Most algae are capable of forming visible 'blooms' under the proper growing conditions. This page includes the Great Lakes species which commonly form floating mats. Mat or scum? - run your fingers through it (wear gloves if toxins suspected) if it coats like paint it's a scum, if it lifts basically in one piece (like hair) it's a mat.
Naturally occuring organic matter in the water can sometimes produce a foam; this is not an algae bloom, though it may be associated with a dying bloom. Pollen, bacteria, calcium carbonate precipitate, and oil can produce slicks on the water surface which look similar to an algae scum.
a link to a non-GLERL NOAA website.
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