Quantifying spatiotemporal variability of glacier algal blooms and the impact on surface albedo in southwestern Greenland

Shujie Wang,Xavier Fettweis,Min Xu,Patrick Alexander,Marco Tedesco

doi:10.5194/tc-14-2687-2020

Abstract

Abstract. Albedo reduction due to light-absorbing impurities can substantially enhance ice sheet surface melt by increasing surface absorption of solar energy. Glacier algae have been suggested to play a critical role in darkening the ablation zone in southwestern Greenland. It was very recently found that the Sentinel-3 Ocean and Land Colour Instrument (OLCI) band ratio R709 nm∕R673 nm can characterize the spatial patterns of glacier algal blooms. However, Sentinel-3 was launched in 2016, and current data are only available over three melting seasons (2016–2019). Here, we demonstrate the capability of the MEdium Resolution Imaging Spectrometer (MERIS) for mapping glacier algae from space and extend the quantification of glacier algal blooms over southwestern Greenland back to the period 2004–2011. Several band ratio indices (MERIS chlorophyll a indices and the impurity index) were computed and compared with each other. The results indicate that the MERIS two-band ratio index (2BDA) R709 nm∕R665 nm is very effective in capturing the spatial distribution and temporal dynamics of glacier algal growth on bare ice in July and August. We analyzed the interannual (2004–2011) and summer (July–August) trends of algal distribution and found significant seasonal and interannual increases in glacier algae close to the Jakobshavn Isbrae Glacier and along the middle dark zone between the altitudes of 1200 and 1400 m. Using broadband albedo data from the Moderate Resolution Imaging Spectroradiometer (MODIS), we quantified the impact of glacier algal growth on bare ice albedo, finding a significant correlation between algal development and albedo reduction over algae-abundant areas. Our analysis indicates the strong potential for the satellite algal index to be used to reduce bare ice albedo biases in regional climate model simulations.

Highlights

Snow and ice play a critical role in regulating the global energy balance through high surface albedos (Skiles et al, 2018; Warren, 1982)
The differences illustrate that pixels classified as dark ice can have different spectral properties, and in particular differences associated with reflectance characteristics of chlorophyll a
We find that the 2BDA, 3BDA, and normalized difference chlorophyll index (NDCI) indices are most suitable for detection of chlorophyll a, given their specificity to chlorophyll a signal bands, the sensitivity of the impurity index to liquid water, and the sensitivity of the maximum chlorophyll index (MCI) index to the bare ice spectrum

Summary

Introduction

Snow and ice play a critical role in regulating the global energy balance through high surface albedos (Skiles et al, 2018; Warren, 1982). S. Wang et al.: Spatiotemporal variability of Greenland glacier algal blooms regulating surface melt processes through the reduction in snow and ice albedo resulting from dark algae pigmentation (Lutz et al, 2014; Remias et al, 2012; Stibal et al, 2017; Yallop et al, 2012). The primary glacier algal species are Ancylonema nordenskiöldii, Mesotaenium berggrenii, and Cylindrocystis brebissonii, which are green microalgae and produce pigments including chlorophyll a, chlorophyll b, beta-carotene, lutein, and violaxanthin. It has been suggested that this purpurogallin pigment accounts for the brownish-grey color of the algae-laden ice (Remias et al, 2012; Yallop et al, 2012)

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