Species-specific monitoring of Skeletonema blooms in the coastal waters of Ariake Sound, Japan

Skeletonema is a cosmopolitan diatom observed in coastal waters that forms extensive blooms, largely underpinning coastal ecosystems and fisheries. Recent revisions to the phylogeny of Skeletonema have shown multiple pseudocryptic species within the type species S. costatum. We developed a novel species-specific quantitative PCR (qPCR) method to track the annual dynamics of 7 Skeletonema species, including the pseudocryptic species, in the coastal waters of Ariake Sound, Japan. Fortnightly monitoring revealed that each Skeletonema species had different seasonality and occurrence patterns. The significant findings related to: (1) the species-specific ecological strategies and (2) the ecophysiology of the 5 pseudocryptic species assigned as S. costatum (sensu lato). Three Skeletonema species (S. costatum [sensu stricto], S. menzelii, and S. tropicum) formed blooms in summer, while the copy number of other species increased in winter (S. ardens, S. dohrnii/marinoi, S. grevillei, and S. japonicum). Bloom dynamics also differed among species, with either single intense bloom events or sustained moderate blooms. Summer species bloomed successively under the highly dynamic summer conditions (high freshwater input and anticyclonic tropical typhoons). By contrast, in winter, multi-species blooms were observed under conditions of low sea surface temperature and solar insolation. The single-peak species intensely bloomed during an ephemeral optimal time whereas others sustained their biomass in the pax between the intense blooms. Our in situ species-specific monitoring study connects ex situ physiology and in situ distributions, highlighting the diverse ecophysiology within this cosmopolitan diatom genus.