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Protistan algae (phytoplankton) dominate coastal upwelling ecosystems where they form massive blooms that support the world's most important fisheries and constitute an important sink for atmospheric CO 2 . Bloom initiation is well understood, but the biotic and abiotic forces that shape shortterm dynamics in community composition are still poorly characterized. Here, high-frequency (daily) changes in relative abundance dynamics of the metabolically active protistan community were followed via expressed 18S V4 rRNA genes (RNA) throughout two algal blooms during the spring of 2018 and 2019 in 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 relatively low inorganic nitrogen conditions following diatom bloom senescence in 2019 but not 2018. Network analysis revealed associations between diatoms and cercozoan putative parasitic taxa and syndinean parasites during 2019 that may have influenced the demise of the diatoms, and the transition to a dinoflagellatedominated bloom.

Aim To improve our understanding of how parasitism interacts with geographical range expansions by quantifying diversity and abundance of parasites in 25 populations of a large marine snail, Kellet's whelk (Kelletia kelletii), throughout its historical and recently expanded range, which are separated by a well‐known biogeographical boundary. Location California coast (western North America). Methods Parasitological examinations were conducted on 199 whelks from 25 subtidal reefs throughout its expanded and historical ranges. We calculated infection risk, parasite intensity, and parasite species diversity. Abiotic (temperature, latitude, distance from range limit) and biotic (host density) variables were analysed as potential drivers of differential parasitism between expanded‐ and historical‐range populations. Results Compared with historical‐range whelks, expanded‐range whelks were 20% as likely to be infected by parasites, and those that were infected had 6% the number of individual parasites. On average, expanded‐range whelks had 14% the number of parasite species than the historical‐range whelks. The marked decrease in species richness of parasites infecting expanded‐range whelks was only partly explained by the low numbers of parasites. The reduced parasite abundance and diversity in the expanded‐range whelks was not explained by the examined abiotic factors or by whelk density. Main conclusions Expanded‐range populations of Kellet's whelk experience substantially lower parasite abundance and diversity than the historical‐range populations, despite relatively poor demographic performance. The reduced parasitism observed resembles the enemy escape typically characterizing invasive species. A possible explanation for the observed ‘parasite escape’ is that the biogeographical boundary limits the movements or drives the low abundance of other host species (elasmobranchs) required to complete the life cycles of the ‘missing’ parasites. We suggest that parasite escape may generally characterize range‐margin expansions and be important in permitting expansions into what may otherwise be marginal habitats. This parasite escape may directly counter the spread of infectious diseases associated with global warming‐induced range shifts.