Seasonal dynamics of new production from Trichodesmium N2 fixation and nitrate uptake in the upstream Kuroshio and South China Sea basin

Abstract

The seasonal dynamics of primary production in the upstream Kuroshio was studied to understand why pelagic fish are relatively populous in this oligotrophic tropical ocean region. The relative importance of new production from nitrate uptake vs. Trichodesmium N 2 fixation was compared to differentiate the seasonal production dynamics in the upstream Kuroshio from that in the neighboring and interacting northern South China Sea (SCS). Unlike the SCS, which was most productive in the winter, seasonal fluctuation of primary production (0.51-0.53 g C m 22 d 21 ) was not apparent in the upstream Kuroshio. The seasonality in new production in the Kuroshio, in contrast, was pronounced. Its nitrate-uptake-based new production was high in the winter (0.27 g C m 22 d 21 ) and low in the summer (0.16 g C m 22 d 21 ); conversely, Trichodesmium N 2 -fixation-based new production was low in the winter (2.4 mmol N m 22 d 21 ) and high in the summer (168.1 mmol N m 22 d 21 ). In the summer when nitrate-uptake-based new production was low due to nitrate depletion, the Kuroshio remained productive because of enhanced Trichodesmium N 2 fixation from strong stratification and a deepened nitracline. The standing crop of Trichodesmium was consistently much higher in the Kuroshio (34.61 3 10 6 and 0.49 3 10 6 trichomes m 22 in the summer and winter, respectively) than in the SCS (4.87 3 10 6 and 0.15 3 10 6 trichomes m 22 , respectively). In both regions, surface water temperature correlated significantly with increased Trichodesmium stocks. At a given temperature, higher Trichodesmium standing crop in the Kuroshio than the SCS was related to the deeper nitracline and ensuing decrease in nitrate availability in the Kuroshio.The Kuroshio and South China Sea (SCS) are two neighboring and interacting waterbodies in the western North Pacific. The Kuroshio spans a wide range of latitudes from tropical 15uN to temperate 40uN. Before passing southeastern Taiwan, the upstream Kuroshio flows by the Luzon Strait, which is the deepest passage between the western Pacific and the SCS. A branch of the Kuroshio intrudes through the southern Luzon Strait into SCS at a depth between 1,500 and 1,900 m. Its upper water mass mixes upward and eventually flows out of the SCS basin through the northern Luzon Strait at an intermediate depth of 500-1,000 m before being reunited with the main stream Kuroshio (Gong et al. 1992). As the upstream Kuroshio enters the SCS, the ensuing upwelling brings nutrient-laden water closer to the surface. The nutricline in the northern SCS is uplifted by as much as 100 m relative to the West Philippine Sea as well as the upstream Kuroshio (Gong et al. 1992). The SCS is one of the major marginal seas on Earth, extending from the equator to 22uN. Its deep basin (5,000 m) is located in the northeast. The SCS is heavily influenced seasonally by monsoons. The northeastern monsoon, which corresponds to the formation of cyclonic gyres, prevails in winter and spring; the southwestern monsoon in summer and autumn creates anti...

Isotope constraints on particulate nitrogen source and dynamics in the upper water column of the oligotrophic South China Sea

“…The geographical setting of the South China Sea (SCS), the second largest marginal sea in the world [Liu et al, 2010], offers favorable environmental features for nitrogen fixation [Chao et al, 1996;Wong et al, 2002]. Among the relatively small number of measurements on the rates of nitrogen fixation in the world ocean, the rates observed in the SCS were among the highest values reported [Capone et al, 1997, and references therein], and comparably high rates were also observed in the adjacent areas, like the Kuroshio Current (KC) and East China Sea [Chen et al, 2008;Shiozaki et al, 2010]. 1 Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan.…”

“…Five of Saino and Hattori [1987] are also included for discussion. The seasonal variation of the concentration-weighted average d 15 N PN from the lower 100 m mimics that of the top 100 m as shown in Figure 5c implying that the surface isotopic signals have been [43] Diazotrophs in the SCS include all forms of N 2 fixers, filamentous (e.g., Trichodesmium), unicellular and symbiotic cyanobacteria [Chen et al, 2008;Chou et al, 2006]. Aside from the symbiotic diazotrophs, the rest do not make it into the sinking particles readily.…”

Nitrogen Fixation by Trichodesmium and unicellular diazotrophs in the northern South China Sea and the Kuroshio in summer

“…Compared with previous studies in the East China Sea 24 , the SCS 4,15,25 and the Kruoshio 26 have been studied deficiently. The present study was conducted to investigate the horizontal and vertical variation of Trichodesmium abundance and it’s in situ nitrogen fixation rates in the nSCS and the Kuroshio.…”

“…The nSCS exhibits strong variations of temperature and salinity from coastal region to the open sea. The surface water of the nSCS and the Kuroshio is generally warm, stratified and oligotrophic which is supposedly an ideal habitat for N-fixers especially in summer 4 . The Kuroshio and nSCS are two neighboring and interacting waterbodies.…”

Re-evaluation of the diversity and distribution of diazotrophs in the South China Sea by pyrosequencing the nifH gene

“…Nitrogen availability affects the seasonal dynamics of the phytoplankton biomass, production and community structure (Chen et al 2008). The recognition of nitrogen fixation as a major source of new nitrogen for the oceanic ecosystem has prompted the need for a better understanding of the diversity, abundance and dynamics of N 2 -fixing organisms (Campbell et al 2005).…”