“Blue-green algae is/are present in the lake”?
The National Geographic Style Manual at http://stylemanual.ngs.org/home/A/alga-algae has (selected extracts):
Alga is the singular noun and requires a singular verb
Algae is the plural form and requires a plural verb: When the algae die, they fall to the bottom.
Occasionally algae may be considered a collective [mass] noun and treated as singular: A map shows how algae periodically blooms in the bay. Look! There it has drifted under the bridges, and out into the ocean, where it hovers over the reef.
However, there is an inconsistency in their approach; the example they give for the use of the singular noun is:
A colony of Licmophora fans itself across another alga, Bonnemaisonia. This should surely be another mass-noun usage, but they're using the singular, alga. Their example should read: A colony of Licmophora fans itself across a colony of another alga, Bonnemaisonia.
The trouble here is that alga is rarely considered as a single particle - more often as a single strain. One virion or one bacterium are reasonably commonly considered.
Algae are is correct.
Per Dorland's Illustrated Medical Dictionary, 31st Edition (bold mine):
alga — any individual organism of the algae
algae — a large group included in the kingdom Protista, now classifed into several differnt phyla. They are cryptogamous plantlike organisms in which the body is unicellular or consists of a thallus. Algae include the seaweeds and many unicellular fresh-water plants.
I would say in your two links, the more scholarly sites and publications use plural verbs with algae. The public, when asking questions about things like blue-green algae in a backyard pool, seem more likely to use a singular verb.
While algae is plural in form, in English, it is usually not a count noun: for example, we can say one cow or two cows. We don't usually say one alga or two algae: unless you're talking about different types of algae. So, if you're saying something like "the algae covers everything", the singular would be OK. But, if you're saying, "algae are divided into five species...." then it should be plural.