Abstract
Gas exchange reduction (GER) at the air-sea interface is positively related to the concentration of organic matter (OM) in the top centimetre of the ocean, as well as to phytoplankton abundance and primary production. The mechanisms relating OM to GER remain unclear, but may involve mechanical (rheological) damping of turbulence in the water immediately below the surface microlayer, damping of ripples and blocking of molecular diffusion by layers of OM, as well as electrical effects. To help guide future research in GER, particularly of CO2, we review published rheological properties of ocean water and cultures of phytoplankton and bacteria in both 3D and 2D deformation geometries, in water from both the surface layer and underlying water. Production of foam modulates air-sea exchange of many properties and substances, perhaps including climate-changing gases such as CO2. We thus also review biological modulation of production and decay of whitecaps and other sea foam. In the ocean literature on biological production of OM, particularly that which associates with the sea surface, the terms “surfactant” and “surface-active” have been given a variety of meanings that are sometimes vague, and may confuse. We therefore propose a more restricted definition of these terms in line with usage in surface science and organic chemistry. Finally, possible changes in OM-modulated GER are presented in relation to predicted global environmental changes.
Highlights
For over a century, CO2 levels in the atmosphere have been increasing at an accelerating rate
As addition of oil was observed to spread rapidly on the surface to form what could be calculated as a molecular monolayer, early models of ripple and wave damping, as well as gas exchange by oils, solvents and natural organic matter (OM) were based on the concept of a monolayer of OM at the surface
A few nanometres above the surface microlayer (SML), between the water surface and the air, sits an extremely hydrophobic layer. The dynamics of this layer may be affected by factors such as turbulence in both the air and the water, as well as density stratification and the composition and vertical distribution of hydrophilic, hydrophobic and surfactant molecules in the SML
Summary
CO2 levels in the atmosphere have been increasing at an accelerating rate. After first clarifying the sometimes conflicting meanings of the key terms “surfactant” and “surface-active” (see Table 1 for a glossary of other terms used in this review), we treat OM in the oceans, in association with the SML. The third pool consists of less than 1% of the total OM in the ocean, but it is very labile and so represents a large C flux (Hansell et al, 2009) We suggest that this fraction, generally close in space and time to areas of high productivity, is the most rheologically active, both in the SML (Pogorzelski et al, 2005) and in the bulk phase (Jenkinson and Sun, 2010; Jenkinson et al, 2015)
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