Introduction

Due to the enrichment of chemicals and biota within the sea-surface microlayer, there is the widely held presumption that the surface microlayer could act as a highly efficient and selective microreactor, effectively concentrating and transforming materials brought to the interface from the atmosphere and oceans by physical processes. Rapid photochemical, chemical, and biological reactions within the microlayer could produce a variety of interesting feedbacks. For example, photochemical reactions might destroy (or produce) surface-active species, thereby altering surface wave damping and gas exchange rates. Elevated levels of highly reactive intermediates produced within this zone could present a ‘reaction barrier’ to the transport of some chemicals and trace gases across the air–sea interface, thus affecting their flux to the atmosphere or ocean. Further, reactions occurring within the microlayer potentially could enhance (or deplete) the surface concentrations of certain gases relative to those of bulk seawater, chemically modify compounds during their transport across the interface, alter the redox state, speciation and biological availability of trace metals deposited by the atmosphere to the interface, as well as influence the types and distributions of microlayer materials introduced to the atmosphere by bubble injection and to the deep ocean by particle settling.

Although these processes are very intriguing and potentially of great importance, in many instances evidence supporting the existence of them is lacking.

Abstract

Due to the photochemical production and atmospheric deposition of highly reactive species at the sea surface, the microlayer could well act as a highly efficient microreactor, effectively sequestering and transforming select materials brought to the interface from the atmosphere and oceans by physical processes. However, very little is known about the optical and photochemical properties of this regime. Based on the measured enrichments of light absorbing material in the microlayer and employing photochemical quantum yields obtained for bulk waters, photochemical production rates and fluxes are estimated for the microlayer. The microlayer fluxes are generally small with respect to atmospheric deposition and the water column fluxes. This result argues that the microlayer is unlikely to act as a ‘reaction barrier’ to the exchange of trace gases across the interface. However, the higher photochemical production rates at the surface should lead to the more rapid oxidative turnover of materials at the interface and potentially to reactions and processes not observed in bulk waters.

Introduction

The sea-surface microlayer acts as a dynamic interface mediating the exchange of matter, heat, momentum, and electromagnetic radiation between the earth's oceans and atmosphere. The accumulation of surface-active material within this thin oceanic layer – defined operationally as the top 0.03 to 500 μm depending on the sampling method employed (Liss, 1986; Hunter and Liss, 1981; Carlson, 1982b) –has been shown by workers over the last several decades to alter the physical and chemical properties of the interface.

Abstract

Various parameterizations of gas exchange with wind speed at the ocean surface are poorly constrained by field measurements using natural and artificial tracers. One of the factors leading to uncertainty for in situ estimates of the gas transfer velocity is the presence of organic films at the air–sea interface. Such films are derived from bulk seawater dissolved organic matter, from terrestrial sources (natural and anthropogenic) and from petroleum seeps and spills. The ubiquitous background of degraded natural biopolymeric and geopolymeric materials in the sea potentially contributes to surface accumulations of organic matter even in very oligotrophic waters. Specific inputs during phytoplankton blooms and from neuston in the microlayer also contribute to the enrichment of surface-active matter at the interface.

Organic films can affect air–sea gas exchange through both static and dynamic mechanisms. The static effect arises from the presence of additional mass transfer resistance due to the physical barrier provided by the film. This effect is not considered to be important at the sea surface, since it requires the presence of condensed, solid type surfactant films that are easily dispersed under typical oceanic conditions of wind and waves. Much more significant is the hydrodynamic effect of a film that arises from the viscoelastic property of a surfactant-influenced interface.

Abstract

Laboratory measurements of air–water gas transfer rates for cleaned and film-covered water surfaces have shown that the presence of soluble and insoluble surfactants can inhibit air–water gas fluxes. Naturally occurring surface-active material is known to concentrate in the marine surface microlayer and form films and slicks. It is reasonable that oceanic slicks and films may lower in situ gas transfer rates compared with air–sea gas exchange through a clean ocean surface. Here, a simple model of gas transfer through clean and surfactant-influenced water surfaces is used to develop parameterizations of liquid-phase, and gas-phase, rate-controlled gas transfer velocities through clean and surfactant-influenced ocean surfaces. The parameterization for liquid-phase, rate-controlled processes is used to estimate the effect of naturally occurring surface films on the net global flux of carbon dioxide. The gas-phase, rate-controlled relations are used to study the impact of films on the flux of ammonia from the central Pacific Ocean.

