"What does a layperson HAVE TO know about?"
* During the Jurassic and (especially) the Cretaceous, a major transformation of marine life occurred. Green-algae phytoplankton were displaced by red- and brown-algae (which continue to dominate modern marine ecosystems).
I thought this change from "green oceans" to "red oceans" happened at the P-Tr boundary mass extinction?
BTW, there are no planktonic brown algae (Phaeophyta), they're all multicellular and benthic. However, some of their fellow chromists (eukaryotes with a red alga as an endosymbiont) are planktonic and brown, such as the pelagophytes that make brown tides today.
Not literally brown planktonic chromists include the haptophytes, to which the coccolithophores belong. Chalk only forms in "red oceans"!