Abstract
Climate change at the Western Antarctic Peninsula (WAP) is predicted to cause major changes in phytoplankton community composition, however, detailed seasonal field data remain limited and it is largely unknown how (changes in) environmental factors influence cell size and ecosystem function. Physicochemical drivers of phytoplankton community abundance, taxonomic composition and size class were studied over two productive austral seasons in the coastal waters of the climatically sensitive WAP. Ice type (fast, grease, pack or brash ice) was important in structuring the pre-bloom phytoplankton community as well as cell size of the summer phytoplankton bloom. Maximum biomass accumulation was regulated by light and nutrient availability, which in turn were regulated by wind-driven mixing events. The proportion of larger-sized (> 20 µm) diatoms increased under prolonged summer stratification in combination with frequent and moderate-strength wind-induced mixing. Canonical correspondence analysis showed that relatively high temperature was correlated with nano-sized cryptophytes, whereas prymnesiophytes (Phaeocystis antarctica) increased in association with high irradiance and low salinities. During autumn of Season 1, a large bloom of 4.5-µm-sized diatoms occurred under conditions of seawater temperature > 0 °C and relatively high light and phosphate concentrations. This bloom was followed by a succession of larger nano-sized diatoms (11.4 µm) related to reductions in phosphate and light availability. Our results demonstrate that flow cytometry in combination with chemotaxonomy and size fractionation provides a powerful approach to monitor phytoplankton community dynamics in the rapidly warming Antarctic coastal waters.
Highlights
Global climate change will have consequences for marine ecosystems throughout the world’s oceans (Boyce et al 2010; Hallegraeff 2010)
Definitions for ice type were as follows: fast ice is a solid sheet of ice attached to the land, pack ice is a large area of sea ice that is not land fast, brash ice is small fragments of floating ice, grease ice is a very thin layer of frazil ice and pancake ice is round pieces of newly formed ice
At the beginning of S2, grease ice was the dominated ice type, but ice cover rapidly changed to pack ice and declined in extent to 10% by 17th November
Summary
Global climate change will have consequences for marine ecosystems throughout the world’s oceans (Boyce et al 2010; Hallegraeff 2010). Decreased ice cover has been associated with a reduction in photosynthetic efficiency (Schofield et al 2018) and several studies have described a reduction in overall phytoplankton biomass and a shift from large phytoplankton (diatoms) to smaller flagellated species. These shifts have been associated with reduced salinities, higher temperatures and stronger vertical stratification (Moline and Prezelin 1996; Moline et al 2004; MontesHugo et al 2009; Venables et al 2013; Mendes et al 2017; Rozema et al 2017c). Our current understanding of the drivers associated with the seasonal progression within the phytoplankton community (Henley et al 2019), the pico- (≤ 3 μm) and nano-sized (> 3–20 μm) phytoplankton, limits our ability to accurately predict how polar marine systems will respond to future climate change with implications for food-web dynamics and the marine carbon cycle (e.g. Pinhassi et al 2004; Conan et al 2007; Christaki et al 2008; Obernosterer et al 2008; Agustí and Duarte 2013; Evans et al 2017)
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