By relating the fractional area coverage of surfactant-influenced sea surface to a global map of net synthetic primary production, the model shows that surface films can increase or decrease the net global oceanic carbon dioxide flux, depending on the regional film coverage.

Introduction

Motivation and purpose

There is little doubt that under laboratory conditions both soluble and insoluble surfactants suppress gas–liquid mass transfer rates.

Abstract

The upper metre of the ocean is a transition zone between the atmosphere and deeper water and can be subdivided into strata (nanolayer, microlayer, millilayer and centilayer) with different chemical and biological characteristics. A variety of techniques has been used to collect samples from different depths for chemical and biological analysis. Autotrophic and heterotrophic neuston (surface dwelling biota) range in size from less than 2 μm (piconeuston) to a metre or more (macroneuston) and are represented by, perhaps, several thousand species worldwide. They occur in much greater densities than their sub-surface counterparts, the plankton and nekton.

Anthropogenic organic compounds and metals frequently occur in greater concentrations in the microlayer (upper 10-6 m) than in deeper layers. These surface enrichments originate from a variety of sources, but in offshore and in some coastal areas atmospheric deposition is particularly important. Enrichment factors (microlayer concentration/bulk-water concentration) for Pb, Cd, Cu, Zn, some radionuclides and aromatic hydrocarbons are typically 10 to 102 and for chlorinated organics may be 103 or more.

Increases in global sea-surface contamination and/or ultraviolet radiation could threaten important sea-surface biological communities and processes. Contaminated films deposited on intertidal beaches during receding tides could negatively impact shellfish and infauna. The majority of marine fish has floating eggs or larvae which could be adversely affected.

Introduction

The physics, chemistry and biology of the sea surface are closely interrelated. Plankton in the water column produce an abundance of particulate and dissolved organic material, some of which is transported to the surface either passively by floatation or actively by bubble transport. Atmospheric deposition also enriches the sea surface with natural and anthropogenic compounds, which often accumulate there in relatively high concentrations compared with those in the water column. The abundance of organic matter at the sea surface provides a substrate for the growth of organisms that inhabit the sea surface microlayer: the neuston. Most studies suggest that the sea surface represents a highly productive, metabolically active interface. Organisms from most major divisions of the plant and animal kingdoms either live, reproduce or feed in the surface layers. Of particular interest are the microneuston, which may be involved in biogeochemical cycling, and neustonic eggs and larvae of commercially important fish and shellfish.

The quantities and types of anthropogenic chemicals entering the earth's atmosphere continue to grow. Many of these chemicals, some of which are highly toxic, are now globally distributed in the atmosphere and deposit to the sea surface even in remote areas. Due to stratospheric ozone depletion, ultraviolet-b (UV-B) radiation reaching the sea surface is increasing annually.

Abstract

The thermodynamics and kinetics of air–sea exchange are discussed in terms of the underlying theory and on the basis of numerical calculations. The thermodynamic driving force for gas exchange is dependent on the air–sea temperature difference as well as on the partial pressure or concentration difference across the interface. The kinetics of the exchange process are strongly affected by the surface temperature of the water, as controlled by the fluxes of sensible and latent heat. The results of calculations for a model which incorporates a turbulent air layer are compared with the experimental data of Liss et al. (1981), Smith and Jones (1985) and Smith et al. (1991). This comparison clearly demonstrates the importance of coupling, in the sense of Onsager's irreversible thermodynamics, of the fluxes of sensible heat and matter across the interface. The calculations also suggest a possible new approach to the measurement of air–water exchange rates for trace gases such as carbon dioxide.

Introduction

The rate of air–sea exchange of carbon dioxide is a topic which has been the subject of some controversy in recent years. This exchange rate, which is of great practical importance in connection with global warming, is very difficult to determine experimentally, and the difficulty is compounded by the need to obtain values for both the long-term average of the rate on a global scale and the local exchange rate at a particular instant.

Abstract

The gas flux of weakly soluble gases through the air–sea interface is controlled by the transport mechanism in the aqueous diffusive boundary layer. The combination of molecular and turbulent transport and of secondary motions near the interface determines the exchange rate. This layer is difficult to access experimentally, so a combination of observation and physical interpretation is necessary.

Typical modes of fluid motion at the interface and their potential to further gas exchange are reviewed: organized motions, like cell and helicoidal rolls or Langmuir circulation on one hand, waves and wind induced shear flow on the other. Special attention is given to wave dissipation in the form of wave breaking. The secondary flow and irregular motions of breaking waves, as well as possible rolling motions of smaller waves on the slopes of larger waves, are seen as enhancing gas transfer through surface renewal.

