AUTHOR=Kuroda Hiroshi , Taniuchi Yukiko , Watanabe Tsuyoshi , Azumaya Tomonori , Hasegawa Natsuki 

TITLE=Distribution of Harmful Algae (Karenia spp.) in October 2021 Off Southeast Hokkaido, Japan

JOURNAL=Frontiers in Marine Science

VOLUME=9

YEAR=2022

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2022.841364

DOI=10.3389/fmars.2022.841364

ISSN=2296-7745

ABSTRACT=<p>An unprecedented large-scale outbreak of harmful algae, including <italic>Karenia selliformis</italic> and <italic>Karenia mikimotoi</italic>, was reported in mid-September 2021 in the northwest Pacific Ocean off southeastern Hokkaido, Japan. It inflicted catastrophic damage on coastal fisheries in the ensuing months. To understand the spatiotemporal distribution of <italic>Karenia</italic> spp. abundance, we conducted extensive ship-based surveys across several water masses during 4–14 October, 2021 and analyzed <italic>in-situ</italic> data in combination with Sentinel-3-derived ocean color imagery with a horizontal resolution of 300 m. High chlorophyll-<italic>a</italic> concentrations (exceeding 10 mg m<sup>–3</sup>) were identified mainly in coastal shelf–slope waters of &lt;1,000-m water depth occupied by Surface Coastal Oyashio Water or Modified Soya Warm Current Water. <italic>Karenia</italic> spp. abundance was strongly correlated with chlorophyll-<italic>a</italic> concentration, which typically had a shallow vertical maximum within the surface mixed layer. Large- and small-scale distributions of <italic>Karenia</italic> spp. abundance at the ocean surface were estimated from two satellite-imagery products: maximum line height and red-band difference. Maps generated of <italic>Karenia</italic> spp. abundance revealed snapshots of dynamic <italic>Karenia</italic> bloom distributions. Specifically, the cores of <italic>Karenia</italic> blooms were located on continental shelves, sometimes locally exceeded 10<sup>4</sup> cells mL<sup>–1</sup>, and seemed to be connected intermittently to very nearshore waters. Relatively high-abundance areas (&gt;10<sup>3</sup> cells mL<sup>–1</sup>) of <italic>Karenia</italic> spp. on the shelf were characterized by submesoscale (i.e., 1–10 km) patch- or streak-like distributions, or both. Within a roughly 24-h period from 12 to 13 October, <italic>Karenia</italic>-spp. abundances averaged over the shelf abruptly increased more than doubled; these abundance spikes were associated with the combined effects of physical advection and algal growth. The obtained maps and features of <italic>Karenia</italic> spp. abundance will provide basic estimates needed to understand the processes and mechanisms by which algal blooms can inflict damage on regional fisheries.</p>