It was a normal weekday in sunny Los Angeles, about 3:31 pm, sometime during the month of April, 2018. Dave had left the lab, and was now driving to his home in Topanga, CA. As he reached coastal Pacific Coast Highway (PCH) he became aware that something was different in the air. But not just something in the air. It was the air itself. The wind along the coast was unusually high, causing the palm trees to bend and drop leaves. The signal was clear: A Spring bloom is coming. And if we were going to catch it, we had better start sampling now.
April wind speed in Santa Monica Bay in 2018 (A) and 2019 (B). Vertical dashed lines indicate the start of sampling during each year.
For the next 15 days my team and I would collect daily samples of seawater from the Southern California coast—off the Santa Monica Pier (SMP) to be exact. By accomplishing our immediate goal—to record the physical, chemical, and biological cascade associated with the algal bloom we were about to watch rise and fall—we aimed to integrate these data to improve our ability to predict when blooms will occur and which species will dominate. During this year, we recorded the events leading up to a diatom bloom and its eventual collapse. We repeated the process the following year to record another spring bloom for comparison. Poised to capture another diatom spring bloom for comparison; we were surprised that what we got was not the typical diatom spring bloom. It was instead a massive and prolonged dinoflagellate bloom that required us to extend our sampling period to 22 days (a week longer than the previous year), an unusual phenomena during the cold nutrient-laden waters of the spring.
With these datasets, I revealed notable environmental factors and species interactions that may have produced these contrasting bloom events:
Changes in the relative proportion of major protistan taxonomic groupings during the sampling periods in 2018 (A) and 2019 (B).
First, there is a core community of taxa that are present every year in the spring from which the dominant species is selected. I think of this in terms of any team sport like baseball. The same teams compete every year, and from those teams one makes it to the top after the World Series.
Second, there were subtle differences in nutrient availability. Stronger wind during the spring of 2018 produced deeper nutrient upwelling and a more turbid water column, and turbidity is what keeps sinking diatoms in the light.
Upwelling was weaker during the spring of 2019, a year that was less windy in general. So less nutrients and less physical mixing to keep diatoms from sinking. But also network analysis and microscopy revealed parasites attacking a dominant diatom species (Guinardia) during this year but not 2018 that may have further impeded diatom success and facilitated the dinoflagellate bloom by making space or by producing “old nutrients”.
Lysis of diatoms by their cercozoan parasites may have released organic material useful for dinoflagellates to bloom without further upwelling. Accordingly, the dinoflagellates that bloomed (Akashiwo and Margalefidinium) are known mixotrophic taxa that are able to use organic forms of nitrogen and phosphorus.
Images taken of the dominant protistan taxa from the 2018 and 2019 blooms via light microscopy. Black scale bar = 20 µm. A) Cryothecomonas sp. (Cercozoa) infecting Guinardia sp., B) Pseudo-nitzschia sp., C) Rhizosolenia sp., D) Akashiwo sp., E) Thalassiosira eccentrica, F) Margalefidinium fulvenscens
More work is needed to examine the physiology of the dominant species to determine what they were consuming and why they died; however this part 1 of this study—examination of community dynamics (18S sequencing and microscopy) in conjunction with physical and chemical measurements—was accepted for publication in 2022 (Environmental Microbiology): Environ Microbiol. 2022 Dec; 24(12): 6033–6051.