Observations of surface streaming obtained by dying the sea surface are discussed in terms of the above-mentioned models of surface renewal. A set of observations by Gemmrich is used to assess the effectiveness of secondary motions, as found at a given wind speed at sea, to enhance gas transfer. It is found for the natural mix of wind speeds (e.g. for the North Atlantic Ocean) that enhancement of gas transfer, compared with undisturbed boundary-layer flow, occurs in about 20–25% of cases.

Surface films

Sources, sinks, and properties of surface films

Sea-surface films are derived from multiple sources, both in the sea and on land. Even in oligotrophic waters where biological productivity is low, the concentration of dissolved organic matter, including degraded biopolymers and geopolymeric materials, is sufficient to produce surface enrichments of organic matter under favourable physical conditions. Specific inputs from phytoplankton blooms and from neuston indigenous to the microlayer also contribute to the enrichment of surface-active matter at the interface. Terrestrial sources include both natural and anthropogenic contributions. Terrestrial plant-derived materials are released directly to the atmosphere and introduced via dry and wet deposition, or enter the ocean environment via riverine inputs as decay products of vegetation. Anthropogenic contributions include point sources related to industrial processes, agricultural runoff, and spills of petroleum products (catastrophic and chronic). In addition, municipal wastewater discharges are frequently highly enriched in surfactants that ultimately enter coastal seas and sediments. In shallow coastal environments, resuspension of sediments and release of sediment pore-water materials are other potential sources of surfactants.

The relative importance of these sources is not known. A major source of surfactants is thought to be production by phytoplankton, which exude natural surfactants as metabolic by-products (Zutic et al., 1981).

Abstract

Bubbles can be generated at the sea surface by many mechanisms, but the main source is by the entrainment of air in breaking waves. Bubbles will scavenge material from the surrounding water, thus contributing to the cycling of dissolved and particulate organic material. When they burst at the sea surface these bubbles generate a sea salt aerosol contaminated with material scavenged from the sea-surface microlayer and below. Gases will be exchanged between a bubble and the surrounding water while it is submerged. In addition, the breaking waves and surfacing bubble plumes disrupt the surface microlayer, and this may enhance transfer of gases directly through the sea surface.

The net transfer of a gas between a bubble and the surrounding water, from entrainment until the bubble bursts or fully dissolves, contributes to the total transfer of that gas between atmosphere and ocean. This bubble-mediated transfer has some special properties that set it apart from direct air–sea transfer of poorly soluble gases. Bubble-mediated air–sea transfer velocities depend on the solubility in addition to the molecular diffusivity of the gas in seawater. Bubble-mediated transfer is not proportional to air–sea concentration difference, but is biased toward injection and the forcing of supersaturation. The entrainment of air by breaking waves increases rapidly in intensity with higher wind speeds.

Abstract

The surface film of water (the laminar layer about 0.5 mm thick and the intermediate layer about 5 mm thick) is regarded as a ‘bottleneck’ for heat and mass exchange between the atmosphere and natural water bodies. The dependence of the surface film temperature on air and water temperature and humidity under laboratory conditions is described. As demonstrated, replacing a ‘warm’ by a ‘cold’ surface film results in the oxygen transfer rate increasing by 8%.

The surface film of natural water bodies is inhabited by specific neuston organisms. The freshwater zooneuston of large Siberian reservoirs is described, and their general similarity with the marine neuston of the Sea of Japan is shown. The statement is made that there are two ways in which the biota influence the properties of surface films: 1. mechanical – by providing turbulence in the laminar layer by the swimming action of small zooneuston organisms; and 2. chemical – by the influence of biogenic surfactants on the water film. Experimental evidence of the biotic influence is demonstrated.

Introduction

From the hydrophysical or thermophysical point of view the surface film of water is envisaged as consisting of a laminar layer through which heat and mass transfer takes place by molecular diffusion, and an intermediate layer in which the rates of diffusion increase from molecular to turbulent scales (Figure 10.1).

The sea-surface microlayer has often been operationally defined as roughly the top 1 to 1000 micrometres of the ocean surface. There has been considerable new research in this area over the past 5–10 years. The microlayer is known to concentrate, to varying degrees, many chemical substances, particularly those that are surface active, and many organisms live and/or find food there. It is clearly the interface through which all gaseous, liquid and particulate material must pass when exchanging between the ocean and the atmosphere. It also plays a vital role in the transfer of various forms of energy (momentum, heat) between the two media.

