A massive Lingulodinium bloom erupted on the Southern California coast, early April 2020. The so-called ‘red tide’ extended from at least the coast of Los Angeles (Santa Monica) to Baja, Mexico. Reports that this bloom was bioluminescent —a result of a chemical reaction within Lingulodinium cells that emits blue light when the cells are agitated— circulated through the now very bored community like a deadly virus. In midst of the darkness of the COVID-19 pandemic, people flocked to the beach at night to catch a glimpse of the bright blue bioluminescent waves despite the recent stay home mandate and beach closures. The bloom lasted for several weeks and disappeared as quick as toilet paper at the beginning of the pandemic.
The red tide has been well documented by both amateur and professional plankton enthusiasts. Photos and video of Lingulodinium under the microscope, the red seawater, bioluminescence, satellite imagery illustrating the sheer magnitude of the bloom along the California coast… —the works. Even photographs of the marine life left dead in the wake of the bloom can easily be found on the web. A really well written, detailed summary has been provided by Clarissa Anderson and Megan Hepner-Medina of the Southern California Coastal Observing System HERE.
The only thing missing from the world wide web and most summaries is the reason for the bloom’s demise. I present a scintilla of evidence that says it was (at least in part) Noctiluca, an algal grazer.
Marine algae live in a state of chronic starvation; there is rarely too much ‘food’ (nutrients). Algal blooms —the sudden proliferation of algae in an area— are typically an opportunistic response to a sudden increase in resources like nutrients (N, P, Fe, B12, etc.) or sunlight, albeit a sudden decrease in predators may have the same effect. Relatively high concentrations of nutrients are ‘upwelled’ into the sun-lit depths where the alga live every spring on the California Coast. During this time, a few of the impoverished algal species thrive and proliferate as a response. The battle these few successful species won against the thousands of others is a topic for another discussion. Anyway… all good things come to an end, and innocent algae are eaten every day by a protistan grazer. But you don’t hear about that on the news.
Unicellular ‘grazers’ of algae are important consumers of phytoplankton in marine ecosystems. Not only do they check algal populations, but they are also a necessary link in the food chain that carries energy and organic material up to higher trophic levels like fish. Grazers also live in a state of near starvation, limited by the availability of resources —food in their case. And like algae, they will respond to increases in the availability of resources in the form of prey. Algal blooms represent a surge in prey abundance.
Algae consumed by grazers can be seen under the microscope as a chlorophyll a containing ball inside of a protistan predators.
Artist rendition of a heterotrophic protist (coleps) filled with and digesting grazed algal prey.
I went out to collect samples off the California coast in the San Pedro Bay as part of the San Pedro Ocean Time Series, which was around the time of the bloom’s demise. To examine the nano and micro planktonic protistan species associated with the bloom and its demise I collected a sample of the nano plankton (>20µm) using a plankton net.
Examining the contents of the plankton net at 200-400X magnification (inverted microscope) I saw that there was still an abundance of Lingulodinium, however they weren’t free. They were mostly trapped inside of the bellies of Noctiluca, a species of Lingulodinium predator.
Noctiluca collected from the San Pedro Bay, June 2020. Nodules the shape of Lingulodinium can be seen inside of Noctiluca. A free-living Lingulodinium cell can also be seen in bottom right corner of the rightmost photograph.
Without higher resolution temporal data (Daily sampling) on the community and physicochemical dynamics, it’s not possible to say Noctiluca destroyed the bloom. However, I speculate they had a major role in influencing Lingulodinium biomass and the local community structure as a result.
Regardless of the direct cause, the sudden appearance of a protistan grazer pigging out on large quantities a previously blooming alga is a cogent example of the role of unicellular heterotrophs in the oceanic food web.