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 colour 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.
Highlights
Phytoplankton are responsible for more than 45% of the global primary production (Field et al, 1998) and the sequestration of 5–10 Gt of carbon per year in the ocean through the biological pump and play an important role in controlling the concentration of atmospheric carbon dioxide
Based on observations performed during the Heard Earth-Ocean-Biosphere Interactions (HEOBI) voyage, we investigated a number of possible controls on biomass accumulation and changes in the phytoplankton community structure, including dilution of biomass, light limitation enhanced by the presence of resuspended sediments in the coastal shallow areas as well as the removal of phytoplankton by zooplankton grazing
The median chl-a inventories at transect B and C stations were 114.0 and 54.5 mg m−2, respectively, whereas for Heard and McDonald Islands they were 17.1 and 20.3 mg m−2, respectively (Figure 7C). This difference between the MLD vs. full water column chlorophyll inventories arises because significant portions of the phytoplankton biomass at the transect B and C stations lies below the shallow mixed layer depths defined by the 0.2◦C temperature threshold criterion
Summary
Phytoplankton are responsible for more than 45% of the global primary production (Field et al, 1998) and the sequestration of 5–10 Gt of carbon per year in the ocean through the biological pump and play an important role in controlling the concentration of atmospheric carbon dioxide. In some regions, e.g., downstream of islands or in coastal polynyas, surface waters are naturally fertilized by iron, relaxing the iron limitation and leading to the occurrence of extensive phytoplankton blooms in Phytoplankton in Iron-Fertilized Region these areas This iron supply can come from a number of different sources, including a small input from atmospheric deposition (e.g., Jickells et al, 2005; Mahowald et al, 2005), entrainment from the deep ocean during winter mixing (e.g., Blain et al, 2008; Tagliabue et al, 2014), riverine input associated with snowmelt (van der Merwe et al, 2015), melting of sea ice and icebergs (Sedwick and DiTullio, 1997), release from sediments (e.g., de Jong et al, 2012; Hatta et al, 2013), and underwater hydrothermal activity (Holmes et al, 2017 and references therein). In cases when silica was limiting, no transition to diatom domination could be observed (e.g., Coale et al, 2004; Peloquin et al, 2011)
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