Net heterotrophy in High Arctic first-year and multi-year spring sea ice

Karley Campbell,P Tremblay,R Gradinger,S Duerksen,S Rysgaard,M Nicolaus,P Anhaus,J C Landy,M Tranter,C Michel,B A Lange,C Katlein,C Haas,J Charette,I Matero

doi:10.1525/elementa.2021.00040

Abstract

The net productivity of sea ice is determined by the physical and geochemical characteristics of the ice–ocean system and the activity of organisms inhabiting the ice. Differences in habitat suitability between first-year and multi-year sea ice can affect the ice algal community composition and acclimation state, introducing considerable variability to primary production within each ice type. In this study, we characterized the biogeochemical variability between adjacent first-year and multi-year sea ice floes in the Lincoln Sea of the Canadian High Arctic, during the May 2018 Multidisciplinary Arctic Program—Last Ice sampling campaign. Combining measurements of transmitted irradiance from a remotely operated underwater vehicle with laboratory-based oxygen optode incubations, this work shows widespread heterotrophy (net oxygen uptake) in the bottom 10 cm of both ice types, particularly in thick multi-year ice (&gt;2.4 m) and early morning of the 24-h day. Algal acclimation state and species composition varied between ice types despite similar net community production due to widespread light and nutrient limitation. The first-year ice algal community was increasingly dominated over spring by the potentially toxin-producing genus Pseudonitzschia that was acclimated to high and variable light conditions characteristic of a thinner ice habitat with mobile snow cover. In comparison, the multi-year ice harbored more shade-acclimated algae of mixed composition. This work highlights the potential for heterotrophy in sea ice habitats of the High Arctic, including first measurements of such O2-uptake in multi-year ice floes. Observed differences in photophysiology between algae of these sea ice types suggests that a shift toward higher light availability and a younger sea ice cover with climate change does not necessarily result in a more productive system. Instead, it may favor future sea ice algal communities of different species composition, with lower photosynthetic potential but greater resilience to stronger and more variable light conditions.

Highlights

Algae inhabiting sea ice account for a significant portion of total primary production in the Arctic marine system, in the High Arctic (e.g., Arctic Basin) where the contribution from multi-year sea ice (MYI; see Text S1 for list of abbreviations) algae to total production can be as high as 60% (Gosselin et al, 1997; Fernandez-Mendez et al, 2015)
Physical site characteristics The physical characteristics of transect locations on the first-year sea ice (FYI) and MYI floes have been described in detail by Lange et al (2019), with FYI sites of approximately 140-cm ice thickness and 25-cm snow depth that were consistently thinner than the MYI floe of 300-cm ice and 45-cm snow thickness
In comparison to transect locations that were intentionally sampled over a range of snow depths, the additional nontransect ice-core sites for this study targeted comparatively thinner snow covers of about 10– 20 cm (Table 1), with the exception of relatively thick snow sites on FYI during May 23 sampling (16 cm) and MYI on May 10 (24 cm)

Summary

Introduction

Algae inhabiting sea ice account for a significant portion of total primary production in the Arctic marine system, in the High Arctic (e.g., Arctic Basin) where the contribution from multi-year sea ice (MYI; see Text S1 for list of abbreviations) algae to total production can be as high as 60% (Gosselin et al, 1997; Fernandez-Mendez et al, 2015). Campbell et al: Net heterotrophy in sea ice ice-covered Arctic Ocean functions to capture or contribute to rising atmospheric concentrations of CO2. FYI is typically characterized by a relatively uniform thickness within a given region and wind-drifted snow cover (Iacozza and Barber, 1999; Webster et al, 2018), with the latter predominantly controlling the magnitude of light transmission through the snow and sea ice to the ocean below (Grenfell and Maykut, 1977; Perovich, 1990; Nicolaus et al, 2012; Katlein et al, 2019). Annual melt and freeze cycles create substantial heterogeneity across the MYI cover, with surface elevations (hummocks, pressure ridges) and depressions (non-hummocked ice and melt ponds) that either reduce or enhance, respectively, the accumulation of light-attenuating snow (Iacozza and Barber, 1999; Lange et al, 2017)

Methods

Results

Discussion

Conclusion