Diatom resting spore ecology drives enhanced carbon export from a naturally iron‐fertilized bloom in the Southern Ocean

Abstract

Southern Ocean Island systems sustain phytoplankton blooms induced by natural iron fertilization that are important for the uptake of atmospheric carbon dioxide and serve as analogues for past and future climate change. We present data on diatom flux assemblages and the biogeochemical properties of sinking particles to explain the enhanced particulate organic carbon (POC) export fluxes observed in response to natural iron supply in the Crozet Islands region (CROZeX). Moored deep‐ocean sediment traps (>2000 m) were located beneath a naturally fertilized island bloom and beneath an adjacent High Nutrient Low Chlorophyll (HNLC) control site. Deep‐ocean carbon flux from the naturally‐fertilized bloom area was tightly correlated (R = 0.83, n = 12, P < 0.0006) with the resting spore flux of a single island‐associated diatom species,Eucampia antarctica var. antarctica. The unusually well preserved state of the Eucampia‐associated carbon flux, determined by amino acid studies of organic matter degradation, was likely influenced by their ecology, since diatom resting spores are adapted to settle rapidly out of the surface ocean preserving viable cells. The naturally fertilized bloom enhanced carbon flux and the resulting Si/C and Si/N ratios were 2.0–3.4‐fold and 2.2–3.5‐fold lower than those measured in the adjacent HNLC control area. The enhanced carbon export and distinctive stoichiometry observed in naturally fertilized systems is therefore largely not attributable to iron relief of open ocean diatoms, but rather to the advection and growth of diatom species characteristic of island systems and the subsequent flux of resting spores. Carbon export estimates from current natural iron fertilization studies therefore represent a highly specific response of the island systems chosen as natural laboratories and may not be appropriate analogues for the larger Southern Ocean response. The broader implications of our results emphasize the role of phytoplankton diversity and ecology and highlight the need for a species‐centered approach in order to understand the regulation of biogeochemical fluxes.

The reproductive ecology of deep-sea ophiuroids around the Crozet plateau, Southern Indian ocean, under contrasting productivity regimes

“…At +Fe the total flux recorded by sediment traps was 2.4 times greater than at HNLC (Wolff et al 2011;Salter 2012). In addition, the period over which the flux of POM occurred extended over c. 150 days at +Fe, as opposed to just 14 days at HNLC.…”

“…Sediment trap moorings deployed at the two sites showed distinct differences with greater, and longerlasting, particulate flux at the +Fe site (Salter et al 2012). The organic carbon flux at the +Fe site was about three times greater than at the HNLC site (41.1 mmol m −2 and 14.1 mmol m −2 over 355 and 352, respectively) (Wolff et al 2011 Figure 2; Salter et al 2012). Additionally, the chemical composition of the particulate material at the +Fe site was of a better quality, with greater concentrations of labile lipids, polyunsaturated fatty acids (PUFAs) and monounsaturated fatty acids (MUFAs) (Wolff et al 2011).…”

“…Occasionally, though, mesoscale eddies spin off from the Crozet Plateau to the south in some years (Pollard et al 2007a). These eddies may lead to sporadic, short and intense POM fluxes to the seabed, as seen in early 2005 (Wolff et al 2011;Salter et al 2012). No POM flux event was evident in the sediment trap record at the HNLC site before the traps were recovered in early January 2006 (i.e.…”

“…Sediment trap moorings deployed at the two sites showed distinct differences with greater, and longerlasting, particulate flux at the +Fe site (Salter et al 2012). The organic carbon flux at the +Fe site was about three times greater than at the HNLC site (41.1 mmol m −2 and 14.1 mmol m −2 over 355 and 352, respectively) (Wolff et al 2011 Figure 2; Salter et al 2012).…”

“…The two sites were at the same depth, in the same water mass and there was no topographic barrier between them. They differed only in total POM flux and in the intensity, and potentially the regularity, of seasonal inputs (Wolff et al 2011;Salter et al 2012). …”