Massive circumpolar biomass of Southern Ocean zooplankton: Implications for food web structure, carbon export, and marine spatial planning

Abstract

With rapid, sector‐specific climatic changes impacting the Southern Ocean, we need circumpolar‐scale biomass data of its plankton taxa to improve food web models, blue carbon budgets and resource management. Here, we provide a new dataset on mesozooplankton biomass with 2909 records spanning the last 90 yr, and describe, in comparable carbon units, their circumpolar distribution alongside those of phytoplankton, Antarctic krill, and salps. With our datasets, we estimate total summer carbon biomasses for phytoplankton (36 MT), mesozooplankton (67 MT), krill (30 MT), and salps (1.7 MT). The mesozooplankton value is much higher than previously reported and, added to that of krill and salps, points to an enormous overall biomass of zooplankton in the Southern Ocean. This means that the pyramids of biomass are often inverted, with higher biomass of zooplankton than of phytoplankton. Such high biomasses suggest key roles of grazers in nutrient cycling and we estimate an export of ~ 50 Mt C yr−1, solely from mortality of overwintering zooplankton that typically reside at depth. Deep lipid respiration (the lipid pump), for example, would increase this export even further. While inverted biomass pyramids prevailed at mid latitudes (50°–70°S), the balance of taxa differed regionally: for example, with biomass dominance by phytoplankton (highest latitudes and Pacific sector), mesozooplankton (Kerguelen Plateau), krill (north and east Scotia Sea), and salps (Crozet area). In light of contrasting climate change impacts between these sectors, we provide data that will underpin biogeochemical and food web models, blue carbon budgets, and the planning of marine protected areas.

Valuing carbon sequestration by Antarctic krill faecal pellets

“…Our study does though highlight the importance of Antarctic krill, and likely other components of the biological pump, in sequestering carbon which is on par with coastal blue carbon stores. Evidence from this study and others 14,38,39 are increasingly showing the importance of krill and other zooplankton in structuring Southern Ocean food webs and activating a strong biological pump. Understanding the relative roles of the key phyto-, zooplankton and micronekton functional groups in carbon sequestration is therefore an urgent priority.…”

Climate-driven variability of the Southern Ocean CO 2 sink

“…Current generation GOBMs represent ecosystems that are largely driven by upwelled nutrients and do not yet include the more complex ecosystem responses, such as vertical migrations and salps/krill dipole that characterize the Southern Ocean (e.g. [ 57 ]). The importance of these processes for the variability in CO 2 and O 2 fluxes has not yet been examined.…”

Omnivorous summer feeding by juvenile Antarctic krill in coastal waters

“…Krill in the Scotia Sea and near South Georgia during January-February had the highest 15 N enrichment in glutamic acid and alanine relative to phenylalanine (Schmidt et al 2006) (Supplementary Fig. S11), possibly attributable to increased regional abundance of mesozooplankton prey (Yang et al 2022) and the inclusion of larger krill, which occupy higher trophic levels (Polito et al 2013). Female krill collected near the South Shetland Islands in late summer (March) exhibited higher relative 15 N enrichment in glutamic acid compared to alanine when both were normalized to δ 15 N Phe (Schmidt et al 2004) (Supplementary Fig.…”