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

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

Upper ocean biogeochemistry of the oligotrophic North Pacific Subtropical Gyre : from nutrient sources to carbon export

Subtropical gyres cover 26–29% of the world’s surface ocean and are conventionally regarded as ocean deserts due to their permanent stratification, depleted surface nutrients, and low biological productivity. Despite tremendous advances over the past three decades, particularly through the Hawaii Ocean Time-series and the Bermuda Atlantic Time-series Study, which have revolutionized our understanding of the biogeochemistry in oligotrophic marine ecosystems, the gyres remain understudied. We review current understanding of upper ocean biogeochemistry in the North Pacific Subtropical Gyre, considering other subtropical gyres for comparison. We focus our synthesis on spatial variability, which shows larger than expected dynamic ranges of properties such as nutrient concentrations, rates of N2 fixation, and biological production. This review provides new insights into how nutrient sources drive community structure and export in upper subtropical gyres. We examine the euphotic zone in subtropical gyres as a two-layered vertically structured system: a nutrient-depleted layer above the top of the nutricline in the well-lit upper ocean and a nutrient-replete layer below in the dimly lit waters. These layers vary in nutrient supply and stoichiometries and physical forcing, promoting differences in community structure and food webs, with direct impacts on the magnitude and composition of export production. We evaluate long-term variations in key biogeochemical parameters in both of these euphotic zone layers. Finally, we identify major knowledge gaps and research challenges in these vast and unique systems that offer opportunities for future studies

On contributions by wind-induced mixing and eddy pumping to interannual chlorophyll variability during different ENSO phases in the northern South China Sea

The chlorophyll in the northern South China Sea (NSCS) shows strong interannual variability between different phases of the El Nino-Southern Oscillation (ENSO), primarily due to the influence from Kuroshio intrusion. Chlorophyll observations also reveal that significant year-to-year variation remains in the same ENSO phase, but its controlling mechanism is unknown. In this study, we examined driving mechanisms for such regional ecosystem variability on a year-to-year timescale. Using observational data and modeling results, we found that both cyclonic eddies (CEs) and wind-induced mixing affect phytoplankton variability, but the former is the dominant factor regulating the interannual variability of chlorophyll during La Nina years, while the latter becomes the dominant one during El Nino years. The underlying mechanisms leading to the contrast in biological responses are attributable to changes in background nutricline depth and mixed-layer depth (MLD) during different ENSO phases. During La Nina, both thermocline and nutricline are deepened in the NSCS. Thus, wind mixing is less effective in non-eddy areas due to the deepened nutricline, while the CE-induced subsurface nutrient anomaly associated with increased MLD is able to control the interannual variability of chlorophyll. During El Nino, wind-induced mixing is more effective than CEs because the surface wind can influence a larger area

Dynamics of Ammonium Biogeochemistry in an Oligotrophic Regime in the South China Sea

Ammonium (NH4+) dynamics in the oligotrophic South China Sea (SCS) was examined via a large data set developed during summer 2014 and spring 2016. A sensitive fluorometric technique allowed measuring NH4+ concentrations down to 0.7 nmol L-1. The NH4+ inventory of the upper 100 m showed large inter-annual variation, averaging 3.46 mmol m-2 in 2014 and almost doubling in 2016. This could be attributed to seasonality or more likely to influence of Kuroshio Current intrusion, prominent within El Niño years, as in 2016. Indeed, Kuroshio-influenced stations, yielded elevated NH4+ and lower NO3- inventories. Higher NH4+ levels at the Kuroshio-SCS frontal zone were consistent with prior observations of enhanced microbial consumption of Kuroshio-derived dissolved organic nitrogen. Two patterns in vertical NH4+ distributions were observed: a subsurface NH4+ maximum (AM; 30.1-241 nmol L-1), usually occurring at 50-100 m and closely coupled with the deep chlorophyll maximum, and one where NH4+ concentrations were low and generally vertically uniform. Dynamics of the AM appears to be controlled by the euphotic depth, position of the nitracline, as well as the difference in the affinity to NH4+ between phytoplankton and nitrifiers. Results demonstrate that precise measurements of NH4+ concentrations at nanomolar levels facilitate improved understanding of coastal nitrogen biogeochemistry.Ministry of Science and Technology of the People's Republic of China (MOST): 2015CB954003; National Natural Science Foundation of China (NSFC): 41606089; National Natural Science Foundation of China (NSFC): 9185810