Introduction
Estuarine ecosystems are characterized by a constantly
changing mixture of saltwater and freshwater (McLusky,
1989). The flux of saltwater and freshwater presents physiological challenges to estuarine organisms. Gradients of
salinity, for example, restrict many animal species in the
adjacent sea or rivers from entering estuaries. In highly
stratified estuaries, denser marine water entering an estuary along the bottom gradually mixes vertically with the
outgoing surface flow of freshwater. Environmental gradients in estuaries depend on seasonal and daily fluctuations
of abiotic factors.
Environmental gradients
Abiotic factors
Fairbridge (1980) defined an estuary as an inlet of the sea
reaching into a river valley, as far as the upper limit of tidal
rise, usually being divisible into three sectors: (1) a marine
or lower estuary, in free connections with the open sea,
(2) a middle estuary subject to strong saltwater and freshwater mixing, and (3) an upper or fluvial estuary, characterized by freshwater but subject to strong tidal action.
The limits between these sectors are variable and subject
to constant seasonal changes in the river discharge.
Estuaries may be classified as positive, neutral, or negative depending on their salinity regime and the extent of
evaporation (McLusky and Elliott, 2004). In positive estuaries, the evaporation from the surface of the estuary is less
than the volume of freshwater entering the estuary from
rivers and land drainage. In such an estuary, the outgoing
freshwater floats on top of saline water entering from the
sea. Therefore, the water gradually mixes vertically from
the bottom to the top in the estuarine basin. This type of
estuary is thus characterized by incoming saltwater along
the bottom, with gradual vertical mixing leading to an outgoing stream of fresher surface water (salt wedge). In negative estuaries, evaporation from the surface exceeds the
freshwater runoff entering the estuary, and evaporation
causes the surface salinity to increase. The saltier surface
water is then denser than the water underneath, causing
it to sink. The circulation pattern is thus opposite to that
of a positive estuary because, in a negative estuary, the
seawater and freshwater both enter the estuary on the surface, but after evaporation and sinking, they leave the estuary as an outgoing bottom current. Finally, in neutral
estuaries, freshwater input equals evaporation and, in this
case, a static salinity regime occurs. Depending on the
tidal amplitude and volume of freshwater flow, four main
types of positive estuaries are recognized: highly stratified, fjords, partially mixed, and homogeneous (Dyer,
1973, 1974).
Along an estuarine gradient, there are definite changes
in salinity ranging from seawater to freshwater and associated changes in sedimentary conditions from coarse sediment (sand or gravel) outside the estuaries to fine
sediments (mud) within the estuaries (Miranda et al.,
2002). The horizontal distribution of average salinity in
estuaries typically varies between 1 and 36, indicating that
the saltwater was gradually diluted by the freshwater discharge. Moreover, the mixing and layering processes of
salinity in the estuary depend on its geometry, freshwater
discharge, tidal range, salinity, circulation of the adjacent
oceanic region, and wind acting directly or indirectly on
its surface. Other possible changes in estuarine gradients
are alterations in turbidity of the water column or chemical
composition, including changes in nutrients, dissolved
gases, and trace elements (McLusky and Elliott, 2004).
Biotic factors
Estuarine organisms and habitats often experience steep
gradients in salinity. Estuarine fauna must adapt to the
dynamic environmental conditions. Many fishes, for
example, have the physiological capacity to tolerate high
fluctuations of salinity along an estuarine gradient. Estuarine fauna can be classified based on their salinity tolerances (McLusky and Elliott, 2004). For instance,
oligohaline organisms are those that live in rivers/freshwater and do not tolerate salinities greater than 0.5. Most
fauna with marine affinities that live in the central segment
of estuaries (salinities of 5–18) are the truly estuarine
forms. They can live in the sea, but the estuarine waters
provide space and food resources as well as less competition with other marine species. Euryhaline marine species
constitute the majority of organisms living in estuaries
with their spatial distribution ranging from the sea up to
the central segment of estuaries. Each species has its
own range of salinity tolerance. Stenohaline is a term
describing organisms that cannot tolerate a wide fluctuation in salinity. They are often fish and crabs that spend
only part of their life cycles in estuaries, using these waters
as pathways to and from their breeding areas in rivers or
the open sea. The composition of estuarine fish assemblages is determined by a combination of biotic and abiotic factors, particularly competition for space and food,
tolerance of diel and seasonal changes in salinity and turbidity, and temperature gradients (Barletta and Blaber,
2007).
