Nitrogen Limitation of the Summer Phytoplankton and Heterotrophic Prokaryote Communities in the Chukchi Sea

Abstract

Major changes to Arctic marine ecosystems have resulted in longer growing seasons with increased phytoplankton production over larger areas. In the Chukchi Sea, the high productivity fuels intense benthic denitrification creating a nitrogen (N) deficit that is transported through the Arctic to the Atlantic Ocean, where it likely fuels N fixation. Given the rapid pace of environmental change and the potentially globally significant N deficit, we conducted experiments aimed at understanding phytoplankton and microbial N utilization in the Chukchi Sea. Ship-board experiments tested the effect of nitrate (NO 3 −) additions on both phytoplankton and heterotrophic prokaryote abundance, community composition, photophysiology, carbon fixation and NO 3 − uptake rates. Results support the critical role of NO 3 − in limiting summer phytoplankton communities to small cells with low production rates. NO 3 − additions increased particulate concentrations, abundance of large diatoms, and rates of carbon fixation and NO 3 − uptake by cells >1 µm. Increases in the quantum yield and electron turnover rate of photosystem II in +NO 3 − treatments suggested that phytoplankton in the ambient dissolved N environment were N starved and unable to build new, or repair damaged, reaction centers. While some increases in heterotrophic prokaryote abundance and production were noted with NO 3 − amendments, phytoplankton competition or grazers likely dampened these responses. Trends toward a warmer more stratified Chukchi Sea will likely enhance summer oligotrophic conditions and further N starve Chukchi Sea phytoplankton communities.

Environmental drivers of under-ice phytoplankton bloom dynamics in the Arctic Ocean

“…Given that the C:N between different phytoplankton varies much less than either C:Si or C:P, diatoms can likely grow to greater biomass on the same N inventory than other taxa, and thus both winter water Si(OH) 4 and PO 4 3concentrations, and Si(OH) 4 :NO 3 and NO 3 -:PO 4 3drawdown ratios, are good predictors of bloom magnitude. That maximum chl a was predicted best by Si(OH) 4 and PO 4 3is surprising, given that NO 3 is generally considered the nutrient that limits Arctic phytoplankton growth (Tremblay and Gagnon, 2009;Ardyna et al, 2011;Mills et al, 2018;Randelhoff et al, 2020). We suspect that, while NO 3 is typically the first nutrient depleted during typical Arctic UIBs, the inventory of nutrients (specifically Si(OH) 4 ) allows for the growth of diatoms which can attain higher biomass than other taxa on the same inventory of NO 3 -.…”

Primary Productivity Dynamics in the Summer Arctic Ocean Confirms Broad Regulation of the Electron Requirement for Carbon Fixation by Light-Phytoplankton Community Interaction

“…Ice cover loss leads to increased light availability and the phytoplankton growing season, thus primary productivity in the Arctic Ocean (Arrigo et al, 2008;Hill et al, 2017). However, ice melt also intensifies freshening and in turn stronger stratification that may act to suppress upward fluxes of nutrients leading to decreased primary productivity and an increase in the fraction of picophytoplankton, typical for low-nutrient environments (Li et al, 2009;Coupel et al, 2015;Mills et al, 2018) through selection for low nutrient pico-phytoplankton (see Li et al, 2009). The non-uniform trends associated with changes in sea-ice and primary productivity (Brown and Arrigo, 2012;Fernández-Méndez et al, 2015) fundamentally require more data across a wide variety of assemblages that can more broadly resolve this issue.…”

Nitrate supply and uptake in the Atlantic Arctic sea ice zone: seasonal cycle, mechanisms and drivers

“…PSW occupied the upper water column down to approximately 40 m in late December and early January, and either shoaled above 21 m or was displaced by deeper waters to the surface for the rest of the deployment. BSW contributes more than 50% over the full 4), which is sufficiently low to limit phytoplankton growth [71,72]. Nitrate was replenished slowly over autumn and winter, and did not reach seasonal maximum values (greater than 10 µmol l −1 ) until February.…”