This is an excerpt of the June 14, 2021 edition of What's Up With Water. Microscopic phytoplankton, which are the foundation of the marine food chain, are some of the world’s most abundant and ancient organisms. Though essential to ocean life, they produce plenty of drawbacks, too. When they cluster along the coast, certain species paint the nearshore waters in a palette of fiery reds and mossy greens. Other toxin-producing species cause beach closures, kill fish, and lead to restrictions on harvesting clams and oysters. Anecdotal evidence suggests that these marine harmful algal blooms are on the rise. But are they really increasing globally? According to a first-ever assessment, the answer is no.