Abstract
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 VT 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.
Highlights
Biological particle production and sedimentation out of the euphotic layer to underlying waters is the major mechanisms for atmospheric CO2 removal and the redistribution of carbon and associated nutrients in the ocean
The water column was characterized by a deep mixed layer (∼ 150 m) during the pre-bloom and early bloom surveys, with a range of 120 to 171 m (Figs. 1 and 2)
The chlorophyll a (Chl a) profile determined using bottle samples for Station A3-2 was characterized by a subsurface maximum at 170 m, at the bottom of the mixed layer (Fig. 3)
Summary
Biological particle production and sedimentation out of the euphotic layer to underlying waters is the major mechanisms for atmospheric CO2 removal and the redistribution of carbon and associated nutrients in the ocean. The fate of this exported particulate carbon is a function of the plankton community producing it in the upper layer and particle transformation by microbes and zooplankton during their descent to the deep sea. Physical aggregation of particles is one key process in this transformation and transport and can explain the rapid formation and export of large particles during bloom conditions. Since the HNLC regions result from low supplies of the crucial nutrient iron, the hypothesis is that these blooms are supported by natural sources of iron, most likely supplied from local islands and shallow sediment (Moore and Abbott, 2002; Tyrrell et al, 2005; Blain et al, 2007; Pollard et al, 2007)
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