Abstract
The accuracy of sea ice algal production estimates is influenced by the range of melting procedures used in studies to obtain a liquid sample for incubation, particularly in relation to the duration of melt and the approach to buffering for osmotic shock. In this research, ice algal photophysiology from 14C incubations was compared in field samples prepared by three melt procedures: i) a rapid ≤ 4 h melt of the bottommost (< 1 cm) ice algal layer scraped into a large volume of filtered seawater (salinity 27 - 30), ii) melt of a bottom 5 cm section diluted into a moderate volume of filtered seawater over 24 h (salinity 20 - 24), and iii) melt of a bottom 5 cm section without any filtered seawater dilution over about 48 h (salinity 10 -12). Maximum photosynthetic rate, photosynthetic efficiency and production at zero irradiance were significantly affected by the melt treatment employed in experiments. All variables were greatest in the highly diluted scrape sample and lowest in the bulk-ice samples melted in the absence of filtered seawater. Laboratory experiments exposing cultures of the common sea ice diatom Nitzschia frigida to different salinities and light conditions suggested that the field-based responses can be attributed to the rapid (< 4 h) adverse effects of exposing cells to low salinities during melt without dilution. The observed differences in primary production between melt treatments were estimated to account for over 60% of the variability in production estimates reported for the Arctic. Future studies are strongly encouraged to replicate salinity conditions representative of in situ values during the melting process to minimize hypoosmotic stress, thereby most accurately estimating primary production.
Highlights
Algae colonizing the brine network and bottom layer of sea ice are estimated to account for 3– 25% of annual primary production in the Arctic Ocean (Subba Rao and Platt, 1984; Legendre et al, 1992), greater contributions have been documented in high Arctic regions (Gosselin et al, 1997)
Our field-based measurements clearly demonstrate that sample melting procedure affects the magnitude of many photophysiological parameters subsequently measured on the incubated meltwater
Lab experiments indicated that the differences observed between melting procedures can be primarily attributed to different levels of hypoosmotic stress
Summary
Algae colonizing the brine network and bottom layer of sea ice are estimated to account for 3– 25% of annual primary production in the Arctic Ocean (Subba Rao and Platt, 1984; Legendre et al, 1992), greater contributions have been documented in high Arctic regions (Gosselin et al, 1997). There are renewed efforts to expand the spatial coverage of sea ice production measurements as recent studies have shown that ice algae in previously under sampled regions may be greatly underestimated (Lange et al, 2017; Fernandez-Mendez et al, 2018) Completion of such assessments of sea ice algal communities are required to predict the consequences of climate warming on ice algae, including the impact of ongoing changes to sea ice volume, seasonality and areal coverage (Vihma, 2014; Simmonds, 2015). Effective studies of sea ice biogeochemistry rely on accurate measurements of ice algal production, which can be done by monitoring oxygen evolution or carbon uptake in situ or via incubation of samples in a closed system In situ approaches, such as quantifying oxygen flux across the ocean-ice diffusive boundary layer (McMinn et al, 2000; Rysgaard et al, 2001; McMinn and Hegseth, 2007), arguably provide estimates under the most natural conditions. There are concerns related to gas and tracer diffusion when measuring algal production within the solid ice matrix (Mock and Gradinger, 1999)
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