Abstract
Marine aggregates formed through particle coagulation, large ones (>0.05 cm) also called marine snow, make a significant contribution to the global carbon flux by sinking from the euphotic zone, impacting the Earth’s climate. Since aggregate sinking velocity and carbon content are size-dependent, understanding the physical mechanisms controlling aggregate size distribution is fundamental to determining the biological carbon pump efficiency. Theoretical, laboratory and in-situ studies of flocculation have suggested that turbulence in the benthic boundary layer is important for aggregate formation and destruction, but the small number of field observations has limited our understanding of the role of turbulence on aggregation processes in the ocean surface layer away from energetic boundaries. Using simultaneous field observations of turbulence and aggregates, we show how aggregate formation, destruction, morphology and size distribution in the ocean surface layer (10–100 m) are mediated by interactions between turbulence and aggregate concentration. Our findings suggest that turbulence enhances aggregate formation up to a critical turbulent kinetic energy dissipation rate of 10−6 (W kg−1), above which the smallest turbulent eddies limit aggregate size.
Highlights
The conclusion from these studies[18,19] was that disaggregation by turbulence was relatively unimportant in the upper ocean
The relationship observed between turbulence and aggregates in highly localized bottom boundary layers[20,21] and energetic coastal waters[22,23] are not likely to be representative of processes occurring in the water column interior of the upper ocean that occupies most of the world ocean
Our observations provide a comprehensive set of simultaneous measurements of aggregate concentrations as a function of size that resolve the full range of turbulent intensities, ε = 10−10 – 10−6 W kg−1, found within the upper ocean away from energetic near surface and bottom boundary layers (~10–100 m depth)
Summary
The conclusion from these studies[18,19] was that disaggregation by turbulence was relatively unimportant in the upper ocean. The composition of aggregates formed in the ocean surface layer contains an increased fraction of organic material, including living and dead phytoplankton[1,2], fecal pellets[3] and extracellular polymeric substances (EPS)[29] Another important difference between laboratory experiments, energetic coastal environments (e.g., bottom boundary layer, tidal channels) and the upper water column of the open ocean is the intensity of turbulence. The relationship observed between turbulence and aggregates in highly localized bottom boundary layers[20,21] and energetic coastal waters[22,23] are not likely to be representative of processes occurring in the water column interior of the upper ocean that occupies most of the world ocean These uncertainties support the necessity of field measurements in the upper ocean to develop our understanding of the relationship between turbulence and aggregates and its implications for the biological pump under climate change[32,33]. We explore how aggregate size and other related properties, such as morphology and volume concentration, are affected by turbulence in the sunlit upper layer of the world ocean where particles are formed by primary production
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