Seasonal and Spatial Changes in Trichodesmium Associated With Physicochemical Properties in East China Sea and Southern Yellow Sea

Abstract

Trichodesmium is broadly distributed and occasionally blooms in the East China Sea (ECS) and southern Yellow Sea, where it contributes to local N and C budgets. However, its population structure, spatiotemporal distribution, controlling factors, and N2 fixation remain poorly documented. Here we provide high spatial resolution data sets of Trichodesmium during the four seasons of 2011–2012 using water‐ and net‐collected methods. The net‐collected method captures colonial trichomes of Trichodesmium effectively but results in an underestimation of free trichomes. Colonies are rarely observed and occur only on the ECS shelf, which are easily missed in water‐collected samples. Depth‐integrated densities of Trichodesmium were found to be significantly higher in warm seasons than in cold seasons. Maximum densities in the water column were generally found at depths of 10–50 m. Trichodesmium thrives on the oligotrophic, warm, offshore ECS shelf (controlled by the Kuroshio and Taiwan Warm Current), but restrains in the cold southern Yellow Sea and the eutrophic, inshore ECS. Seasonal and spatial variations in Trichodesmium are closely correlated with physicochemical properties (mainly temperature and P), which are primarily controlled by circulation alteration and water mass movement. The N2 fixation rates of Trichodesmium in the ECS in summer and autumn (>20°C) are roughly estimated at 17.1 and 41.7 μmol N m−2 d−1 under nonbloom conditions, which potentially contribute to 81% and 57% of biological N2 fixation, respectively. Compared with historical data since the 1970s, Trichodesmium densities have increased considerably in all seasons, and the distribution boundary has shifted northward under regional warming and hydrological changes.

Kuroshio Shape Composition and Distribution of Filamentous Diazotrophs in the East China Sea and Southern Yellow Sea

“…Previous studies have observed high abundance of Trichodesmium in Kuroshio and bifurcation regions (Chen et al, ; Jiang et al, , ; Marumo & Asaoka, ; Shiozaki, Takeda, et al, ; Zhang et al, ). Similarly, high‐density Richelia / Calothrix were observed in/near upstream Kuroshio (Chen et al, ), the Kuroshio Extension (Kitajima et al, ), and the Tsushima Strait affected by Kuroshio branch (Shiozaki et al, ) through fluorescence microscopy.…”

“…Shiozaki et al () found relatively abundant Richelia in Kuroshio from northeastern Taiwan to south of mainland Japan. Several studies have suggested that abundant Trichodesmium can be transported to vast regions by Kuroshio (Jiang et al, ; Marumo & Asaoka, ; Shiozaki, Takeda, et al, ; Zhang et al, ). In this context, we hypothesize that the incursion of Kuroshio may profoundly influence composition and distribution of filamentous diazotrophs in the ECSYS.…”

Impact of Hydrological Conditions on the Biogeochemical Dynamics of Suspended Particulate Organic Matter in the Upper Mixed Layer of the Southern East China Sea

“…This contrast probably suggests that biological fixed nitrogen in the ECS shelf is not mainly produced by diazotroph living in the continental shelf of the ECS but overwhelmingly produced in the Kuroshio water, which are transported to the ECS shelf as Kuroshio intrudes. Consistently, based on the nitrogen budget for the ECS (Zhang et al, ) and the conservative estimation that 20% of the nitrogen in the upwelled Kuroshio water is from nitrogen fixation (Liu et al, ), the biological fixed nitrogen by Kuroshio input to the continental shelf of the ECS are estimated to be 10 (0.30 versus 0.03 × 10 12 g) and 2.6 (0.13 versus 0.05 × 10 12 g) times higher than local N 2 fixation fluxes (Jiang et al, ) during autumn and summer, respectively.…”

“…By contrast, our study probably benefited from the relatively small area and the well‐mixed water column. The scattering of δ 15 N with temperature (Figure c) may be caused by additional factors influencing nitrogen fixation rate, such as dissolved phosphorus concentration (Mulholland & Bernhardt, ) and some other nitrogen fixers like the unicellular diazotroph which behave differently with temperature compared to Trichodesmium (Chen et al, ; Jiang et al, ; Moisander et al, ). Although nitrification, a biological oxidation of NH 4 + to NO 3 − via NO 2 − , more strongly correlates with the water temperature than nitrogen fixation (Sugimoto et al, ), it would not contribute to such a negative correlation between δ 15 N of planktonic OM and temperature due to the N depletion in the study area.…”

“…Moreover, the fraction of biologically fixed N to total bioavailable nitrogen and marine productivity is higher in autumn than in summer, as evidenced from the more negative δ 15 N of suspended POM and a better correlation between δ 15 N and temperature in autumn (Figure c). However, the Trichodesmium abundances (Zhang et al, ) and their nitrogen fixation rates (Jiang et al, ) are reported to be more than 2 times higher in summer than in autumn. This contrast probably suggests that biological fixed nitrogen in the ECS shelf is not mainly produced by diazotroph living in the continental shelf of the ECS but overwhelmingly produced in the Kuroshio water, which are transported to the ECS shelf as Kuroshio intrudes.…”