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Basin-scale distribution of salps and doliolids in the transition region of the North Pacific Ocean in summer: Drivers of bloom occurrence and effect on the pelagic ecosystem

2022; Elsevier BV; Volume: 204; Linguagem: Inglês

10.1016/j.pocean.2022.102793

1873-4472

Nurul Huda Ahmad Ishak, Kentaro Motoki, Hiroomi Miyamoto, Taiki Fuji, Yukiko Taniuchi, Shigeho Kakehi, Hiroshi Kuroda, Takashi Setou, Kazutaka Takahashi,

Isotope Analysis in Ecology

• The large-scale distribution of thaliacean blooms in the North Pacific transition region was investigated in early summer. • The significant drivers on bloom formation were salinity, pico-sized chlorophyll a , phosphate, silicate, and seawater density. • The most dominant species, Thalia democratica thrived in the area south of the subarctic boundary with the influence of the HNLC water mass from the subarctic region. • Doliolids generally occur in area where regeneration production prevails, indicating that they are important components of the microbial food web. The large-scale distribution of thaliaceans in relation to environmental variables was investigated over 4500 km in the transition region of the North Pacific (36°N–43.6° N, 141.5° E–164.5° W) in the summer of 2016–2019. Three main patterns occurred in the thaliacean bloom formations with specific and annual variabilities. Thalia democratica , the most dominant thaliacean species in this region, formed an intensive bloom (>100 ind m −3 ) of up to 300 km, mainly south of the subarctic boundary between 160° E and 180°. The bloom was accompanied by a dense water mass which was associated with high nutrient concentration and N/P ratio, indicating that this species thrived under the influence of the HNLC water mass from the subarctic region. A Dolioletta gegenbauri bloom was observed east of 180° and the north of the subarctic boundary in association with higher chlorophyll a levels. Doliolum denticulatum and Doliolum nationalis increased with sea surface temperature and pico-sized chlorophyll a , suggesting adaptation to oligotrophic subtropical waters. Estimated grazing and downward flux by Thalia democratica bloom corresponded to 63–80% and 20–26% of daily primary production, respectively, indicating that the bloom enhanced the biological pump in the HNLC-like environment dominated by small phytoplankton. Doliolids function as important components of the microbial food web through the consumption of varied organic matter. Thaliaceans utilise the specific niche which suspension-feeding copepods find difficult to colonise, indicating that bottom-up control of oceanographic conditions determines the mesozooplankton community and food-web structure in the transition region.