Abstract
Ice-associated microalgae make a significant seasonal contribution to primary production and biogeochemical cycling in polar regions. However, the distribution of algal cells is driven by strong physicochemical gradients which lead to a degree of microspatial variability in the microbial biomass that is significant, but difficult to quantify. We address this methodological gap by employing a field-deployable hyperspectral scanning and photogrammetric approach to study sea-ice cores. The optical set-up facilitated unsupervised mapping of the vertical and horizontal distribution of phototrophic biomass in sea-ice cores at mm-scale resolution (using chlorophyll a [Chl a] as proxy), and enabled the development of novel spectral indices to be tested against extracted Chl a (R2 ≤ 0.84). The modelled bio-optical relationships were applied to hyperspectral imagery captured both in situ (using an under-ice sliding platform) and ex situ (on the extracted cores) to quantitatively map Chl a in mg m−2 at high-resolution (≤ 2.4 mm). The optical quantification of Chl a on a per-pixel basis represents a step-change in characterising microspatial variation in the distribution of ice-associated algae. This study highlights the need to increase the resolution at which we monitor under-ice biophysical systems, and the emerging capability of hyperspectral imaging technologies to deliver on this research goal.
Highlights
Ice-associated microalgae make a significant seasonal contribution to primary production and biogeochemical cycling in polar regions
While most of the algal biomass is concentrated at the ice-water interface, microbes are present in the interior of the ice matrix, which adds to the biocomplexity of the system[2,8]
The sea ice across the study area had a thickness of 180 ± 1 cm, except for occasional ridged or cracked areas
Summary
Ice-associated microalgae make a significant seasonal contribution to primary production and biogeochemical cycling in polar regions. The distribution of algal cells is driven by strong physicochemical gradients which lead to a degree of microspatial variability in the microbial biomass that is significant, but difficult to quantify We address this methodological gap by employing a fielddeployable hyperspectral scanning and photogrammetric approach to study sea-ice cores. The use of under-ice optical sensing from UUVs has extended the spatial coverage of algal surveys (e.g., covering meso- to floe-scale areas), but resolutions still remain coarse as a result of large footprints of underwater radiance (or irradiance) sensors[10,16,22] This shortcoming demonstrates a gap in field-sampling techniques that permit the quantification of horizontal and vertical distributions and temporal dynamics of ice algae biomass at relevant spatial scales
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