Sustained Upwelling of Subsurface Iron Supplies Seasonally Persistent Phytoplankton Blooms Around the Southern Kerguelen Plateau, Southern Ocean

Abstract

Although the supply of iron generally limits phytoplankton productivity in the Southern Ocean, substantial seasonal blooms are observed over and downstream of the Kerguelen plateau in the Indian sector of the Southern Ocean. Surprisingly, of the oceanic blooms, those associated with the deeper southern plateau last much longer (~3 months) than the northern bloom (~1‐month downstream of northern plateau). In this study, iron supply mechanisms around the southern plateau were investigated, obtaining profiles of dissolved iron (<0.2 μm, dFe) to 2,000‐m deep at 25 stations during austral summer 2016. The dFe concentrations in surface waters (≤100‐m depth) ranged from below the detection limit (DL, median of 0.026 nmol/kg) to 0.34 nmol/kg near the Antarctic shelf, with almost half the data points below detection. These low and—with few exceptions—largely spatially invariant concentrations, presumably driven by seasonal drawdown of this essential micronutrient by phytoplankton, could not explain observed patterns in chlorophyll a. In contrast, dFe concentrations (0.05–1.27 nmol/kg) in subsurface waters (100–800 m) showed strong spatial variations that can explain bloom patterns around the southern Kerguelen plateau when considered in the context of frontal locations and associated frontal processes, including upwelling, that may increase the upward supply of dFe in the region. This sustained vertical dFe supply distinguishes the southern blooms from the bloom downstream of the northern Kerguelen plateau and explains their persistence through the season.

Southern Ocean Phytoplankton Blooms Observed by Biogeochemical Floats

“…The second float (Float ID: 5904683) is located downstream of the Kerguelan Plateau and drifts along the Subantactic Front (SAF; Figures b and S3a). This is a region of vigorous eddy activity, and the presence of the Kerguelan Islands and hydrothermal vents can act as a source of lithogenic/benthic iron here (Ardyna et al, ; Blain et al, ; Bowie et al, ; Gille et al, ; Schallenberg et al, ). This float happened to see a short‐lived period of growth in July, but the onset to apex is from August till the end of December.…”

Decoupling Between Phytoplankton Growth and Microzooplankton Grazing Enhances Productivity in Subantarctic Waters on Campbell Plateau, Southeast of New Zealand

“…Several physical processes including internal waves and increased vertical eddy diffusivity (Blain et al, ), upwelling (de Baar et al, ; Korb & Whitehouse, ; Schallenberg et al, ), or vertical mixing (Lam & Bishop, ; Tagliabue et al, ; Tyrrell et al, ) have been shown to enhance vertical iron supply to surface HNLC waters. The SAW on the plateau are characterized by weak stratification and are prone to deep winter mixing caused by the strong thermal contrasts and winds that are characteristic of the region (Morris et al, ; Neil et al, ; and references therein).…”

Trophodynamics of Southern Ocean pteropods on the southern Kerguelen Plateau

“…Linear regressions revealed a statistically significant positive relationship between δ 13 C and longitude for C. pyramidata, which may relate to the cluster of enriched values overlapping the same region (just north of the sACCf on the Plateau) we measured high abundances of this species, toward the eastern flank of the Plateau. Particulate organic nitrogen (PON) measurements revealed enriched values along this region, indicating nitrate uptake which suggests new production was occurring here (Schallenberg et al, ). However, we also detected a statistically significant negative relationship between δ 13 C and longitude for C. antarctica , whose highest abundances generally overlapped with those of C. pyramidata .…”

“…Regions along the transect where abundances were lowest (between 70°E to 80°E) overlapped with the lowest δ 13 C and δ 15 N values measured for all species, combined. Incidentally, this region, which covers the western area of the Plateau, also measured the most 15 N‐depleted in PON, suggesting nitrogen recycling and ammonia uptake (Schallenberg et al, ).…”