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

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 (>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.

Variability in sea ice carbonate chemistry: a case study comparing the importance of ikaite precipitation, bottom-ice algae, and currents across an invisible polynya

“…It may, however, be more instructive to consider the slope of the bottom horizon alone (Table 4), as the overall slopes are biased by the high number of samples collected at the surface and bottom despite representing only a small portion of the overall ice volume. In the bottom horizon, we do see lower slope values (0.67 at TFYI, 0.50 at POLY), suggesting a shift towards the net respiration line, consistent with past studies that have found net heterotrophy in ice algae communities during the bloom (Campbell et al, 2017(Campbell et al, , 2022Rysgaard and Glud, 2004). In the 2014 spring, Campbell et al (2017) observed net heterotrophic conditions in the bottom ice of Dease Strait during a period of carbon accumulation before switching to autotrophic conditions around 8 May.…”

Mapping Potential Timing of Ice Algal Blooms From Satellite

“…Studies have suggested the PAR value needed for ice algal blooms to occur can range significantly, with bottom‐ice algal blooms found under comparatively lower (e.g., PAR < 0.17 μmol photons m −2 s −1 , Hancke et al., 2018) or higher (e.g., PAR >20 μmol photons m −2 s −1 , Campbell et al., 2022) light levels depending on algal acclimate state. A lower light threshold may be most realistic given multiple scattering within the sea ice that can increase cellular absorption of light (Ehn & Mundy, 2013) as well as the shade adaptations of ice algae (Johnsen & Hegseth, 1991).…”

Overview of the MOSAiC expedition: Ecosystem

“…Different sensitivities to climate change drivers by various ecosystem components may cause mismatches between trophic levels, such as algae blooms occurring earlier than the zooplankton life stages depending on them as food (Søreide et al, 2010). Also, the shift from a dominance of a multi-year ice (MYI) or second-year ice (SYI) to a first-year ice (FYI) regime will likely impact sea ice biota; however, evidence for change is patchy due to the limited availability of sufficiently long time-series data (Campbell et al, 2022). Comparisons between FYI and MYI diversity of sea ice protists indicate substantially lower (by 39%) diversity in FYI compared to MYI (Hop et al, 2020).…”