Effects of Nitrogen Limitation on Phytoplankton Physiology in the Western Arctic Ocean in Summer

Abstract

Phytoplankton in the Arctic Ocean are subject to nitrogen limitation in the summer, however, how severely the nitrogen stress affects phytoplankton physiology remains largely unknown. In the summers of 2015-2018, we examined the distribution of phytoplankton photophysiological properties across two contrasting regions of the Arctic Ocean with distinctly different levels of nitrogen availability in the upper water column. We quantified the extent of nitrogen stress using a highly sensitive fluorescence induction and relaxation system to obtain continuous underway measurements and via discrete sample analyses of phytoplankton physiology, as well as nutrient enrichment incubations. The results revealed vast regions in the Chukchi Sea where phytoplankton photosynthesis was severely nitrogen-stressed. Thereby, the maximum quantum yield of photochemistry in photosystem II showed only a small decrease (12 ± 9%) relative to its nutrient-replete values, while the maximum photosynthetic electron transport rates under saturating irradiance were impaired to a greater extent (40 ± 17%). This phytoplankton photosynthesis response is indicative of a severe nitrogen limitation, which results in dramatic reduction in growth and net primary production rates. Nutrient enrichment incubations also revealed a marked increase in large-size phytoplankton growth (>10 μm) after the nitrogen stress was alleviated, suggesting that the larger cells were more susceptible to nitrogen stress. These results are important for understanding how regional nitrogen fluxes control variability in the primary production and phytoplankton community structure and how these processes might change with rapid climate changes in the Arctic Ocean. Plain Language Summary Nutrient availability is the main bottom-up controls of phytoplankton physiology and growth in the upper ocean. The distribution of nutrient limitation in the global ocean varies greatly in space and time, so phytoplankton responses to this factor are essential for understanding the marine ecosystem. Although nitrogen limitation was previously shown in the Arctic Ocean in the summer, how nitrogen stress affects phytoplankton physiology remains largely unknown. This study investigates, with high spatial resolution, the distribution of phytoplankton physiological status and quantifies the effects of nitrogen stress in the western Arctic Ocean. Our results revealed severe nitrogen limitation in the summer that results in dramatic reduction in growth and net primary production in this region of the ocean. Therefore, alterations in nitrogen fluxes along with climate change in the Arctic Ocean would be important for controlling phytoplankton growth and primary production in this region.

Chlorophyll fluorescence-based estimates of photosynthetic electron transport in Arctic phytoplankton assemblages

“…Previous studies of late-summer Arctic photophysiology have reported low background F v / F m values, as observed over most of our sampling transect. At the same time, these prior studies have also demonstrated increased F v / F m in response to nitrate (but not phosphate) enrichments [5], [50]. This result, coupled with the nutrient depletion observed at profiling stations along our cruise track (Table 2), suggest that the low F v / F m values we observed likely reflect nitrogen deficiency.…”

“…Nutrient enrichment experiments have demonstrated increasing τ Qa with nutrient deficiency and elevated actinic irradiance [30], [50]. Moreover, σ PSII can also change with nutrient availability, but the observed percent change in σ PSII following short-term nitrate enrichment is small compared with that of F v / F m [5], [50]. Baseline fluorescence, may also influence the terms used to define ETR k :ETR a .…”

“…This phenomenon represents a non-variable contribution to the Chl fluorescence signal, which is understood to reflect the presence of energetically decoupled light harvesting complexes under nutrient limitation or photoinhibitory stress [31]. High baseline fluorescence decreases the amplitude of fluorescence transients, making F v / F m a useful gauge of phytoplankton physiological stress [5], [62], [63]. Analysis of our data revealed a strong correlation between F v / F m and (F’q/F’m) Emax , ( r = 0.93, p < 0.001, n = 25), suggesting (F’q/F’m) Emax is similarly affected by baseline fluorescence and reflective of physiological status.…”

“…In late summer, when sea ice cover is at a minimum and the mixed layer is shallow and highly stratified, phytoplankton are exposed to high solar irradiances and low nutrient concentrations [2]. Under these conditions, the growth and photosynthetic efficiency of Arctic phytoplankton becomes nitrogen-limited [3]- [5]. However, localized regions of elevated productivity can persist where various processes transport nutrients into the mixed layer, including upwelling, tidal mixing, and freshwater input from rivers and glaciers [6]- [8].…”

Particulate Export of PAHs Firstly Traced by 210Po/210Pb Disequilibrium: Implication on the “Shelf Sink Effect” in the Arctic Ocean

“…

The Arctic Ocean is undergoing rapid changes such as temperature rising, river runoff increasing, and sea ice decreasing, which profoundly impact the rapid-progressing biogeochemical cycle in the polar environments (Dai et al, 2019;Ko et al, 2020). The loss of sea ice increases photosynthesis and thus enhances the biological pump and primary production, while the increase of freshwater enhances the stratification in the ocean's upper layer, hindering the vertical mixing and material supply from the deep layer (Garnett et al, 2019;Morison et al, 2012;Nishino et al, 2011;Underwood et al, 2019).

…”