Abstract
Unprecedented large-scale harmful algae blooms (HABs) were reported in coastal waters off the south-eastern coast of Hokkaido, Japan, in mid-to-late September 2021, about a month after very intense and extensive marine heatwaves subsided. To understand the physical–biological processes associated with development of the HABs, we conducted analyses via a combination of realistic ocean circulation models, particle-tracking simulations, and satellite measurements. The satellite-derived chlorophyll concentrations (SCCs) and areal extent of the high SCCs associated with the HABs were the highest recorded since 1998. More specifically, the extent of SCCs exceeding 5 or 10 mg m−3 started to slowly increase after 20 August, when the marine heatwaves subsided, intermittently exceeded the climatological daily maximum after late August, and reached record-breaking extremes in mid-to-late September. About 70% of the SCCs that exceeded 10 mg m−3 occurred in places where water depths were <300 m, i.e., coastal shelf waters. The high SCCs were also tightly linked with low-salinity water (e.g., subarctic Oyashio and river-influenced waters). High-salinity subtropical water (e.g., Soya Warm Current water) appeared to suppress the occurrence of HABs. The expansion of the area of high SCCs seemed to be synchronized with the deepening of surface mixed layer depths in subarctic waters on the Pacific shelves. That deepening began around 10 August, when the marine heatwaves weakened abruptly. However, another mechanism was needed to explain the intensification of the SCCs in very nearshore waters off southeast Hokkaido. Particle-tracking simulations based on ocean circulation models identified three potential source areas of the HABs: the Pacific Ocean east of the Kamchatka Peninsula, the Sea of Japan, and the Sea of Okhotsk east of the Sakhalin Island. Different processes of HAB development were proposed because distance, time, and probability for transport of harmful algae from the potential source areas to the study region differed greatly between the three source areas.
Highlights
IntroductionHarmful algal blooms (HABs) constitute a critical worldwide problem and have devastating impacts on marine ecosystems, aquaculture, fisheries, and local economies, as well as on human health and wellbeing [1,2,3,4,5,6]
satellite-derived chlorophyll concentrations (SCCs) between the western and eastern side, respectively, of the sharp salinity front salinity front south of Cape Erimo. These results suggested that the high SCCs associated south of Cape Erimo. These results suggested that the high SCCs associated with harmful algae blooms (HABs) with HABs were less intense in subtropical, high-salinity waters
How can we most efficiently develop an understanding of the impact of extreme events on marine ecosystems and relate those impacts to environmental conditions? One of the most robust methodologies is analysis by combining near-real-time ocean monitoring with realistic ocean modelling
Summary
Harmful algal blooms (HABs) constitute a critical worldwide problem and have devastating impacts on marine ecosystems, aquaculture, fisheries, and local economies, as well as on human health and wellbeing [1,2,3,4,5,6]. HABs are natural phenomena, but they can be stimulated by the impacts of anthropogenic activities on coastal ecosystems, primarily nutrient over-enrichment, e.g., [4,7]. Local oceanographic conditions tightly linked with basin-scale climate changes have sometimes triggered HABs [8]. Examples include the extreme blooms of Pseudochattonella cf verruculosa that occurred in the coastal waters of western Patagonia during the summer of 2016 due to the strong El Niño and the positive phase of the Southern Annular Mode [9], and massive Pseudo-nitzschia blooms
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