The global ocean is solar powered. Unicellular eukaryotes (protists) numerically dominate coastal upwelling ecosystems where they form massive spring algal blooms that support the world’s most important fisheries, and constitute an important sink for atmospheric CO2. Spring bloom initiation is well understood; however, how community interactions and physicochemical forces intersect to determine species dominance (among dozens of potentially harmful and benign species) and the resulting ecological impact is not fully resolved. In this study, daily changes in relative abundances of the metabolically active protistan community were followed via 18S V4 rRNA transcripts (RNA, not DNA) throughout two algal blooms during spring 2018 and 2019 in the Santa Monica Bay (central Southern California Bight). A diatom bloom formed after wind-driven, nutrient upwelling events in both years, but different taxa dominated each year. Whereas diatoms bloomed following elevated nutrients and declined after depletion each year, a massive dinoflagellate bloom manifested under low inorganic nitrogen following diatom bloom senescence in 2019 but not 2018. Network analysis combined with light microscopy revealed interactions between diatoms and cercozoan parasites during 2019 that may have influenced the demise of certain diatoms, and the transition to a dinoflagellate-dominated bloom. The full study can be found in Environmental Microbiology.