Phytoplankton photophysiology varies depending on nitrogen and light availability at the subsurface chlorophyll maximum in the northern Chukchi Sea

Abstract

Vertical distributions of phytoplankton in the Arctic Ocean are characterized by a very narrow subsurface chlorophyll maximum (SCM) layer formed every summer after the sea ice retreats. Despite the prevalence of this narrow SCM layer, phytoplankton photosynthetic response to climate change remains to be elucidated. Here, we examined the photophysiological properties of phytoplankton in the SCM layer in the northern Chukchi Sea during the summers of 2015–2018. There was a significant difference in the SCM depth between the northwestern and northeastern Chukchi Sea determined by the distribution of Pacific Summer Water (PSW) around the SCM layer (34 ± 14 m vs. 49 ± 10 m, respectively). The maximum quantum yield of photochemistry in photosystem II (Fv/Fm) in the SCM phytoplankton was high (Fv/Fm ≥ 0.54) and similar in both regions until 2016; however, since then, Fv/Fm in the northeastern Chukchi Sea has decreased by approximately 10%. This decrease was accompanied by a marked decrease in the fraction of microplankton, which are known to be susceptible to nutrient limitation. This result suggests a reduction in nitrogen availability in the SCM layer in the northeastern Chukchi Sea. Meanwhile, the maximum electron transfer rate (ETRmax) did not have a significant relationship with the nitrogen availability and phytoplankton community size structure in the SCM layer; however the improved light conditions (with an approximately two-fold increase in the relative ratio of surface PAR reaching the SCM layer) increased ETRmax by up to 30% in the SCM phytoplankton in the northwestern Chukchi Sea. Therefore, these results provide a better understanding of how changes in nitrogen and light availability could affect phytoplankton photosynthesis and primary production in the Arctic Ocean.

Revealing the seasonal cycles of Arctic phytoplankton: insights from year-round chlorophyll monitoring

“…After sea ice retreat at KAMS1 and KAMS4, the Chl-a concentration near the SCM layer reached its maximum after 2-4 weeks, indicating a period of SCM layer formation (figure 2 and table 1). During SCM layer development, nutrient availability in the SCM layer at KAMS1 was higher than that at KAMS4 (Gal et al 2022, Ko et al 2022. This difference may contribute to variations in maximum Chl-a concentration between the two mooring sites (3.04 ± 0.71 vs. 1.63 ± 0.29 mg m −3 ).…”

“…The extracted samples were analyzed using a fluorometer (Trilogy, Turner Designs) calibrated with a purified Sigma Chla standard solution (Parsons et al 1984). The Chl-a fluorescence profile at each mooring site was obtained during summer field surveys using a fluorometer (ECO FL, WET Labs), which was calibrated using the Chl-a concentration sampled in a Niskin bottle (Ko et al 2022). The subsurface chlorophyll maximum (SCM) depth was determined based on the depth at which the highest value was observed for the Chl-a fluorescence profile in summer filed surveys.…”

Seasonal and Spatial Variability of Phytoplankton Primary Production in a Shallow Temperate Coastal Lagoon (Ria Formosa, Portugal)

“…Co-limitation by N and P is frequently observed throughout the seasonal cycle in the Ria Formosa coastal lagoon [46], and higher nutrient concentrations are associated with higher phytoplankton growth [28]. Nutrient, particularly nitrogen, availability is commonly a major factor regulating primary production and photosynthetic activity in different types of ecosystems [23,47], but is particularly relevant in shallow ecosystems such as the Ria Formosa, where light limitation seldom occurs [27].…”