Plankton Community Respiration and Particulate Organic Carbon in the Kuroshio East of Taiwan

Abstract

Biological organic carbon production and consumption play a fundamental role in the understanding of organic carbon cycling in oceans. However, studies on them in the Kuroshio, the western boundary current in the North Pacific Ocean, are scarce. To better understand the variations of plankton community respiration (CR) and particulate organic carbon (POC), eight cruises. which covered four seasons over a 2-year period, were surveyed across the Kuroshio at the KTV1 transect east of Taiwan. Spatially, a coastal uplift of isotherms (i.e., onshore lifting and offshore deepening) was observed along the KTV1 transect. During the uplift, the cold and nutrient-rich deep waters shoal to shallow water and enhance phytoplankton growth, resulting in higher values of phytoplankton, POC, and plankton CR on the onshore side. In this study, phytoplankton was dominated by picophytoplankton including Prochlorococcus, Synechococcus, and picoeukaryotes. Plankton CR was low, and its mean depth-normalized integrated rate (the upper 100 m water depth) ranged from 7.07 to 22.27 mg C m−3 d−1, to which the picophytoplankton and heterotrophic bacteria contributed the most. The mean depth-normalized integrated value of POC ranged from 12.7 to 21.6 μg C L−1. POC is mainly associated with phytoplankton biomass with a mean carbon ratio of chlorophyll a/POC » 1.03. All results suggest that plankton CR and POC variations may be associated with picoplankton dynamics in the Kuroshio.

Seasonal Patterns of Picocyanobacterial Community Structure in the Kuroshio Current

“…The Kuroshio current is an important western boundary current in the Northwest Pacific Ocean. More than half of Chl a in the Kuroshio current is contributed by picoplankton [29]. However, in comparison with other highly oligotrophic oceanic currents (i.e., the Gulf of Mexico), few studies have focused on prokaryotic picoplankton in the Kuroshio current [54][55][56].…”

“…In the subtropical segment of the Kuroshio, its temperature and salinity have no significant seasonal variation and remain at 26 • C to 30 • C and approximately 34.5, respectively [27]. The Kuroshio water in the euphotic zone is characterized as ultraoligotrophic [28,29]. However, when the Kuroshio current flows near the east coast of Taiwan, upwelling along the coastal region transports deep nutrients into the surface [29,30].…”

“…The Kuroshio water in the euphotic zone is characterized as ultraoligotrophic [28,29]. However, when the Kuroshio current flows near the east coast of Taiwan, upwelling along the coastal region transports deep nutrients into the surface [29,30]. The uplift of deep seawater provides a large amount of nutrients to the Kuroshio euphotic zone when the Kuroshio invades the East China Sea shelf [31].…”

“…These nutrient sources support primary productivity in the Kuroshio region [32,33]. The surface Kuroshio is oligotrophic and has low chlorophyll a (Chl a) concentrations of less than 0.2 mg m −3 [29,[34][35][36][37]. The yearly primary production in the subtropical Kuroshio current is approximately 0.91 ± 0.47 mg carbon m −3 day −1 [36].…”

Seasonal Assemblages of picocyanobacteria in the subtropical segment of the Kuroshio Current

“…These nutrient sources support primary productivity in the Kuroshio region [30][31][32]. The surface Kuroshio is oligotrophic and has low chlorophyll a (Chl a) concentrations of less than 0.2 mg m − 3 [27,[33][34][35][36]. The yearly primary production in the subtropical Kuroshio current is approximately 0.91 ± 0.47 mgC/m 3 /day [33].…”