Changes in Under‐Ice Primary Production in the Chukchi Sea From 1988 to 2018

Abstract

Changes in sea ice thickness and extent have corresponded with substantial changes in net primary production (NPP) in the Arctic Ocean. In recent years, observations of massive phytoplankton blooms under sea ice have upended the previous paradigm that Arctic NPP was driven largely by growth in the marginal ice zone and open water periods. Here, a new 1‐D biogeochemical model capable of simulating ice algal and phytoplankton dynamics both under the ice and in open waters is applied in the northern Chukchi Sea for the years 1988–2018. Over this period, substantial under‐ice (UI) blooms were produced in all but four years and were the primary drivers of interannual variation in total NPP. While NPP in the UI period was highly variable interannually due to fluctuations in ice thickness and the length of the UI period, UI NPP accounted for nearly half of total NPP between 1988 and 2018. Further, years with high UI NPP had reduced annual zooplankton grazing, indicating an intensification in the mismatch between phytoplankton and zooplankton populations and possibly altering the partitioning of food between benthic and pelagic ecosystems. These results demonstrate that the often‐overlooked ice covered period can be highly productive in the Arctic Ocean, and that the northern Chukchi Sea has been amenable to UIB formation since at least 1988.

Increases in Benthic Particulate Export and Sedimentary Denitrification in the Northern Chukchi Sea Tied to Under‐Ice Primary Production

“…CAOS-GO produced excess N in line with these values, ranging from 2.4 to 4.6 mmol m −3 between 1988 and 2018 (mean = 3.6 ± 0.6 mmol m −3 ). Further, our results indicate that denitrification rates increased annually by 1.3 mmol m −2 yr −1 over this timeframe, indicating that the sediments of the northern Chukchi Sea may contribute even more to future N loss than previously calculated (Chang & Devol, 2009 Much of this modeled increase in denitrification was attributable to the increase in annual NPP (Payne et al, 2021) and PON exported to the benthos. A 3.8 mmol m −2 yr −1 increase in N assimilation by microalgae between 1988 and 2018 corresponded to a 1.8 mmol m −2 yr −1 increase in PON export to the benthos.…”

“…Additionally, we report concentrations and rates taking place in thin layers in the sediments in nmol cm −3 but in mmol m −3 in the water column and for depth-integrated sedimentary processes (units are equivalent). For more complete model details, see Payne et al (2021).…”

“…Wintertime 𝐴𝐴 NO − 3 concentration shows minimal spatial variability across the Chukchi shelf; surface 𝐴𝐴 NO − 3 concentrations averaged 14.0 ± 1.91 mmol m −3 across the region in 2014 (Arrigo et al, 2017). To test the effect of pre-bloom winter 𝐴𝐴 NO − 3 concentration on rates of sedimentary denitrification, we ran the model for the year 2011, a year with a moderate UIB (Payne et al, 2021)…”

“…Phytoplankton blooms in the under-ice (UI) period are dominated by diatoms (Laney & Sosik, 2014) which sink rapidly and contribute disproportionately to global particulate organic carbon (POC) export due to their highly silicified cell walls and large cell size (Smetacek, 1999). If NPP during the UI period is substantial, reduced grazing pressure due to cold water temperatures (Campbell et al, 2001;Coyle et al, 2007;Huntley & Lopez, 1992) is likely to increase POC and particulate organic N (PON) export to the sediments (Payne et al, 2021). Observations in the northern Chukchi Sea indicate that algal export during the UI period can be high (with daily chlorophyll (Chl) a export to the benthos of up to 5 mg m −3 d −1 ), is almost entirely diatom-dominated (93%-100%), and has increased over time (Lalande et al, 2007(Lalande et al, , 2020Szymanski & Gradinger, 2016).…”

“…ODU is an abbreviation for the OMEXDIA model state variable of oxygen demand units. FromPayne et al (2021).…”