10
Lipids in Water-Surface Microlayers
and Foams
Guillermo E. Napolitano and Daniel S. Cicerone
10.1. Introduction
The water-surface microlayers oflakes and streams are unique environments with
a different chemical composition and physical properties from the underlying
water. The surface layer acts as an interface to the exchange of gases between air
and water and is a vehicle for the transport of inorganic and organic materials
between the atmosphere and the water column. The definition, boundaries, and
the measurement of the thickness of the water microlayer have been elusive, but
the microlayer is generally acknowledged to be limited to the uppermost 30-100
J.1m of the water (Hardy et aI., 1988, and references therein). These dimensions,
however, reflect more the characteristics and capabilities of the sampling devices
than rigorous measurements of the microstructure of the surface waters.
Dissolved organic matter originates from several autochthonous and allochthonous sources, including the hydrolysis of animal and plant materials,
metabolic products of algae and vascular plants. It has been estimated that 1030% of the aquatic primary production is "lost" as exudates, contributing to the
pool of dissolved organic matter (Fogg, 1977; Sharp, 1977). Autochthonous
sources of dissolved and particulate organic material determine, to a large extent,
the characteristics of the marine surface microlayer (Meyers and Kawka, 1982). In
the freshwater environment, however, the properties of the watershed and hydrological regimes probably have a stronger influence on the microlayer composition
than does the contribution from aquatic primary production.
The surface micro layer of streams and lakes accumulates biogenic surfactants,
primarily consisting of humic and fulvic materials, leached from soils by precipitation events (Wissmar and Simenstad, 1984; Pojasek and Zajicek, 1978). The
presence of these natural surfactants is important because they favor the uptake of
biogenic lipids and hydrophobic pollutants by the water microlayer. In fact, an
extensive database now exists that shows an enrichment of natural and anthropogenic hydrophobic compounds in the surface micro layer with respect to the
underlying water (Napolitano and Richmond, 1995; Hardy et aI., 1988; Sodergren, 1979; Duce et aI., 1972). Many studies have also demonstrated that the
concentrations of metals of environmental concern (e.g., Zn, Cd, Pb, Cu) (Elzer235
Lipids in Water-Surface Microlayers
and Foams
Guillermo E. Napolitano and Daniel S. Cicerone
10.1. Introduction
The water-surface microlayers oflakes and streams are unique environments with
a different chemical composition and physical properties from the underlying
water. The surface layer acts as an interface to the exchange of gases between air
and water and is a vehicle for the transport of inorganic and organic materials
between the atmosphere and the water column. The definition, boundaries, and
the measurement of the thickness of the water microlayer have been elusive, but
the microlayer is generally acknowledged to be limited to the uppermost 30-100
J.1m of the water (Hardy et aI., 1988, and references therein). These dimensions,
however, reflect more the characteristics and capabilities of the sampling devices
than rigorous measurements of the microstructure of the surface waters.
Dissolved organic matter originates from several autochthonous and allochthonous sources, including the hydrolysis of animal and plant materials,
metabolic products of algae and vascular plants. It has been estimated that 1030% of the aquatic primary production is "lost" as exudates, contributing to the
pool of dissolved organic matter (Fogg, 1977; Sharp, 1977). Autochthonous
sources of dissolved and particulate organic material determine, to a large extent,
the characteristics of the marine surface microlayer (Meyers and Kawka, 1982). In
the freshwater environment, however, the properties of the watershed and hydrological regimes probably have a stronger influence on the microlayer composition
than does the contribution from aquatic primary production.
The surface micro layer of streams and lakes accumulates biogenic surfactants,
primarily consisting of humic and fulvic materials, leached from soils by precipitation events (Wissmar and Simenstad, 1984; Pojasek and Zajicek, 1978). The
presence of these natural surfactants is important because they favor the uptake of
biogenic lipids and hydrophobic pollutants by the water microlayer. In fact, an
extensive database now exists that shows an enrichment of natural and anthropogenic hydrophobic compounds in the surface micro layer with respect to the
underlying water (Napolitano and Richmond, 1995; Hardy et aI., 1988; Sodergren, 1979; Duce et aI., 1972). Many studies have also demonstrated that the
concentrations of metals of environmental concern (e.g., Zn, Cd, Pb, Cu) (Elzer235