Influence of Coastal Oyashio water on massive spring diatom blooms in the Oyashio area of the North Pacific Ocean

Abstract

We hypothesized that the characteristics of Coastal Oyashio water in the North Pacific Ocean during winter–spring have a major influence on the generation of massive spring diatom blooms throughout the Oyashio area. To test this hypothesis, we analyzed in situ data from May during 1990–2016 along a transect known as the “A-line” to monitor conditions in the Oyashio off the southeast coast of Hokkaido, Japan. Chlorophyll-a concentrations were high (≥5 mg m−3) on the shelf and in the offshore Oyashio areas around the First and Second Oyashio Intrusions. About 80% of the high chlorophyll-a concentrations in the Oyashio area were linked with low-salinity (≤33.0) water that was associated with the Coastal Oyashio water and its modified waters. For the remaining Oyashio area, the so-called “Oyashio water” with a high salinity of 33.0–33.7 was near the sea surface and associated with low chlorophyll-a concentrations with a frequency distribution mode at 1–2 mg m−3. Diatom communities with high abundance and diversity were also connected (disconnected) between the shelf and the offshore Oyashio areas with low (high) salinity and high (low) chlorophyll-a concentrations. Analysis of historical temperature–salinity data near the sea surface revealed that the Coastal Oyashio water and its modified waters appear most frequently around the shelf along Hokkaido and the Kuril Islands, and that the frequency of appearance decreases with increasing distance offshore. We suggest that the transport and pathway around the islands of the Coastal Oyashio water and its modified waters, which contain high nutrient concentrations and the seeds for massive spring blooms, are the primary factors regulating the generation and spatial extent of the massive spring blooms over the Oyashio area.