes.