Mesoscale Variability of Conditions Favoring an Iron‐Induced Diatom Bloom Downstream of the Kerguelen Plateau

Abstract

Heterogeneity in phytoplankton distribution is related to spatial and temporal variations in biogeochemical and ecological processes. In the open ocean, the interaction of these processes with meso‐ and submeso‐scale dynamics (1‐100 km, few days to months) gives rise to complex spatio‐temporal patterns, whose characterization is difficult without extensive sampling efforts. In this study, we integrate pigment sampling and multisatellite data to assess the link between iron enrichment and diatom dominance in the open ocean region east of the Kerguelen Islands (Indian Sector of the Southern Ocean). In this region, the High Nutrient Low Chlorophyll conditions typical of the Southern Ocean are alleviated by the transport of iron off the Kerguelen Plateau, resulting in a plume of chlorophyll that extends 1,000 km downstream. We show that in situ concentrations of the diatom‐associated pigment fucoxanthin and ocean‐color‐derived estimates of diatom dominance correlate with the “water age”, i.e., the time since the respective water parcel departed the Kerguelen Plateau. We propose a “threshold model” linking diatom ecological success and iron availability of downstream‐advected water parcels. The pattern of diatom dominance generated by this model predicts the extent and spatial structure of satellite‐based estimates at the regional scale (∼100s of km) and describes the mesoscale distribution of diatom dominance in the proximity of the plateau. However, the complexity of diatom dominance patterns further away from the plateau indicates that other physical and ecological mechanisms may drive phytoplankton dominance downstream.

Global Connectivity of Southern Ocean Ecosystems

“…However, latitudinal movements and variation of frontal positions and associated meandering and eddy shedding, particularly within the Polar Frontal Zone (PFZ), facilitate cross-frontal exchanges (Lutjeharms and Baker, 1980;Bryden, 1983;Heywood et al, 1985;Lutjeharms et al, 1985;Pakhomov et al, 1997;Chiba et al, 2001;Hunt et al, 2001Hunt et al, , 2002Bernard et al, 2007;Foppert et al, 2017). Eddies that drift some distance from their origin provide an effective means of transporting faunas into different water masses (Atkinson et al, 1990;Bernard et al, 2007) and can enhance zooplankton productivity and abundance, particularly at their peripheries (Bernard et al, 2007;Della Penna et al, 2018). Planktonic larvae of benthic species can cross such frontal zones in eddies (Clarke et al, 2005), and also have new and increasing passive opportunities on floating debris such as plastic (Barnes, 2002;Horton and Barnes, 2020).…”

Marked spatiotemporal variations in small phytoplankton structure in contrasted waters of the Southern Ocean (Kerguelen area)

“…As chain-forming and large-diatom blooms occur during relatively short periods, it has been hypothesized that smallsize cells are an essential element of Antarctic food webs, especially during winter and periods preceding blooms (Detmer and Bathmann 1997). Satellite data and sampling has indicated that phytoplankton communities on the plateau switch to an ecosystem dominated by small phytoplankton before (Rembauville et al 2017), and also likely, after the bloom (Penna et al 2018). As such, the plateau of Kerguelen could also include a persistent and functionally important small phytoplankton community, on which diatom blooms superimpose themselves (Smetacek et al 1990).…”

Physical Drivers of Biogeochemical Variability in the Polar Front Meander

“…This area is far away from terrestrial iron sources and the influence of sea ice (Bowie et al, 2009;Cavalieri et al, 1996;Lannuzel et al, 2011), suggesting that these eddies are produced without iron input from any islands (Moreau et al, 2017). This is in contrast to the situation where high chlorophyll eddies observed near islands like the Kerguelen Plateau, have productivity that is likely enhanced by terrestrial iron (Della-Penna et al, 2018;Grenier et al, 2015).…”