Nutrient and Silicon Isotope Dynamics in the Laptev Sea and Implications for Nutrient Availability in the Transpolar Drift

Abstract

Realistic prediction of the near‐future response of Arctic Ocean primary productivity to ongoing warming and sea ice loss requires a mechanistic understanding of the processes controlling nutrient bioavailability. To evaluate continental nutrient inputs, biological utilization, and the influence of mixing and winter processes in the Laptev Sea, the major source region of the Transpolar Drift (TPD), we compare observed with preformed concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP), silicic acid (DSi), and silicon isotope compositions of DSi (δ30SiDSi) obtained for two summers (2013 and 2014) and one winter (2012). In summer, preformed nutrient concentrations persisted in the surface layer of the southeastern Laptev Sea, while diatom‐dominated utilization caused intense northward drawdown and a pronounced shift in δ30SiDSi from +0.91 to +3.82‰. The modeled Si isotope fractionation suggests that DSi in the northern Laptev Sea originated from the Lena River and was supplied during the spring freshet, while riverine DSi in the southeastern Laptev Sea was continuously supplied during the summer. Primary productivity fueled by river‐borne nutrients was enhanced by admixture of DIN‐ and DIP‐rich Atlantic‐sourced waters to the surface, either by convective mixing during the previous winter or by occasional storm‐induced stratification breakdowns in late summer. Substantial enrichments of DSi (+240%) and DIP (+90%) beneath the Lena River plume were caused by sea ice‐driven redistribution and remineralization. Predicted weaker stratification on the outer Laptev Shelf will enhance DSi utilization and removal through greater vertical DIN supply, which will limit DSi export and reduce diatom‐dominated primary productivity in the TPD.

Effects of sea ice retreat and ocean warming on the Laptev Sea continental slope ecosystem (1993 vs 2012)

“…An increased variability of stratification has been reported in the region for the period after the mid-2000s, with the region of freshwater influence being determined by summer winds in the ice-free areas (Janout et al, 2020); trends in stratification are thus difficult to generalize. Riverine influence is expected to be rather low for most of the stations investigated here, as indicated by surface salinities reported in previous studies (Janout et al, 2020;Laukert et al, 2022).…”

Atypical Seasonality of the Silicon Cycle in the Yellow River Estuary and Bohai Sea Revealed by Stable Silicon Isotopes

“…This leads to a Si isotope fractionation factor from 3.9‰ to 0.53‰ during diatom production (De La Rocha et al, 1997;Frings et al, 2016;Schmidtbauer et al, 2022;Sun et al, 2014;Sutton et al, 2013Sutton et al, , 2018. The Si isotope values of DSi (expressed as δ 30 Si DSi ) in coastal areas are thus generally higher than those of diatoms, ranging from +0.49‰ to +3.82‰ (summarized from around 600 values measured by published data and this study; Cao et al, 2012Cao et al, , 2015Cassarino et al, 2017;Ehlert et al, 2012;Laukert et al, 2022;Zhang et al, 2019;Zhang, Cao, et al, 2020;.…”

“…High-latitude systems exhibit even larger seasonal variations. In the Laptev Sea, the δ 30 Si DSi values varied from +0.91 to +3.82 ‰ in summer, while only from +1.48 to +1.74 ‰ in winter (Laukert et al, 2022). Cassarino et al (2017) found that the δ 30 Si DSi values in the Marguerite Bay on the west side of the Antarctic Peninsula increased from +1.5‰ to +2.0‰ in summer.…”

“…One useful way to track the biogeochemical Si cycle in coastal areas is to use stable Si isotopes (e.g., Ehlert et al, 2012;Laukert et al, 2022;, which include three naturally existing isotopes: 28 Si (92.23%), 29 Si (4.67%), 30 Si (3.10%) (Sutton et al, 2018). According to Frings et al (2016) and references therein, the δ 30 Si values of diatoms vary from ∼ 1‰ to ∼ +3‰ (n = 95).…”

“…High‐latitude systems exhibit even larger seasonal variations. In the Laptev Sea, the δ 30 Si DSi values varied from +0.91 to +3.82 ‰ in summer, while only from +1.48 to +1.74 ‰ in winter (Laukert et al., 2022). Cassarino et al.…”