Cladophora (green algae) and dreissenid mussels over a nutrient loading gradient on the north shore of Lake Ontario

Benthic algae, much of it the green alga Cladophora, blanket the nearshore lakebed of the north shore of Lake Ontario. Nearshore field studies in Toronto, Ajax, Oshawa and Cobourg in 2012 and 2013, and Toronto in 2015 examined the distribution of Cladophora over a nutrient gradient on the north shore of the lake. Concentrations of total phosphorus, dissolved phosphorus and total inorganic nitrogen in the water column decreased from west to east over the gradient corresponding with decreasing watershed development and size. However, high surface cover and nuisance levels of Cladophora biomass were found across the gradient, including the least developed study area where total phosphorus concentrations were similar to those in the open lake. The abundance of Dreissena, high in all areas, was measured concurrently with Cladophora biomass and correlated positively at depths of 6 to 10 m. External loading of phosphorus provides a basis for abatement of Cladophora; however, the influences of enrichment along the shoreline, internal loading at the lakebed and lake trophic status in sustaining growth remain obscure. Nuisance levels of Cladophora are not isolated to locations experiencing elevated external loading and should be viewed in the context of interacting area-specific and lake-wide nutrient supplies.