Wind-driven upwelling of iron sustains dense blooms and food webs in the eastern Weddell Gyre

Abstract

The Southern Ocean is a major sink of anthropogenic CO₂ and an important foraging area for top trophic level consumers. However, iron limitation sets an upper limit to primary productivity. Here we report on a considerably dense late summer phytoplankton bloom spanning 9000 km² in the open ocean of the eastern Weddell Gyre. Over its 2.5 months duration, the bloom accumulated up to 20 g C m^-2 of organic matter, which is unusually high for Southern Ocean open waters. We show that, over 1997-2019, this open ocean bloom was likely driven by anomalies in easterly winds that push sea ice southwards and favor the upwelling of Warm Deep Water enriched in hydrothermal iron and, possibly, other iron sources. This recurring open ocean bloom likely facilitates enhanced carbon export and sustains high standing stocks of Antarctic krill, supporting feeding hot spots for marine birds and baleen whales.

Fig. 1. A large phytoplankton bloom in the eastern Weddell Gyre.

a Sentinel satellite image of the open ocean phytoplankton bloom on March 8, 2019. The positions of underway water samples are indicated where measurements of phytoplankton physiology and microscopic counts and identification of phytoplankton species were carried out (inverted black triangles). Also indicated are the 100 m depth integrated POC (g m⁻²) for each glider profile and the locations of Maud Rise, Astrid Ridge and the Fimbul Ice Shelf. b Upper 100 m integrated POC (g m⁻²) and Chlorophyll a (mg m⁻²) from each glider profile, c contoured Chl a (mg m⁻³), d contoured POC (mg m⁻³), and e contoured temperature (°C) for the glider transect. In panels c–e, the black line with crosses indicates the depth of the surface Mixed Layer.

Fig. 2. Composite anomalies in the presence and absence of the open ocean bloom.

a Mean satellite-derived Chl a (at 8-day intervals) averaged over 4°–8° E and 67.8°–68.4° S between 1997 and 2019 (all circles). Red circles correspond to when the mean satellite derived Chl a was larger than the 23-year long mean Chl a ± 1 standard deviation averaged over that area (i.e., 1.14 mg m⁻³) and are considered as bloom occurrences. Composite of b the Chl a concentration, d the zonal wind component and f sea-ice concentration de-seasoned anomalies (mean seasonal cycle removed) in the presence of a bloom (the weeks/years indicated by red circles in (a)) between 4°–8° E and 67.8°–68.4° S. Composite of c the Chl a concentration, e the zonal wind component and g sea-ice concentration de-seasoned anomalies in the absence of a bloom (the weeks indicated by blue circles in (a)) between 4°–8° E and 67.8°–68.4° S. In panels b–g, the black cross denotes the position of the glider deployment.

Fig. 3. Hydrothermal influence in the Eastern Weddell Gyre.

a Map of the locations of isotopic helium (δ³He) data in the water column from the GLODAP database and the Transektokt 2020/21, at or close to the Prime Meridian (green squares), following a transect along 6° East (red squares) and a transect along 30° East (blue squares). Also indicated by colored lines and arrows are the tectonics plates boundaries (black line) and a sketched circulation of the eastern Weddell Gyre from Ryan et al. who described a two-core pathway of the northeastern inflow of WDW at about 20° E, the northern pathway being driven by eddy mixing in the northeastern corner of the Weddell Gyre (brown arrow), the southern one being an advective route which forms the southern branch of the inflow and extends beyond 30° E before turning westward (red arrow). The yellow arrow indicates the predominantly westward coastal flow regime south of 65° S. The presence of the bloom is indicated by a green oval. The yellow star indicates the presence of hydrothermal vents at the Southwest Indian Ridge. Contoured and scatter plots of δ³He (%) at or close to the Prime Meridian (b), following a transect along 6° East (c), and along 30° East (d). Density isopycnal contours are also indicated. Note the different latitude range used for the 6° East transect.

Fig. 4. The open ocean bloom attracted a rich ecosystem in the open Southern Ocean.

a Abundance of krill (nautical area scattering coefficient (NASC) values integrated over 500 m distance) retrieved from the ship’s echosounders during the ecosystem cruise. Also indicated are the two krill trawling stations inside the bloom area (red diamonds). b Abundance of humpback whales (number of individuals per sighting event) and Antarctic and Snow petrels (total daily sightings) observed during the ecosystem cruise. c Heat map of the utilization distribution, (i.e., the probability density that an animal is found at a point according to its geographical coordinates) of Svarthamaren Antarctic petrels averaged over the summers 2012, 2013, 2014, 2016 and 2018 (see “Methods”). In panels a and b, the Sentinel satellite image of the open ocean phytoplankton bloom on March 8th, 2019 is used.