Role of zooplankton dynamics for Southern Ocean phytoplankton biomass and global biogeochemical cycles

Abstract

Abstract. Global ocean biogeochemistry models currently employed in climate change projections use highly simplified representations of pelagic food webs. These food webs do not necessarily include critical pathways by which ecosystems interact with ocean biogeochemistry and climate. Here we present a global biogeochemical model which incorporates ecosystem dynamics based on the representation of ten plankton functional types (PFTs): six types of phytoplankton, three types of zooplankton, and heterotrophic procaryotes. We improved the representation of zooplankton dynamics in our model through (a) the explicit inclusion of large, slow-growing macrozooplankton (e.g. krill), and (b) the introduction of trophic cascades among the three zooplankton types. We use the model to quantitatively assess the relative roles of iron vs. grazing in determining phytoplankton biomass in the Southern Ocean high-nutrient low-chlorophyll (HNLC) region during summer. When model simulations do not include macrozooplankton grazing explicitly, they systematically overestimate Southern Ocean chlorophyll biomass during the summer, even when there is no iron deposition from dust. When model simulations include a slow-growing macrozooplankton and trophic cascades among three zooplankton types, the high-chlorophyll summer bias in the Southern Ocean HNLC region largely disappears. Our model results suggest that the observed low phytoplankton biomass in the Southern Ocean during summer is primarily explained by the dynamics of the Southern Ocean zooplankton community, despite iron limitation of phytoplankton community growth rates. This result has implications for the representation of global biogeochemical cycles in models as zooplankton faecal pellets sink rapidly and partly control the carbon export to the intermediate and deep ocean.

The Weddell Gyre, Southern Ocean: Present Knowledge and Future Challenges

“…Our present understanding of phytoplankton diversity and productivity in the WG, and Southern Ocean in general, is based upon bottom‐up rather than top‐down processes, in spite of the importance of grazing in the polar carbon cycle (le Quéré et al, ; Smetacek et al, ). Field experiments indicate that viral lysis of phytoplankton (both picoplankton and nanoplankton) can be relatively minor in the WG, while grazing by microzooplankton exerts significant control of phytoplankton growth (Agustí & Duarte, ; Evans & Brussaard, ; Froneman & Perissinotto, ; Henjes et al, ; Jacobsen et al, ).…”

Biogeochemical modelling of dissolved oxygen in a changing ocean

“…1c). Elevated remineralisation driven alteration to the DIC profile at intermediate depths is associated with high POC sinking speeds and model underestimation of upper ocean bacterial biomass (0 -200 m; [35]). …”

“…PlankTOM10 is a global ocean biogeochemistry model which describes lower-trophic level ecosystem dynamics explicitly based on PFTs [35,54,55]. It includes ten PFTs: picophytoplankton, N2-fixers, coccolithophores, mixed phytoplankton, diatoms, colonial Phaeocystis, bacteria, protozooplankton, mesozooplankton and macrozooplankton.…”

How much longer can Antarctica’s hostile ocean delay global warming?

“…Nor is Le Quéré convinced that the new network of floats will provide enough detail. In a paper published in July 4 , she found that models of carbon uptake by the Southern Ocean depend strongly on assumptions about the structure of the food web there. She says that climate scientists need to improve their understanding of the type and timing of phytoplankton and zooplankton blooms if they are going to get their climate projections right.…”