Abstract
Abstract. A massive diatom bloom is observed each year in the surface waters of the naturally Fe-fertilized Kerguelen Plateau (Southern Ocean). We measured biogenic silica production and dissolution fluxes (ρSi and ρDiss, respectively) in the mixed layer in the vicinity of the Kerguelen Plateau during austral spring 2011 (KEOPS-2 cruise). We compare results from a high-nutrient low-chlorophyll reference station and stations with different degrees of iron enrichment and bloom conditions. Above the plateau biogenic ρSi are among the highest reported so far in the Southern Ocean (up to 47.9 mmol m−2 d−1). Although significant (10.2 mmol m−2 d−1 on average), ρDiss were generally much lower than production rates. Uptake ratios (ρSi : ρC and ρSi : ρN) confirm that diatoms strongly dominate primary production in this area. At the bloom onset, decreasing dissolution-to-production ratios (D : P) indicate that the remineralization of silica could sustain most of the low silicon uptake and that the system progressively shifts toward a silica production regime which must be mainly supported by new source of silicic acid. Moreover, by comparing results from the two KEOPS expeditions (spring 2011 and summer 2005), we suggest that there is a seasonal evolution of the processes decoupling Si and N cycles in the area. Indeed, the consumption of H4SiO4 standing stocks occurs only during the growing stage of the bloom when strong net silica production is observed, contributing to higher H4SiO4 depletion relative to NO3−. Then, the decoupling of H4SiO4 and NO3− is mainly controlled by the more efficient nitrogen recycling relative to Si. Gross Si : N uptake ratios were higher in the Fe-rich regions compared to the high-nutrient low-chlorophyll (HNLC) area, likely due to different diatom communities. This suggests that the diatom responses to natural Fe fertilization are more complex than previously thought, and that natural iron fertilization over long timescales does not necessarily decrease Si : N uptake ratios as suggested by the silicic acid leakage hypothesis. Finally, we propose the first seasonal estimate of the Si biogeochemical budget above the Kerguelen Plateau based on direct measurements. This study points out that naturally iron-fertilized areas of the Southern Ocean could sustain very high regimes of biogenic silica production, similar to those observed in highly productive upwelling systems.
Highlights
Covering 20 % of the World Ocean, the Southern Ocean is considered a crucial component of the climate system since it represents a net sink for atmospheric CO2 (Takahashi et al, 2009)
The marine Si biogeochemical cycle is dominated by biogenic silica production and dissolution in the surface mixed layer, and one atom of Si will undergo a cycle of biological uptake by diatoms and subsequent dissolution about 25 times before being removed to the seabed (Tréguer and De La Rocha, 2013)
The KErguelen Ocean and Plateau compared Study (KEOPS)-2 cruise was conducted in the Indian sector of the Southern Ocean during the austral spring 2011 on board the R/V Marion Dufresne (TAAF/IPEV) and was focused on the ironfertilized blooms observed around the Kerguelen Plateau region
Summary
Covering 20 % of the World Ocean, the Southern Ocean is considered a crucial component of the climate system since it represents a net sink for atmospheric CO2 (Takahashi et al, 2009) It plays a key role in the global silicon (Si) biogeochemical cycle because diatoms, a siliceous phytoplankton group, are one of the major primary producers in Published by Copernicus Publications on behalf of the European Geosciences Union. I. Closset et al.: Seasonal evolution of net and regenerated silica production this area (Buesseler et al, 2001; Quéguiner and Brzezinski, 2002; Tréguer and De la Rocha, 2013). Closset et al.: Seasonal evolution of net and regenerated silica production this area (Buesseler et al, 2001; Quéguiner and Brzezinski, 2002; Tréguer and De la Rocha, 2013) As their cell wall is composed of biogenic silica The number of D : P estimates, due to a small number of Si uptake measurements and an even lower number of Si dissolution measurements, is insufficient compared to the high variability observed regionally and seasonally in the ocean, implying high uncertainty of the global D : P estimate and of the marine silicon budget overall
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