Climate drivers of Southern Ocean phytoplankton community composition and potential impacts on higher trophic levels

Abstract

Southern Ocean phytoplankton production supports rich Antarctic marine ecosystems comprising copepods, krill, fish, seals, penguins, and whales. Anthropogenic climate change, however, is likely to drive rearrangements in phytoplankton community composition with potential ramifications for the whole ecosystem. In general, phytoplankton communities dominated by large phytoplankton, i.e., diatoms, yield shorter, more efficient food chains than ecosystems supported by small phytoplankton. Guided by a large ensemble of Earth system model simulations run under a high emission scenario (RCP8.5), we present hypotheses for how anthropogenic climate change may drive shifts in phytoplankton community structure in two regions of the Southern Ocean: the Antarctic Circumpolar Current (ACC) region and the sea ice zone (SIZ). Though both Southern Ocean regions experience warmer ocean temperatures and increased advective iron flux under 21st century climate warming, the model simulates a proliferation of diatoms at the expense of small phytoplankton in the ACC, while the opposite patterns are evident in the SIZ. The primary drivers of simulated diatom increases in the ACC region include warming, increased iron supply, and reduced light from increased cloudiness. In contrast, simulated reductions in ice cover yield greater light penetration in the SIZ, generating a phenological advance in the bloom accompanied by a shift to more small phytoplankton that effectively consume available iron; the result is an overall increase in net primary production, but a decreasing proportion of diatoms. Changes of this nature may promote more efficient trophic energy transfer via copepods or krill in the ACC region, while ecosystem transfer efficiency in the SIZ may decline as small phytoplankton grow in dominance, possibly impacting marine food webs sustaining Antarctic marine predators. Despite the simplistic ecosystem representation in our model, our results point to a potential shift in the relative success of contrasting phytoplankton ecological strategies in different regions of the Southern Ocean, with ramifications for higher trophic levels.

Extratropical storms induce carbon outgassing over the Southern Ocean

“…Phytoplankton's photo-adaptation strategies are accounted for by variable chlorophyll-a to carbon ratios 84 , phytoplankton's growth can be limited by multiple nutrients (i.e., nitrate, ammonium, phosphate, silicate, and iron), and phytoplankton carbon losses include grazing, mortality, and aggregation. More details on the biogeochemical component of CESMv1 and validation analysis of mean patterns can be found in Krumhardt et al 85 and other references 44,[86][87][88] . The ocean in CESMv1 is also coupled to a land model (the Community Land Model version 4, CLM4 89 ) and a sea-ice model (the Community Ice Code version 4, CICE4 90 ).…”

Forty years of ocean acidification observations (1983–2023) in the Sargasso Sea at the Bermuda Atlantic Time-series Study site

“…On the longer-time scale, coccolithophore calcification in the "Great Calcite Belt" of the Southern Ocean (e.g., Balch et al, 2016) appears to be the important alkalinity control on the subtropical gyres (Krumhardt et al, 2020). If there is a reduction of coccolithophore calcification in the "Great Calcite Belt", model studies indicate increased alkalinity in the subtropical gyres (e.g., Krumhardt et al, 2022).…”

Temperature-enhanced effects of iron on Southern Ocean phytoplankton

“…Depending on the geographical region and the time in the productive season (Thomalla et al, 2023), global warming is predicted to increase wind-induced mixing or strengthen vertical stratification (Bronselaer et al, 2020;De Lavergne et al, 2014;Hillenbrand & Cortese, 2006;Shi et al, 2020). Phytoplankton will bloom earlier in the productive season as a result of decreasing sea ice and consequently higher light (Krumhardt et al, 2022), rapidly drawing down available Fe, followed by stratification, and thus favourable conditions for smaller-sized phytoplankton (Deppeler & Davidson, 2017;Krumhardt et al, 2022). Our study shows, however, that enhanced Fe input in such regions may partly overturn this warming-induced shift, assuming macronutrients will not become limiting.…”