Rapid formation of large aggregates during the spring bloom of Kerguelen Island: observations and model comparisons

Type Article
Date 2014-08-20
Language English
Author(s) Jouandet M. -P.1, Jackson G. A.2, Carlotti F.1, Picheral M.3, Stemmann L.4, Blain S.5, 6
Affiliation(s) 1 : Aix Marseille Univ, CNRS, IRD, MIO,Unite Mixte, F-13288 Marseille 09, France.
2 : Texas A&M Univ, Dept Oceanog, College Stn, TX 77845 USA.
3 : CNRS, UMR7093, LOV, Observ Oceanol, F-06230 Villefranche Sur Mer, France.
4 : Univ Paris 06, Sorbonne Univ, UMR7093, Observ Oceanol,LOV, F-06230 Villefranche Sur Mer, France.
5 : Univ Paris 06, Sorbonne Univ, UMR7621, Lab Oceanog Microbienne,Observ Oceanol, F-66650 Banyuls Sur Mer, France.
6 : CNRS, UMR7621, Observ Oceanol, Lab Oceanog Microbienne, F-66650 Banyuls Sur Mer, France.
Source Biogeosciences (1726-4170) (Copernicus Gesellschaft Mbh), 2014-08-20 , Vol. 11 , N. 16 , P. 4393-4406
DOI 10.5194/bg-11-4393-2014
WOS© Times Cited 20
Note Special issue KEOPS2: Kerguelen Ocean and Plateau Study 2
Abstract While production of aggregates and their subsequent sinking is known to be one pathway for the downward movement of organic matter from the euphotic zone, the rapid transition from non-aggregated to aggregated particles has not been reported previously. We made one vertical profile of particle size distributions (PSD; sizes ranging from 0.052 to several millimeters in equivalent spherical diameter) at pre-bloom stage and seven vertical profiles 3 weeks later over a 48 h period at early bloom stage using the Underwater Vision Profiler during the Kerguelen Ocean and Plateau Compared Study cruise 2 (KEOPS2, October-November 2011). In these naturally iron-fertilized waters southeast of Kerguelen Island (Southern Ocean), the total particle numerical abundance increased by more than fourfold within this time period. A massive total volume increase associated with particle size distribution changes was observed over the 48 h survey, showing the rapid formation of large particles and their accumulation at the base of the mixed layer. The results of a one-dimensional particle dynamics model support coagulation as the mechanism responsible for the rapid aggregate formation and the development of the V-T subsurface maxima. The comparison of VT profiles between early bloom stage and pre-bloom stage indicates an increase of particulate export below 200 m when bloom has developed. These results highlight the role of coagulation in forming large particles and triggering carbon export at the early stage of a naturally iron-fertilized bloom, while zooplankton grazing may dominate later in the season. The rapid changes observed illustrate the critical need to measure carbon export flux with high sampling temporal resolution. Our results are the first published in situ observations of the rapid accumulation of marine aggregates and their export and the general agreement of this rapid event with a model of phytoplankton growth and coagulation.
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