Abstract
A paradox is commonly observed in productive sea ice in which an accumulation in the macro-nutrients nitrate and phosphate coincides with an accumulation of autotrophic biomass. This paradox requires a new conceptual understanding of the biogeochemical processes operating in sea ice. In this study, we investigate this paradox using three time series in Antarctic landfast sea ice, in which massive algal blooms are reported (with particulate organic carbon concentrations up to 2,600 µmol L–1) and bulk nutrient concentrations exceed seawater values up to 3 times for nitrate and up to 19 times for phosphate. High-resolution sampling of the bottom 10 cm of the cores shows that high biomass concentrations coexist with high concentrations of nutrients at the subcentimeter scale. Applying a nutrient-phytoplankton-zooplankton-detritus model approach to this sea-ice system, we propose the presence of a microbial biofilm as a working hypothesis to resolve this paradox. By creating microenvironments with distinct biogeochemical dynamics, as well as favoring nutrient adsorption onto embedded decaying organic matter, a biofilm allows the accumulation of remineralization products (nutrients) in proximity to the sympagic (ice-associated) community. In addition to modifying the intrinsic physicochemical properties of the sea ice and providing a substrate for sympagic community attachment, the biofilm is suggested to play a key role in the flux of matter and energy in this environment.
Highlights
Sea ice plays a significant role in biogeochemical cycles, providing an active biogeochemical interface at the ocean-atmosphere boundary (Vancoppenolle et al, 2013)
We examine data obtained from three time series in Antarctic landfast sea ice and discuss the presence of a microbial biofilm in sea ice as a working hypothesis for the sea-ice nutrient paradox (Figure 1B)
Location of the study sites New data sets sampled during the Year Round survey of Ocean-Sea Ice-Atmosphere Exchanges (YROSIAE) campaign at Cape Evans and a field campaign at Davis Station were combined with published data (Fripiat et al, 2015) from a time series in the vicinity of Dumont d’Urville
Summary
Sea ice plays a significant role in biogeochemical cycles, providing an active biogeochemical interface at the ocean-atmosphere boundary (Vancoppenolle et al, 2013). Permeated by a network of channels, pores, and intracrystalline inclusions, sea ice and its brine-filled spaces are colonized by a sympagic (ice-associated) community that is both taxonomically diverse and metabolically active (Thomas and Dieckmann, 2002). This sympagic community is thought to play a significant role in structuring the biogeochemical dynamics and food webs of polar oceans (e.g., Lannuzel et al, 2020, and references therein). In terms of organism distribution, fluid (and nutrients) transport, and predator–prey interactions, a seawater model is perhaps less useful for conceptualizing the sympagic community than models developed for porous substrates such as soils or sediments (Krembs et al, 2000)
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