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

Abstract

An unprecedented large-scale outbreak of harmful algae, including Karenia selliformis and Karenia mikimotoi, 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 Karenia spp. abundance, we conducted extensive ship-based surveys across several water masses during 4–14 October, 2021 and analyzed in-situ data in combination with Sentinel-3-derived ocean color imagery with a horizontal resolution of 300 m. High chlorophyll-a concentrations (exceeding 10 mg m–3) were identified mainly in coastal shelf–slope waters of <1,000-m water depth occupied by Surface Coastal Oyashio Water or Modified Soya Warm Current Water. Karenia spp. abundance was strongly correlated with chlorophyll-a concentration, which typically had a shallow vertical maximum within the surface mixed layer. Large- and small-scale distributions of Karenia spp. abundance at the ocean surface were estimated from two satellite-imagery products: maximum line height and red-band difference. Maps generated of Karenia spp. abundance revealed snapshots of dynamic Karenia bloom distributions. Specifically, the cores of Karenia blooms were located on continental shelves, sometimes locally exceeded 104 cells mL–1, and seemed to be connected intermittently to very nearshore waters. Relatively high-abundance areas (>103 cells mL–1) of Karenia 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, Karenia-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 Karenia spp. abundance will provide basic estimates needed to understand the processes and mechanisms by which algal blooms can inflict damage on regional fisheries.

Controlling factors of large-scale harmful algal blooms with Karenia selliformis after record-breaking marine heatwaves

“…It is noteworthy that this density was close to that of Karenia spp. (~277000 cells L −1 ) at a depth of 10-m off southeast Hokkaido estimated based on a ship survey in October 2021 (Kuroda et al, 2022). The magnitude of the simulated PK blooms was small compared with the spring bloom of PL, despite previous studies having reported that SCCs in the area where the 2021 HABs occurred exceeded 10 mg m −3 and were comparable with the chlorophyll a concentrations at Stn.…”

“…The magnitude of the simulated PK blooms was small compared with the spring bloom of PL, despite previous studies having reported that SCCs in the area where the 2021 HABs occurred exceeded 10 mg m −3 and were comparable with the chlorophyll a concentrations at Stn. A1 during April (8-15 μg L −1 , see Supplementary Figure 4 for detail) Kuroda et al, 2022). Our model thus underestimated the blooms of K. selliformis in 2021.…”

“…The simulated result that the highest PK concentration occurred in the subsurface layer prior to the main outbreak was consistent with the chlorophyll fluorescence intensity off southeast Hokkaido at the beginning of September 2021 (Misaka and Ando, 2021) and the pattern of K. mikimotoi HABs (Aoki et al, 2017). Kuroda et al (2022) have reported that the SCCs in the area where the 2021 HABs occurred increased greatly from mid-September, and the SCCs remained high until mid-October. The high chlorophyll a concentrations in surface water sampled in a harbor located in southeast Hokkaido during mid-September to mid-November (Hasegawa et al, under review) [1]).…”

“…Horizontal advection as well as vertical processes associated with submesoscale variability were neglected in the one-dimensional simplification. In fact, however, the spatial distribution of K. selliformis abundance is characterized by patch-and streak-like inhomogeneous, submesoscale structures on the shelves (Kuroda et al, 2022). If such submesoscale processes contributed more to the blooms of K. selliformis in 2021 than to the spring blooms of microalgae other than K. selliformis, a one-dimensional simplification may have underestimated the blooms of K. selliformis in 2021.…”

“…This area included some very nearshore areas, where river discharges caused an influx of nutrients and very low-salinity waters. Analyses of SCCs have suggested that very low-salinity waters from rivers (typically, <32.0) would link to high SCCs in very nearshore areas, whereas high-salinity (>33.6), subtropical waters would suppress the occurrence of HABs Kuroda et al, 2022). Thorough understanding of the HAB phenomena would require that the spatial differences of salinity be simulated by a threedimensional model.…”