236
G.E. Napolitano and D.S. Cicerone
man and Armstrong, 1979; Eisenreich et aI., 1978; Szekielda et aI., 1971) and the
densities of certain microorganisms (Johnson et aI., 1989; Means and Wijayarante, 1982) can be up to 4 orders of magnitude higher in the surface microlayers
than in the subsurface waters.
Bubble production is a common natural phenomena at the surface water of
marine (Hunter and Liss, 1981) and freshwater environments and a critical process in surface-layer chemistry. Bubbles may be caused by the activity of organisms, breaking waves, and/or turbulent flows. Experiments have shown that as
bubbles rise through the water column, selected organic molecules and particles
are adsorbed to their surfaces (Blanchard, 1963). At the water surface, the bubbles
may burst, ejecting aerosol droplets into the air. This process has been called the
"bubble microtome" because the droplets eject not only the materials contained in
the bubbles' surface but also those contained in the water-surface microlayer
(MacIntyre, 1968). Under certain conditions, emerging bubbles may not burst
instantly and may accumulate on the water surface, producing foam. Thus, foam
formations represent an excellent opportunity for studying the composition of the
water-surface microlayer and the advection of materials from deeper in the water
column to the surface.
The aqueous phase of foam contains high concentrations of surface active
materials and is therefore expected to concentrate natural lipids and hydrophobic
contaminants. Although we briefly discuss the fate and the distribution of some
biogenic and petroleum hydrocarbons, owing to space limitations we do not cover
the literature on hydrophobic pollutants of the water microlayers (Kucklick and
Bidleman, 1994; Butler and Sibbald, 1987; Duce et aI., 1974, 1972). This chapter
is a critical evaluation of the research on the distribution and dynamics of biogenic
lipids in the surface microlayer and foams of freshwater systems. Attention is
focused on sampling techniques and on the incorporation of biogenic lipids into
these particular environments.
10.2. Basic Physicochemistry of Surface Microlayers
The air-water interface, like all discontinuities in aquatic environments, is a
relative high free-energy system (thermodynamically not stable) in which singularities take place. In its simplest form, the air-water interface of a natural
system can be visualized as a four-layer model consisting (from top to bottom) of
turbulent air, stagnant air, stagnant water, and mixed water (Fig. 10.1). The dimensions of the layers forming the air-water interface presented in Figure 10.1 reflect
typical values found in both laboratory (Schwarzenbach, 1983; Genereux, 1991;
Duran and Hemond, 1984; Wilcock, 1984; Mackay and Yeun, 1983; Rathbun and
Tai, 1983, 1982; Miinnich et aI., 1978; Liss, 1973) and field studies (Kucklick and
Bidleman, 1994; Daumas et aI., 1976; Liss, 1975; Jarvis, 1967; Garret, 1965). The
monomolecular slice of surface active materials and the layer of stagnant water
constitute the actual water-surface microlayer and are the subject of this chapter.
The surface microlayer is a region of 0.01-100 /-Lm of thickness, in which the
G.E. Napolitano and D.S. Cicerone
man and Armstrong, 1979; Eisenreich et aI., 1978; Szekielda et aI., 1971) and the
densities of certain microorganisms (Johnson et aI., 1989; Means and Wijayarante, 1982) can be up to 4 orders of magnitude higher in the surface microlayers
than in the subsurface waters.
Bubble production is a common natural phenomena at the surface water of
marine (Hunter and Liss, 1981) and freshwater environments and a critical process in surface-layer chemistry. Bubbles may be caused by the activity of organisms, breaking waves, and/or turbulent flows. Experiments have shown that as
bubbles rise through the water column, selected organic molecules and particles
are adsorbed to their surfaces (Blanchard, 1963). At the water surface, the bubbles
may burst, ejecting aerosol droplets into the air. This process has been called the
"bubble microtome" because the droplets eject not only the materials contained in
the bubbles' surface but also those contained in the water-surface microlayer
(MacIntyre, 1968). Under certain conditions, emerging bubbles may not burst
instantly and may accumulate on the water surface, producing foam. Thus, foam
formations represent an excellent opportunity for studying the composition of the
water-surface microlayer and the advection of materials from deeper in the water
column to the surface.
The aqueous phase of foam contains high concentrations of surface active
materials and is therefore expected to concentrate natural lipids and hydrophobic
contaminants. Although we briefly discuss the fate and the distribution of some
biogenic and petroleum hydrocarbons, owing to space limitations we do not cover
the literature on hydrophobic pollutants of the water microlayers (Kucklick and
Bidleman, 1994; Butler and Sibbald, 1987; Duce et aI., 1974, 1972). This chapter
is a critical evaluation of the research on the distribution and dynamics of biogenic
lipids in the surface microlayer and foams of freshwater systems. Attention is
focused on sampling techniques and on the incorporation of biogenic lipids into
these particular environments.
10.2. Basic Physicochemistry of Surface Microlayers
The air-water interface, like all discontinuities in aquatic environments, is a
relative high free-energy system (thermodynamically not stable) in which singularities take place. In its simplest form, the air-water interface of a natural
system can be visualized as a four-layer model consisting (from top to bottom) of
turbulent air, stagnant air, stagnant water, and mixed water (Fig. 10.1). The dimensions of the layers forming the air-water interface presented in Figure 10.1 reflect
typical values found in both laboratory (Schwarzenbach, 1983; Genereux, 1991;
Duran and Hemond, 1984; Wilcock, 1984; Mackay and Yeun, 1983; Rathbun and
Tai, 1983, 1982; Miinnich et aI., 1978; Liss, 1973) and field studies (Kucklick and
Bidleman, 1994; Daumas et aI., 1976; Liss, 1975; Jarvis, 1967; Garret, 1965). The
monomolecular slice of surface active materials and the layer of stagnant water
constitute the actual water-surface microlayer and are the subject of this chapter.
The surface microlayer is a region of 0.01-100 /-Lm of thickness, in which the