Trend analysis of time series variations in the marine environments and size-fractionated chlorophyll a in the coastal areas of eastern Hokkaido, southwestern Okhotsk Sea

Abstract

The coastal areas of eastern Hokkaido, located in the southwestern part of the Okhotsk Sea, are unique areas where the Soya Warm Current Water (SWCW) and the East Sakhalin Current Water (cold water) alternate seasonally. They are important sea areas for Japanese fishery production. Observational results off Cape Notoro, eastern Hokkaido, over the last 10 years (2011−2022), showed that the influence of the Tsushima Warm Current Water (TWCW), the origin of the SWCW, has become stronger. The TWCW is dominated by small phytoplankton. An increase in their impact will have a similar effect on the ecosystem in the SWCW, where large phytoplankton dominate, as the shift to small phytoplankton occurs owing to global warming. Although the change in small phytoplankton observed in the current study was not statistically significant, the standing stock of small phytoplankton has been high over the past few years. An increase in the influence of the TWCW and a shift to small phytoplankton were observed in Saroma-ko Lagoon (2009–2022), the largest brackish-water lake in eastern Hokkaido. Data from the past 35 years (1987–2021) indicated a significant increase in the surface water temperature. Furthermore, off Shibetsu in the Nemuro Strait on the east side of the Okhotsk Sea across the Shiretoko Peninsula (2009–2022), the influence of the TWCW strengthened and indications of a shift to small phytoplankton were found. This study showed that in the phytoplankton community, the significance of small phytoplankton as primary producers has risen in response to environmental changes such as global warming and the growing inflow of the TWCW, in the coastal areas of eastern Hokkaido in the Okhotsk Sea. There have been concerns regarding future changes in the ecosystems and fishery production. Therefore, it is essential to continue monitoring studies for sustainable fishery production.