Mapping the in situ microspatial distribution of ice algal biomass through hyperspectral imaging of sea-ice cores

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.

Underwater Hyperspectral Imaging (UHI): A Review of Systems and Applications for Proximal Seafloor Ecosystem Studies

“…Electric rails from which the imaging system can be mounted (Section 3.1.1.) are suitable for analyzing samples extracted from the natural environment (ex-situ) or grown in an artificial medium (in vitro), permitting the calibration and validation of UHI observations [27,66], or for deployment in field laboratories in remote regions (e.g., polar study sites or at sea where working indoors in a lab is preferential) [76]. Laboratory setups for UHI systems also grant efficient temporal quantitative analysis of seafloor processes and their interactions under specific light intensities and spectral qualities (e.g., photosynthetic activity) [80] and capture dimensions that are not visible from the in situ surface perspective.…”

“…Laboratory setups for UHI systems also grant efficient temporal quantitative analysis of seafloor processes and their interactions under specific light intensities and spectral qualities (e.g., photosynthetic activity) [80] and capture dimensions that are not visible from the in situ surface perspective. For example, [27] and [76] have demonstrated the vertical variability found in natural and artificial mediums (e.g., soft substrates, sea-ice), and its influence on UHI analysis and interpretation. Furthermore, ex-situ systems allow the acquisition of spectral information of living organisms for them to be taxonomically identified [64], describe their pigment composition [64,72], or assess individual physiological responses [77].…”

“…Further, Cimoli et al [68] adds that the system capabilities and the "inverted" approach could provide information on more biological features than first thought, such as photo physiology, algae species composition, and habitat features of under-ice grazers. Recently, [76] discussed how UHI approaches combining both in vitro and in situ studies hold great potential for quantitive mapping of sea-ice algae variability, especially when the ice matrix is evaluated both vertically and horizontally to untangle the complexities of sea-ice primary productivity.…”

“…Regarding sediment or other natural substrates (e.g., such as sea-ice) an important feature to be considered during UHI data acquisition, processing, and validation is the spatial and vertical variability of photosynthetic pigments present [27,76]. If the vertical spatial variability of photosynthetic microorganisms in their medium is not accounted for, hyperspectral data analysis with ancillary pigment extraction methods may differ by several tens of percent [27] or might yield an incorrect interpretation of the results [76]. An exemplar study case has been addressed ex-situ by [76] regarding microalgae vertical variability within the ice matrix.…”

“…If the vertical spatial variability of photosynthetic microorganisms in their medium is not accounted for, hyperspectral data analysis with ancillary pigment extraction methods may differ by several tens of percent [27] or might yield an incorrect interpretation of the results [76]. An exemplar study case has been addressed ex-situ by [76] regarding microalgae vertical variability within the ice matrix. This behavior can be turned into an advantage coupled with UHI methods to monitor vertical migration changes/characteristics [27].…”