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...

The primary production (PP), nitrate-uptake-based new production (NO 3 -NP), chlorophyll a concentration, and phytoplankton assemblage in the upstream Kuroshio Current neighboring the northern South China Sea (SCS) before and after the consecutive passage of 3 typhoons during the summer of 2007 were compared. The aim was to elucidate the effects of riverine mixing on phytoplankton dynamics in the oligotrophic Kuroshio, to which northern SCS and Taiwan coastal water spread after the typhoons. Spatial changes in surface salinity were used to differentiate the effects of riverine mixing from wind-induced upwelling. After the typhoons, PP and NO 3 -NP in the Kuroshio both were higher due to enriched nutrients from entrainment of riverine-mixed waters. Abundances of diatoms had increased but the abundances of Trichodesmium spp., Richelia intracellularis, and unicellular potential diazotrophs that typically thrive in the summer were reduced. Specifically, oligotrophic coccolithophores, such as Umbellosphaera tenuis and Discosphaera tubifera, were replaced by Gephyrocapsa oceanica and Emiliania huxleyi, and Prochlorococcus sp. was replaced by Synechococcus spp. The shift of the phytoplankton community from Trichodesmium spp. to diatoms suggests that the biogenic carbon enhanced by the typhoons tended to sink rather than be recycled in the upper-water food web.

No abstract

No abstract

We have investigated the effect of eddies (cold and warm eddies, CEs and WEs) on the nutrient supply to the euphotic zone and the organic carbon export from the euphotic zone to deeper parts of the water column in the northern South China Sea. Besides basic hydrographic and biogeochemical parameters, the flux of particulate organic carbon (POC), a critical index of the strength of the oceanic biological pump, was also measured at several locations within two CEs and one WE using floating sediment traps deployed below the euphotic zone. The POC flux associated with the CEs (85 ± 55 mg-C m−2 d−1) was significantly higher than that associated with the WE (20 ± 7 mg-C m−2 d−1). This was related to differences in the density structure of the water column between the two types of eddies. Within the core of the WE, downwelling created intense stratification which hindered the upward mixing of nutrients and favored the growth of small phytoplankton species. Near the periphery of the WE, nutrient replenishment from below did take place, but only to a limited extent. By far the strongest upwelling was associated with the CEs, bringing nutrients into the lower portion (∼50 m) of the euphotic zone and fueling the growth of larger-cell phytoplankton such as centric diatoms (e.g., Chaetoceros, Coscinodiscus) and dinoflagellates (e.g., Ceratium). A significant finding that emerged from all the results was the positive relationship between the phytoplankton carbon content in the subsurface layer (where the chlorophyll a maximum occurs) and the POC flux to the deep sea.

Supplement. Three tables for the correlation coefficients (Table S1), the principal component analysis (Table S2), and 4 Bacteriastrum-Calothrix associations (Table S3) as well as 2 figures for the vertical distributions of diatoms (Fig. S1) and microscopic images of diatom diazotroph associations (Fig. S2)

Ni is an essential cofactor in NiFe-uptake hydrogenase, an enzyme regulating H 2 metabolism in diazotrophic cyanobacteria, the major H 2 producers in the surface ocean globally. Here, we investigated the effect of Ni supply on H 2 production and N 2 fixation by using a model marine cyanobacterial diazotroph, Cyanothece. By mediating total dissolved Ni concentrations from 100 to 0.03 nmol L −1 in a trace metal-defined culture medium, we demonstrated that Ni deficiency results in H 2 accumulation, coupled with decreasing Ni quotas, growth rates, and occasionally relatively low N 2 fixation rates. These results indicate that Ni deficiency limits the growth of the Cyanothece to some extent, considerably decreases H 2 uptake by hydrogenase and leads to H 2 accumulation and N 2 fixation variation in the diazotroph. The findings show that Ni availability is a critical factor on controlling H 2 production and N 2 fixation in marine diazotrophic cyanobacteria. The information of Ni bioavailability for diazotrophic cyanobacteria is thus essential to evaluate the importance of Ni for H 2 cycling and N 2 fixation in oceanic surface waters.

Actively growing Trichodesmium spp. are known to release their newly fixed nitrogen (N) into the surrounding N-limited water. Although newly released fixed N could potentially enhance the growth of nearby non-diazotrophic phytoplankton, the interactions of Trichodesmium spp. and other co-occurring cells, i.e. diatoms, have never been well documented. We conducted laboratory 15 N tracer experiments to investigate the trophic interactions of T. erythraeum and diatoms in culture. Field studies were carried out to examine spatial-temporal distributions of Trichodesmium spp. and diatoms in the low-latitude Kuroshio Current and the neighboring northern South China Sea (SCS). Enrichment of 15 N occurred in 3 species of cultured diatoms when they were incubated with T. erythraeum IMS101 in culture media enriched with gaseous 15 N 2 , which confirmed the transfer of newly fixed N from T. erythraeum to diatoms. Field results showed that in the Kuroshio, where Trichodesmium spp. flourished in the warm seasons, the abundances of Trichodesmium spp. and diatom cells with >10 µm diameter were positively correlated (p < 0.05). During the warm seasons, diatom vertical distributions showed a surface-abundance maximum that coincided with the depth of highest cell densities of Trichodesmium spp. and a second abundance maximum near the nitracline. The diatom surface-abundance maximum disappeared in the cold seasons, when the Trichodesmium spp. population languished. The bimodal distribution of diatoms and the co-occurrence of diatom and Trichodesmium spp. abundance maxima recurred as Trichodesmium spp. abundance increased when the warm seasons returned. In contrast to the Kuroshio, the SCS, inhabited sparsely by Trichodesmium spp., showed no surface abundance maxima in the vertical distributions of Trichodesmium spp. or diatoms. Both field and laboratory data suggest that the co-occurrence of Trichodesmium spp. and diatoms might be driven by the transfer of N fixed by Trichodesmium spp. to diatoms in the N-limited oligotrophic ocean. Because diatoms are more effective than Trichodesmium spp. in exporting N and carbon to depth, the N exudation by Trichodesmium spp. that supports diatom growth could represent an important pathway of N and carbon export to depth in tropical and subtropical oceans.