Abstract
Snow algae are an important group of terrestrial photosynthetic organisms in Antarctica, where they mostly grow in low lying coastal snow fields. Reliable observations of Antarctic snow algae are difficult owing to the transient nature of their blooms and the logistics involved to travel and work there. Previous studies have used Sentinel 2 satellite imagery to detect and monitor snow algal blooms remotely, but were limited by the coarse spatial resolution and difficulties detecting red blooms. Here, for the first time, we use high-resolution WorldView multispectral satellite imagery to study Antarctic snow algal blooms in detail, tracking the growth of red and green blooms throughout the summer. Our remote sensing approach was developed alongside two Antarctic field seasons, where field spectroscopy was used to build a detection model capable of estimating cell density. Global Positioning System (GPS) tagging of blooms and in situ life cycle analysis was used to validate and verify our model output. WorldView imagery was then used successfully to identify red and green snow algae on Anchorage Island (Ryder Bay, 67°S), estimating peak coverage to be 9.48 × 104 and 6.26 × 104 m2, respectively. Combined, this was greater than terrestrial vegetation area coverage for the island, measured using a normalized difference vegetation index. Green snow algae had greater cell density and average layer thickness than red blooms (6.0 × 104 vs. 4.3 × 104 cells ml−1) and so for Anchorage Island we estimated that green algae dry biomass was over three times that of red algae (567 vs. 180 kg, respectively). Because the high spatial resolution of the WorldView imagery and its ability to detect red blooms, calculated snow algal area was 17.5 times greater than estimated with Sentinel 2 imagery. This highlights a scaling problem of using coarse resolution imagery and suggests snow algal contribution to net primary productivity on Antarctica may be far greater than previously recognized.
Highlights
Blooms of microalgae were noted in Antarctic snow fields by several expeditions in the early 1900s, in a period when scientific discovery became a major driving force for exploration of the continent (Hirano, 1965)
We have made the first use of high-resolution WorldView 2 and 3 imagery from 2017 to identify blooms of red and green snow algae as they grow in the snow pack on Anchorage Island, Antarctica
WorldView imagery provided a significant improvement over previous efforts using Sentinel 2 imagery, with both red and green blooms automatically detected using the methodology developed for Sentinel 2 imagery by Gray et al (2020)
Summary
Blooms of microalgae were noted in Antarctic snow fields by several expeditions in the early 1900s, in a period when scientific discovery became a major driving force for exploration of the continent (Hirano, 1965). Since these early records, blooms of red, green, and orange snow algae in Antarctica have been revealed as diverse ecosystems that play an active role in biogeochemical cycling of Remote Sensing Snow Algae Phenology nutrients and carbon (Ling and Seppelt, 1993; Dierssen et al, 2002; Hodson et al, 2008; Remias et al, 2013; Boetius et al, 2015; Davey et al, 2019; Procházková et al, 2019). The study of snow algae in Antarctica, in particular optimizing approaches to accurately monitor their ecology and influence on snow albedo, is timely and crucial to improve understanding of the effect of climatic changes on their distribution and abundance
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