The highest salinities in the open sea are
found in the dry, hot tropics, where evaporation
is great. Nearer the equator, salinities decrease
because rainfall is heavier. In the cooler middle
latitudes, salinities are relatively low because of
decreased evaporation and increased precipitation. Surface salinities are generally low in the
Arctic and Antarctic waters because of the effect
of melting ice, which dilutes the sea water. In
coastal waters and nearly enclosed seas, the
salinity departs greatly from this pattern. In hot,
dry seas of the Mediterranean and Middle East,
the salinity is greater because the water is subject
to strong evaporation and cannot mix readily
with the open ocean. Near the mouths of large
rivers, and in nearly enclosed seas fed by large
rivers, such as the Baltic Sea (Fig. 2.5), dilution
by fresh water reduces the salinity.
2.2
Types of Hydrologic Regions,
or Oceanic Ecoregions
The interaction of the oceanic macroclimates and
large-scale ocean currents determine the major
hydrologic regions with differing physical and
biological characteristics. These regions are
defined as oceanic ecoregions. Their boundaries
follow the subdivision of the oceans into hydrographic regions by Gu ¨nter Dietrich (1963),
although some modification has been made to
identify regions with high salinity and the zonal
arrangement of the regions. Dietrich’s oceanographic classification and their ecoregion
equivalents are summarized in Tables 2.1 and
2.2. Dietrich’s system was used with similar
results by Hayden et al. (1984) in their proposed
regionalization of marine environments.
2.2.1 Dietrich’s Oceanographic
Classification
Dietrich’s classification takes into account the
circulation of the oceans, the temperature and
salinity, and indirectly, the presence of major
zones of upwelling. The motion of the surface
is emphasized because of its influence on temperature. Salinity is generally higher in areas of
higher temperature and therefore, higher
Fig. 2.4 The distribution of zones of productivity in the oceans is generally related to areas of cool water and
upwelling. From Lieth (1964–1965)
12
2 Oceanic Types and Their Controls
found in the dry, hot tropics, where evaporation
is great. Nearer the equator, salinities decrease
because rainfall is heavier. In the cooler middle
latitudes, salinities are relatively low because of
decreased evaporation and increased precipitation. Surface salinities are generally low in the
Arctic and Antarctic waters because of the effect
of melting ice, which dilutes the sea water. In
coastal waters and nearly enclosed seas, the
salinity departs greatly from this pattern. In hot,
dry seas of the Mediterranean and Middle East,
the salinity is greater because the water is subject
to strong evaporation and cannot mix readily
with the open ocean. Near the mouths of large
rivers, and in nearly enclosed seas fed by large
rivers, such as the Baltic Sea (Fig. 2.5), dilution
by fresh water reduces the salinity.
2.2
Types of Hydrologic Regions,
or Oceanic Ecoregions
The interaction of the oceanic macroclimates and
large-scale ocean currents determine the major
hydrologic regions with differing physical and
biological characteristics. These regions are
defined as oceanic ecoregions. Their boundaries
follow the subdivision of the oceans into hydrographic regions by Gu ¨nter Dietrich (1963),
although some modification has been made to
identify regions with high salinity and the zonal
arrangement of the regions. Dietrich’s oceanographic classification and their ecoregion
equivalents are summarized in Tables 2.1 and
2.2. Dietrich’s system was used with similar
results by Hayden et al. (1984) in their proposed
regionalization of marine environments.
2.2.1 Dietrich’s Oceanographic
Classification
Dietrich’s classification takes into account the
circulation of the oceans, the temperature and
salinity, and indirectly, the presence of major
zones of upwelling. The motion of the surface
is emphasized because of its influence on temperature. Salinity is generally higher in areas of
higher temperature and therefore, higher
Fig. 2.4 The distribution of zones of productivity in the oceans is generally related to areas of cool water and
upwelling. From Lieth (1964–1965)
12
2 Oceanic Types and Their Controls