Abstract
Within the framework of the Marine Ecosystem Assessment for the Southern Ocean (MEASO), this paper brings together analyses of recent trends in phytoplankton biomass, primary production and irradiance at the base of the mixed layer in the Southern Ocean and summarises future projections. Satellite observations suggest that phytoplankton biomass in the mixed-layer has increased over the last 20 years in most (but not all) parts of the Southern Ocean, whereas primary production at the base of the mixed-layer has likely decreased over the same period. Different satellite models of primary production (Vertically Generalised versus Carbon Based Production Models) give different patterns and directions of recent change in net primary production (NPP). At present, the satellite record is not long enough to distinguish between trends and climate-related cycles in primary production. Over the next 100 years, Earth system models project increasing NPP in the water column in the MEASO northern and Antarctic zones but decreases in the Subantarctic zone. Low confidence in these projections arises from: (1) the difficulty in mapping supply mechanisms for key nutrients (silicate, iron); and (2) understanding the effects of multiple stressors (including irradiance, nutrients, temperature, pCO2, pH, grazing) on different species of Antarctic phytoplankton. Notwithstanding these uncertainties, there are likely to be changes to the seasonal patterns of production and the microbial community present over the next 50–100 years and these changes will have ecological consequences across Southern Ocean food-webs, especially on key species such as Antarctic krill and silverfish.
Highlights
Primary production by microalgae communities is the foundation of Southern Ocean food-webs (Deppeler and Davidson, 2017; Boyd et al, 2019), providing the organic matter that sustains Antarctica’s unique marine ecosystems (Loeb et al, 1997; Atkinson et al, 2004; Hill et al, 2006; Mock et al, 2017; Boyd et al, 2019; McCormack et al, in review)
Year-to-year variability in primary production in the Southern Ocean will likely increase with increasing prevalence of marine heatwaves
The importance of local-scale forcing means that forecasting the effects of climate change on coastal primary production
Summary
Primary production by microalgae communities is the foundation of Southern Ocean food-webs (Deppeler and Davidson, 2017; Boyd et al, 2019), providing the organic matter that sustains Antarctica’s unique marine ecosystems (Loeb et al, 1997; Atkinson et al, 2004; Hill et al, 2006; Mock et al, 2017; Boyd et al, 2019; McCormack et al, in review). Increases of atmospheric carbon dioxide (CO2) from ∼400 μatm today beyond 750 μatm by 2100 will likely lead to multifaceted environmental change in the Southern Ocean, including upper-ocean warming, ocean acidification (OA), changes to incident irradiance, increased vertical mixing in the water column, less sea-ice and changed patterns of nutrient input (including iron) (IPCC, 2019; Henley et al, 2020) These environmental and oceanographic changes will affect microbial community composition, patterns of primary production and ecological pathways in Southern Ocean marine ecosystems (Le Quéré et al, 2016; Schofield et al, 2017; Deppeler and Davidson, 2017; Freeman et al, 2019; Johnston et al, in review). Microzooplankton grazing, which can reduce NPP by facilitating nutrient loss through the sinking of particulate detritus (e.g., Cadée et al, 1992; Perissinotto and Pakhomov, 1998; Vernet et al, 2011), will be affected by climate change through changes to grazing rates (Sarmento et al, 2010; Caron and Hutchins, 2013; Behrenfeld, 2014; Biermann et al, 2015; Cael and Follows, 2016) and phytoplankton nutrient density (Finkel et al, 2010; Hixson and Arts, 2016)
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