Radium isotopes as tracers of iron sources fueling a Southern Ocean phytoplankton bloom

Elevated levels of productivity in the wake of Southern Ocean island systems are common despite the fact that they are encircled by high-nutrient low-chlorophyll (HNLC) waters. In the Crozet Plateau region, it has been hypothesized that iron from island runoff or sediments of the plateau could be fueling the austral summer phytoplankton bloom. Here, we use radium isotopes to quantify the rates of surface-ocean iron supply fueling the bloom in the Crozet Plateau region. A 1-D eddy-diffusion-mixing model applied to a 228Ra profile (t1/2=5.75 years) at a station north of the islands suggests fast vertical mixing in the upper 300 m (Kz=11–100 cm2 s−1) with slower mixing between 300 and 1000 m (Kz=1.5 cm2 s−1). This estimate is discussed in the context of Kz derived from the CTD/LADCP data. In combination with the dissolved Fe profile at this location, we estimated a vertical flux of between 5.6 and 31 nmol Fe m−2 d−1. The cross-plateau gradients in the short-lived radium isotopes, 224Ra (t1/2=3.66 d) and 223Ra (t1/2=11.4 d), yielded horizontal eddy diffusivities (Kh) of 39 and 6.6 m2 s−1, respectively. If we assume that the islands (surface runoff) alone were supplying dissolved Fe to the bloom region, then the flux estimates range from 2.3 to 14 nmol Fe m−2 d−1. If the plateau sediments are considered a source of Fe, and conveyed to the bloom region through deep winter mixing combined with horizontal transport, then this flux may be as high as 64–390 nmol Fe m−2 d−1. Combined, these Fe sources are sufficient to initiate and maintain the annual phytoplankton bloom.