44
5 Comparisons of Fronts with Other Boundaries at Sea
been proposed as the cause for the poor development of benthos in those habitats
(Nixon 1988), thus highlighting the importance of benthic currents in the ocean.
5.3 The Sea Surface-Atmosphere Interface
Physical, chemical and biological conditions differ greatly between the uppermost
5 cm of the ocean and the water below, so this thin upper layer constitutes a distinctive biotope (Zaitsev 1997). This layer is occupied by organisms which are
exposed to wind drift because they are fixed to the surface by their own buoyancy
or because they are more or less permanently attached to floating particles; and
also by organisms which stay close to the surface in a more temporary and variable manner (Fig. 5.1) (Hempel and Weikert 1972). In a broad sense the surface
dwelling biota is referred to as neuston (neuston may be divided in different categories, Zaitsev 1997).
Temperature, wave action and solar radiation seem to be the most important
abiotic factors for neuston organisms (Hempel and Weikert 1972; Zaitsev 1997);
though in some circumstances heavy rain could adversely affect them by diluting the
surface waters (Holdway and Maddock 1983). Wave action could have a destructive
effect on some neuston organisms; damage of fish eggs and a decrease in length of
diatom chains has been reported (Hempel and Weikert 1972).
During daytime, solar radiation (particularly the high intensities of its ultraviolet and infrared portions) make the uppermost few centimeters a biotope
quite different from the rest of the water column, where radiation is effectively
decreased by absorption and dispersion. The majority of plankton organisms avoid
the surface during daytime, but representatives of many groups ascend to the surface at night. Under lower radiation situations (e.g. during dim light, at night, in
turbid waters, or at high latitudes) the biotope loses its major characteristic feature. Under such conditions the neuston community becomes less distinct or even
almost identical to the plankton community of the adjacent strata (Hempel and
Weikert 1972; Holdway and Maddock 1983).
Some inorganic and organic material both from the atmosphere and from the
sea may be concentrated in the air-sea interface. In shallow areas, even benthic
particles will be transported up to the surface by turbulence, and may be fixed
there by surface tension. Coastal areas are also particularly rich in insects, plant
pollen, and seeds transported from land by air drift. Colonies of bacteria may grow
attached to these particles, and also to the bubbles of sea foam and to the surface
(Hempel and Weikert 1972).
In clear, tropical and subtropical waters, the first few centimeters under the surface skin will be particularly poor in food. Low nutrient levels and intense radiation
limit phytoplankton growth. Therefore, despite some concentration of food particles
at the very surface, the general picture of the near surface zone is that of a poor
feeding ground. Certain groups of organisms however, have successfully chosen this
biotope as their habitat: larvae of certain fishes; copepods; euphausids; pteropods;
5 Comparisons of Fronts with Other Boundaries at Sea
been proposed as the cause for the poor development of benthos in those habitats
(Nixon 1988), thus highlighting the importance of benthic currents in the ocean.
5.3 The Sea Surface-Atmosphere Interface
Physical, chemical and biological conditions differ greatly between the uppermost
5 cm of the ocean and the water below, so this thin upper layer constitutes a distinctive biotope (Zaitsev 1997). This layer is occupied by organisms which are
exposed to wind drift because they are fixed to the surface by their own buoyancy
or because they are more or less permanently attached to floating particles; and
also by organisms which stay close to the surface in a more temporary and variable manner (Fig. 5.1) (Hempel and Weikert 1972). In a broad sense the surface
dwelling biota is referred to as neuston (neuston may be divided in different categories, Zaitsev 1997).
Temperature, wave action and solar radiation seem to be the most important
abiotic factors for neuston organisms (Hempel and Weikert 1972; Zaitsev 1997);
though in some circumstances heavy rain could adversely affect them by diluting the
surface waters (Holdway and Maddock 1983). Wave action could have a destructive
effect on some neuston organisms; damage of fish eggs and a decrease in length of
diatom chains has been reported (Hempel and Weikert 1972).
During daytime, solar radiation (particularly the high intensities of its ultraviolet and infrared portions) make the uppermost few centimeters a biotope
quite different from the rest of the water column, where radiation is effectively
decreased by absorption and dispersion. The majority of plankton organisms avoid
the surface during daytime, but representatives of many groups ascend to the surface at night. Under lower radiation situations (e.g. during dim light, at night, in
turbid waters, or at high latitudes) the biotope loses its major characteristic feature. Under such conditions the neuston community becomes less distinct or even
almost identical to the plankton community of the adjacent strata (Hempel and
Weikert 1972; Holdway and Maddock 1983).
Some inorganic and organic material both from the atmosphere and from the
sea may be concentrated in the air-sea interface. In shallow areas, even benthic
particles will be transported up to the surface by turbulence, and may be fixed
there by surface tension. Coastal areas are also particularly rich in insects, plant
pollen, and seeds transported from land by air drift. Colonies of bacteria may grow
attached to these particles, and also to the bubbles of sea foam and to the surface
(Hempel and Weikert 1972).
In clear, tropical and subtropical waters, the first few centimeters under the surface skin will be particularly poor in food. Low nutrient levels and intense radiation
limit phytoplankton growth. Therefore, despite some concentration of food particles
at the very surface, the general picture of the near surface zone is that of a poor
feeding ground. Certain groups of organisms however, have successfully chosen this
biotope as their habitat: larvae of certain fishes; copepods; euphausids; pteropods;