Distribution of dissolved iron in the Pearl River (Zhujiang) Estuary and the northern continental slope of the South China Sea

Zhang, Ruifeng; Zhu, Xunchi; Yang, Chenghao; Ye, Liping; Zhang, Guiling; Ren, Jianqiang; Wu, Ying; Liu, Sumei; Zhang, Jing; Zhou, Meng

doi:10.1016/j.dsr2.2018.12.006

Cited by 28 publications

(19 citation statements)

References 62 publications

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“…Although the current study lacked the data to calculate nutrient supply rates into the euphotic zone (matching Fe concentration profiles, euphotic depths), the correlation found between nifH and nitracline depth suggested the potential for the same driver (i.e., Fe:N supply rates) to be operating over this smaller spatial scale. In line with Wen et al (2022), we further hypothesize that the expected significant N supply rate to surface waters of the NSCS (due to a shallower nitracline, alongside riverine and aerosol inputs) reduces, but does not eliminate the competitive ability of diazotrophs, as Fe supply rates to this region are likely also high (Duce et al, 1991;Jickells et al, 2005;Zhang et al, 2019), thereby maintaining Fe:N supply ratios at levels supporting diazotrophs (Ward et al, 2013;Wen et al, 2022). At these Fe:N supply levels, we observed that N2 fixation rates were either (i) 'simultaneously co-limited' by Fe and P (identified at station S1), which represents a state where two, non-substitutable nutrients (in this case, Fe and P) have been drawn down to equally limiting levels (Sperfeld et al, 2016), or (ii) 'independently co-limited' (stations SEATS2016 and S3), which represents a state where the resources are substitutable at biogeochemical (Saito et al, 2008), or community levels (Arrigo, 2005).…”

Section: Discussionsupporting

confidence: 84%

The response of diazotrophs to nutrient amendment in the South China Sea and western North Pacific

Wen

Browning²,

Dai³

et al. 2022

Preprint

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Abstract. The availability of iron (Fe) and phosphorus (P) have been shown to be key factors regulating rates of nitrogen fixation in the western Subtropical Pacific. However, their relative importance at finer spatial scales between the northern South China Sea (NSCS) and the western boundary of the North Pacific is poorly constrained. Furthermore, nutrient limitation of specific diazotroph types has not yet been assessed. Here we investigated these unknowns by carrying out measurements of (i) finer scale spatial variabilities in N2 fixation rates and diazotroph abundances throughout these regions, and (ii) conducting eight additional Fe and phosphate addition bioassay experiments where both changes in N2 fixation rates and the abundances of specific diazotrophs were measured. Overall, nitrogen fixation rates were lower in the NSCS than around the Luzon Strait and the western North Pacific, which we hypothesize was due to lower Fe-to-fixed nitrogen supply ratios that decrease their competitive ability with non-diazotrophic phytoplankton. The nutrient addition bioassay experiments demonstrated that nitrogen fixation rates in the central northern South China Sea (NSCS) were co-limited by Fe and P, whereas in the western boundary of the North Pacific they were P-limited. Changes in the abundances of nifH in response to nutrient addition varied in how well they correlated with changes in nitrogen fixation rates, and the largest responses were always dominated by either Trichodesmium or UCYN-B. In general, nutrient addition had a relatively restricted impact on diazotroph community structure apart from on UCYN-B, which showed increased contribution to the diazotroph community following P addition at sites where N2 fixation rates were P-limited. We further hypothesize the importance of absolute Fe supply rates in regulating spatial variability in diazotroph community structure across the study area.

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Section: Discussionsupporting

confidence: 84%

The response of diazotrophs to nutrient amendment in the South China Sea and western North Pacific

Wen

Browning²,

Dai³

et al. 2022

Preprint

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“…Water samples were collected using Niskin-X bottles at six depths throughout the upper 150 m for the determination of nifH gene abundance and for N 2 fixation and primary production incubations. Samples for nutrient analysis were collected throughout the upper 200 m. Seawater samples for dFe and for the bioassay experiments were collected either using trace-metal-clean Niskin-X bottles or using a trace-metal-clean towed sampling device located around 2 to 5 m depth, with suction provided by a Teflon bellows pump ( 48 ). Seawater was sampled in a dedicated trace-metal-clean sampling container maintained overpressurized by high-efficiency particulate air (HEPA)–filtered air.…”

Section: Methodsmentioning

confidence: 99%

Nutrient regulation of biological nitrogen fixation across the tropical western North Pacific

et al. 2022

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Nitrogen fixation is critical for the biological productivity of the ocean, but clear mechanistic controls on this process remain elusive. Here, we investigate the abundance, activity, and drivers of nitrogen-fixing diazotrophs across the tropical western North Pacific. We find a basin-scale coherence of diazotroph abundances and N 2 fixation rates with the supply ratio of iron:nitrogen to the upper ocean. Across a threshold of increasing supply ratios, the abundance of nifH genes and N 2 fixation rates increased, phosphate concentrations decreased, and bioassay experiments demonstrated evidence for N 2 fixation switching from iron to phosphate limitation. In the northern South China Sea, supply ratios were hypothesized to fall around this critical threshold and bioassay experiments suggested colimitation by both iron and phosphate. Our results provide evidence for iron:nitrogen supply ratios being the most important factor in regulating the distribution of N 2 fixation across the tropical ocean.

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“…Experiments directly followed previously published protocols (Browning et al 2017). Following collection of whole, unfiltered seawater (1 L acid-washed polycarbonate bottles; Nalgene) with a towed sampling device (~2 m depth; Zhang et al 2019), seawater was spiked in triplicate with N (1 μM NO 3 À + 1 μM NH 4 + ), P (0.2 μM) and Fe…”

Section: Methodsmentioning

confidence: 99%

Nutrient co‐limitation in the subtropical Northwest Pacific

Browning

Liu

Zhang

et al. 2021

Limnol Oceanogr Letters

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Constraining which nutrients limit phytoplankton growth is important for understanding ocean productivity, its response to climate change, and providing a benchmark on the accuracy of ocean biogeochemical models. The extensive subtropical Northwest Pacific is assumed to be nitrogen limited, but this is based mostly on indirect evidence. We conducted experiments along an~8000 km cruise track showing a geographic switch from sites that were nitrogen limited and those that were nitrogen-iron co-limited. Co-limited sites showed much larger responses to nutrient supply, due to blooming of diatoms that were initially undetectable. Because diatom growth leads to higher biomass accumulation and more efficient energy transfer up food chains, we hypothesize that this gradient could be important for regulating matching gradients in predatory tuna.

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Distribution of dissolved iron in the Pearl River (Zhujiang) Estuary and the northern continental slope of the South China Sea

Cited by 28 publications

References 62 publications

The response of diazotrophs to nutrient amendment in the South China Sea and western North Pacific

The response of diazotrophs to nutrient amendment in the South China Sea and western North Pacific

Nutrient regulation of biological nitrogen fixation across the tropical western North Pacific

Nutrient co‐limitation in the subtropical Northwest Pacific

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