It is now recognized that important physical, chemical, and biological processes near the air–sea interface are not restricted to what has been traditionally referred to as the ‘microlayer’, but rather occur over gradients of varying thickness. Above the interface is an atmospheric boundary layer of 50–500 μm, where atmospheric turbulence is much reduced. Below the air-water interface the aquatic surface layer contains a series of sublayers (as described by Jack Hardy in Chapter 11). In this book, we use the term ‘microlayer’ in its operational meaning to refer to roughly the uppermost millimetre of the oceans, where properties are most altered relative to deeper waters. We also utilize the following terminology: a ‘film’ refers to a surfactant-influenced surface and a ‘slick’ refers to a visibly surfactant-influenced surface.

Abstract

Hydrocarbons, being minor constituents of dissolved organic matter in seawater, under normal conditions make up a small fraction of the organic surface film. In spill situations, however, they can become principal constituents. Depending on chemical structure, availability of nutrients, enrichment of microorganisms, and light regime, hydrocarbons in the surface microlayer are decomposed either microbially or photochemically. As most biogenic and the majority of fossil hydrocarbons are transparent to solar UV radiation at sea level, sensitizers are required for their photochemical oxidation and decomposition. Sensitizers include natural products, such as humic material, and anthropogenic compounds, such as polycyclic aromatic ketones. Photochemical decomposition products of hydrocarbons include alcohols, aldehydes, ketones, and terminal alkenes. Generation of low-molecular-weight carbonyl compounds by photochemical carbon chain fragmentation has been observed. Microbial decomposition of photooxidation products is often faster than that of the parent hydrocarbons.

Microlayer samplers

The term ‘sea-surface microlayer’ has been defined operationally as that thin layer of water adjacent to and including the air–sea interface which adheres to sampling devices such as wiremesh screens, glass plates, Teflon discs, rotating drums, or collectors for the spray generated by bursting bubbles (Liss, 1975, and references cited therein). Van Vleet and Williams (1980) found preferential uptake of different compound classes to depend upon the type of sampler and the material it is made of (see also Daumas et al., 1976).

Abstract

The first investigations of marine neuston (surface-dwelling organisms) were conducted in the 1950s and focused on the taxonomic diversity and abundance of organisms. Later investigators examined the physics, chemistry and exchange processes between the atmosphere and ocean. Today, we know the ocean–atmosphere interface is important for many biogeochemical processes essential for life.

Physical, chemical and biological conditions differ greatly between the uppermost 5 cm of the ocean and the water below. The marine pleuston includes the larger siphonophores, Physalia and Velella, which float on the surface. Neuston can be divided into epineuston and hyponeuston. The epineuston includes more than 40 species of water striders, Halobates, inhabiting the open ocean and coastal areas. The hyponeuston are organisms in the surface centilayer including hydrozoa, molluscs, copepods, isopods, decapod crustaceans, fishes, and the seaweed Sargassum.

The neuston connect the sea surface and water column as larvae develop and migrate downward, and adult animals visit the surface to feed and reproduce. The sea surface has become a site of significant enrichment of pollutants from terrestrial and atmospheric sources. The spatial coincidence of the maximum pollutant concentrations and the biological sensitivity of its inhabitants creates a critical situation in the marine environment.

High densities of neustonic organisms in the sea surface can influence air–sea exchange processes (as discussed in Chapter 10, this volume).

INTRODUCTION

The alluvial hydrosystems are corridors transferring water, sediment, organic matter and organisms (Décamps & Naiman, 1989). The fluxes of matter flow through the longitudinal axis of the river and also the transversal axis to the riparian zones which constitute the land-water interface (Gregory et al, 1991). These zones are characterized by a large diversity of aquatic and terrestrial ecosystems, due to the geomorphological and hydrological dynamics of the large rivers (Amoros et al, 1988). Permanent interaction exists between both ecosystems, thanks to the vector ‘water’, which allows us to define ecotones, land-water ecotone or water-water ecotone.

INTRODUCTION

Pour l'hydraulicien, la couche superficielle d'un sol agricole sera tout naturellement un écotone, zone d'échange et de ‘mutation’ entre l'eau qui véhicule fertilisants et pesticides, les grains de terre et les racines des plantes, en n'oubliant pas la faune qui catalyse les échanges et les transformations.

Dans ce qui suit, nous allons restreindre notre point de vue á l'examen du devenir des pesticides.

Les produits phytosanitaires sont certes efficaces pour assurer quantité et qualité de la production agricole, mais ils ne sont pas sans danger. Ces risques, difficilement chiffrables, ont cependant donné lieu á des enquêtes.