Overlooked contribution of the biological pump to the Pacific Arctic nitrogen deficit

The nutrient-rich Pacific Ocean seawater that flows through the Bering Strait into the Chukchi Sea is generally considered to be the most important source of nutrients to the Arctic euphotic zone.The inflow is characterized by nitrogen deficit and low nitrate/phosphate(N/P)ratios;this is ascribed to sedimentary denitrification on the Chukchi shelf by preoccupant opinions.However,the Chukchi Sea also has high primary production,which raises the question of whether the biological pump may also significantly modulate nutrient properties of the throughflow.Here,we show that nitrate concentrations of the Pacific inflow gradually decrease northward in association with notable biological utilization.The phytoplankton N/P uptake ratio was 8.8±2.27,higher than the N/P ratio of Pacific inflow water(5-6).This uptake ratio,in combination with efficient vertical nitrogen export,serves to preferentially remove nitrogen(relative to phosphorus)from upper waters,thereby further intensifying the Arctic nitrogen deficit.Accordingly,as large as about 111.7×10^(9)mol N yr^(−1)of nitrate was extra consumed,according to the real N/P uptake ratio rather than the ratio of the Pacific inflow,which may be as great as half the nitrogen loss ascribed to sedimentary denitrification.Our findings suggest that besides sedimentary denitrification,biological disproportionate utilization of nutrients in the Chukchi Sea upper water is another important contributor to the nitrogen limitation and excess phosphorus in the upper Arctic Ocean.In the rapid Arctic change era,the predicted reinforced biological carbon pump could further impact the nutrient dynamics and biogeochemical process of the Arctic Ocean.

Unveiling the secrets of diatom-mediated calcification:Implications for the biological pump

Siliceous diatoms are one of the most prominent actors in the oceans,and they account for approximately 40%of the primary production and particulate organic carbon export flux.It is believed that changes in carbon flux caused by variations in diatom distribution can lead to significant climate shifts.Although the fundamental pathways of diatom-driven carbon sequestration have long been established,there are no reports of CaCO_(3) precipitation induced by marine diatom species.This manuscript introduces novel details regarding the enhancement of aragonite precipitation during photosynthesis in Skeletonema costatum in both artificial and natural seawater.Through direct measurements of cell surfaces via a pH microelectrode and zeta potential analyzer,it was determined that the diatom-mediated promotion of CaCO_(3) precipitation is achieved through the creation of specific microenvironments with concentrated[CO_(3)^(2-)]and[Ca^(2+)]and/or the dehydrating effect of adsorbed Ca^(2+).Based on this mechanism,it is highly plausible that diatom-mediated calcification could occur in the oceans,an assertion that was supported by the significant deviation of total alkalinity(TA)from the conservative TA-salinity mixing line during a Skeletonema costatum bloom in the East China Sea and other similar occurrences.The newly discovered calcification pathway establishes a link between particulate inorganic and organic carbon flux and thus helps in the reassessment of marine carbon export fluxes and CO_(2) sequestration efficiency.This discovery may have important ramifications for assessing marine carbon cycling and predicting the potential effects of future ocean acidification.

Response of phytoplankton community to different water types in the western Arctic Ocean surface water based on pigment analysis in summer 2008

Nutrients and photosynthesis pigments were investigated in the western Arctic Ocean during the 3rd Chinese Arctic Research Expedition Cruise in summer 2008. The study area was divided into five provinces using the K- means clustering method based on the physical and chemical characteristics of the sea water, and to discuss the distribution of the phytoplankton community structure in these provinces. CHEMTAX software was performed using HPLC pigments to estimate the contributions of eight algal classes to the total chlorophyll a （TChl a）. The results showed that on the Chukchi Shelf, the Pacific Ocean inflow mainly controlled the Chl a biomass and phytoplankton communities by nutrient concentrations. The high nutrient Anadyr Water and Bering Shelf Water （AnW and BSW） controlled region have high Chl a levels and the diatom dominated community structure. In contrast, in the region occupied by low-nutrient like Alaska Coastal Water （ACW）, the Chl a biomass was low, with pico- and nano-phytoplankton as dominated species, such as prasinophytes, chrysophytes and cryptophytes. However, over the off-shelf, the ice cover condition which would affect the physical and nutrient concentrations of the water masses, in consequence had a greater impact on the phytoplankton community structure. Diatom dominated in ice cover region and its contribution to Chl a biomass was up to 75%. In the region dose to the Mendeleev Abyssal Plain （MAP）, controlled by sea-ice melt water with relatively high salinity （MW-HS）, higher nutrient and Chl a concentrations were found and the phytoplankton was dominated by pico- and nano-algae, while the diatom abundance reduced to 33%. In the southern Canada Basin, an ice-free basin （IfB） with the lowest nutrient concentrations and most freshened surface water, low Chl a biomass was a consequence of low nutrients. The ice retreating and a prolonged period of open ocean may not be beneficial to the carbon export efficiency due to reducing the Chl a biomass or intriguing smaller size algae growth.