[Advances and Prospect of Sampling Techniques and Analytical Methods for Trace Elements in the Ocean: Progress of Trace Element Platform Construction in Xiamen University].

Trace elements, which are important chemical components in the ocean, generally refer to those chemical elements with concentrations below 10 µmol·kg-1in seawater. Some trace elements, such as Fe and Zn, serve as essential micronutrients for marine organisms, which regulate marine primary productivity and are closely related to the biogeochemical cycle of carbon and nitrogen and therefore affect the global environment and climate change. In contrast, some elements, such as Pb, are anthropogenic pollutants largely released by human activities. In addition, some trace elements and their isotopes can be used as tracers for oceanographic processes and proxies for paleoceanography. However, the high saline matrix and extremely low trace element concentrations in seawater, as well as the contamination from research vessels, sampling equipment, and the surrounding environment during the process of sample collection, pretreatment, and analysis, have restricted researchers from obtaining reliable trace element data in the ocean for a long period of time. Nevertheless, high quality samples and accurate data are prerequisites for investigating the biogeochemical and environmental behavior of marine trace elements. This paper reviews the development of sampling techniques and analytical methods for trace elements in seawater, introduces the research history and platform construction activities in Xiamen University in this field, summarizes the advantages and disadvantages of various sampling and analytical techniques and methods, and presents the perspectives on future developments in the research on trace elements in the ocean.

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 N2 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 N2 fixation rates increased, phosphate concentrations decreased, and bioassay experiments demonstrated evidence for N2 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 N2 fixation across the tropical ocean.

Diagnosis of CO2 dynamics and fluxes in global coastal oceans.

Global coastal oceans as a whole represent an important carbon sink but, due to high spatial-temporal variability, a mechanistic conceptualization of the coastal carbon cycle is still under development, hindering the modelling and inclusion of coastal carbon in Earth System Models. Although temperature is considered an important control of sea surface pCO2, we show that the latitudinal distribution of global coastal surface pCO2 does not match that of temperature, and its inter-seasonal changes are substantially regulated by non-thermal factors such as water mass mixing and net primary production. These processes operate in both ocean-dominated and river-dominated margins, with carbon and nutrients sourced from the open ocean and land, respectively. These can be conceptualized by a semi-analytical framework that assesses the consumption of dissolved inorganic carbon relative to nutrients, to determine how a coastal system is a CO2 source or sink. The framework also finds utility in accounting for additional nutrients in organic forms and testing hypotheses such as using Redfield stoichiometry, and is therefore an essential step toward comprehensively understanding and modelling the role of the coastal ocean in the global carbon cycle.