Factors Controlling the Lack of Phytoplankton Biomass in Naturally Iron Fertilized Waters Near Heard and McDonald Islands in the Southern Ocean

Abstract

The Kerguelen Plateau is one of the regions in the Southern Ocean where spatially large algal blooms occur annually due to natural iron fertilization. The analysis of ocean color data as well as in situ samples collected during the Heard Earth-Ocean-Biosphere Interactions (HEOBI) voyage in January and February 2016, surprisingly revealed that chlorophyll a concentrations in waters located close to Heard and McDonald islands were much lower than those on the central Kerguelen Plateau. This occurs despite high levels of both glacial and volcanic iron supply from these islands. The analysis of pigment and optical data also indicated a shift in the phytoplankton size structure in this region, from a microphytoplankton to nanophytoplankton dominated community. Possible explanations for this high nutrient, high iron (Fe), low chlorophyll (HNHFeLC) phenomenon were explored. Low light availability due to deep mixing and shading by re-suspended sediment particles and augmented by dilution with surrounding low chlorophyll waters in the Antarctic Circumpolar Current was shown to be an important mechanism shaping phytoplankton communities. The competing dynamics between stimulation and limitation illustrate the complexity of short-term responses to our changing climate and cryosphere.

Glacial and Hydrothermal Sources of Dissolved Iron (II) in Southern Ocean Waters Surrounding Heard and McDonald Islands

“…Ascertaining the exact mechanism of the glacial Fe(II) production was outside the scope of this study. However, glacial flour observed at station 24 (van der Merwe et al, 2019) and high concentrations of non-algal particles (Wojtasiewicz et al, 2019) silicates, as has been demonstrated in previous studies (Shoenfelt et al, 2019). These could be important mechanisms in maintaining the observed DFe(II) and DFe enrichment at Heard…”

“…Ascertaining the exact mechanism of the glacial DFe (II) production was outside the scope of this study. However, glacial flour observed at station 24 (van der Merwe et al, 2019) and high concentrations of nonalgal particles (Wojtasiewicz et al, 2019) may indicate that DFe (II) production was the result of physical weathering of underlaying basalt by the glacier (van der Merwe et al, 2019). High silicate concentrations observed at this site associated with lower salinity water (van der Merwe et al, 2019) may also indicate that physical weathering resulted in mobilization of Fe (II) silicates, as has been demonstrated in a previous study of SO marine sediment cores (Shoenfelt et al, 2019).…”

“…A reference site (station 18) was sampled ~100 km to the south of Heard Island, located in waters more representative of HNLC conditions (max 0.81 mg m −3 Chl a in upper 200 m; Wojtasiewicz et al, 2019). Mean currents at this station were associated with the Fawn Trough Current, moving from west (open ocean) to east (plateau; Figure 1; Park et al, 2014).…”

“…Sediments are another potential DFe(II) source in the HIMI region. Relatively high particle backscattering was observed around HIMI (Wojtasiewicz et al, 2019) and wind-driven mixing events often reached the seafloor around HIMI during the voyage resulting in a wellmixed water column homogeneous in properties such as temperature and salinity (R.…”

“…Sediments are another potential DFe (II) source in the HIMI region. Relatively high particle backscattering was observed around HIMI (Wojtasiewicz et al, 2019), and wind-driven mixing events often reached the seafloor around HIMI during the voyage resulting in a well-mixed water column homogeneous in properties such as temperature and salinity (R. Robertson, personal communication, 2016). This mixing could disturb upper oxidizing sediment layers, resuspend sediments, and potentially facilitate the escape of Fe (II) from underlying anoxic sediments (e.g., Elrod et al, 2004).…”