Many studies on fish assemblages in estuaries and their
relationships with habitat types and environmental gradients demonstrate that geology, geomorphology, and more
immediate environmental gradient conditions, such as
salinity and temperature, are associated with fish distribution, species richness, and fisheries catch (Mathieson
et al., 2000; Thiel et al., 2003; Barletta et al., 2005;
Barletta and Blaber, 2007; Barletta et al., 2008). For example, the relative proportions of freshwater and marine
species using estuaries may be different depending on
environmental gradients of salinity and other physicalchemical and geomorphological factors (Barletta et al.,
2000, 2003, 2005, 2008; Blaber, 2000). Many functions
of estuarine fauna, such as breeding, recruitment, nursery,
and food supply, are subject to diel and seasonal fluctuations of environmental gradients. In neotropical estuaries
(Figures 1, 2, and 3), the variation in the seasonal ecocline,
238
ENVIRONMENTAL GRADIENTS
Estuarine ecosystems are characterized by a constantly
changing mixture of saltwater and freshwater (McLusky,
1989). The flux of saltwater and freshwater presents physiological challenges to estuarine organisms. Gradients of
salinity, for example, restrict many animal species in the
adjacent sea or rivers from entering estuaries. In highly
stratified estuaries, denser marine water entering an estuary along the bottom gradually mixes vertically with the
outgoing surface flow of freshwater. Environmental gradients in estuaries depend on seasonal and daily fluctuations
of abiotic factors.
Environmental gradients
Abiotic factors
Fairbridge (1980) defined an estuary as an inlet of the sea
reaching into a river valley, as far as the upper limit of tidal
rise, usually being divisible into three sectors: (1) a marine
or lower estuary, in free connections with the open sea,
(2) a middle estuary subject to strong saltwater and freshwater mixing, and (3) an upper or fluvial estuary, characterized by freshwater but subject to strong tidal action.
The limits between these sectors are variable and subject
to constant seasonal changes in the river discharge.
Estuaries may be classified as positive, neutral, or negative depending on their salinity regime and the extent of
evaporation (McLusky and Elliott, 2004). In positive estuaries, the evaporation from the surface of the estuary is less
than the volume of freshwater entering the estuary from
rivers and land drainage. In such an estuary, the outgoing
freshwater floats on top of saline water entering from the
sea. Therefore, the water gradually mixes vertically from
the bottom to the top in the estuarine basin. This type of
estuary is thus characterized by incoming saltwater along
the bottom, with gradual vertical mixing leading to an outgoing stream of fresher surface water (salt wedge). In negative estuaries, evaporation from the surface exceeds the
freshwater runoff entering the estuary, and evaporation
causes the surface salinity to increase. The saltier surface
water is then denser than the water underneath, causing
it to sink. The circulation pattern is thus opposite to that
of a positive estuary because, in a negative estuary, the
seawater and freshwater both enter the estuary on the surface, but after evaporation and sinking, they leave the estuary as an outgoing bottom current. Finally, in neutral
estuaries, freshwater input equals evaporation and, in this
case, a static salinity regime occurs. Depending on the
tidal amplitude and volume of freshwater flow, four main
types of positive estuaries are recognized: highly stratified, fjords, partially mixed, and homogeneous (Dyer,
1973, 1974).
Along an estuarine gradient, there are definite changes
in salinity ranging from seawater to freshwater and associated changes in sedimentary conditions from coarse sediment (sand or gravel) outside the estuaries to fine
sediments (mud) within the estuaries (Miranda et al.,
2002). The horizontal distribution of average salinity in
estuaries typically varies between 1 and 36, indicating that
the saltwater was gradually diluted by the freshwater discharge. Moreover, the mixing and layering processes of
salinity in the estuary depend on its geometry, freshwater
discharge, tidal range, salinity, circulation of the adjacent
oceanic region, and wind acting directly or indirectly on
its surface. Other possible changes in estuarine gradients
are alterations in turbidity of the water column or chemical
composition, including changes in nutrients, dissolved
gases, and trace elements (McLusky and Elliott, 2004).
Biotic factors
Estuarine organisms and habitats often experience steep
gradients in salinity. Estuarine fauna must adapt to the
dynamic environmental conditions. Many fishes, for
example, have the physiological capacity to tolerate high
fluctuations of salinity along an estuarine gradient. Estuarine fauna can be classified based on their salinity tolerances (McLusky and Elliott, 2004). For instance,
oligohaline organisms are those that live in rivers/freshwater and do not tolerate salinities greater than 0.5. Most
fauna with marine affinities that live in the central segment
of estuaries (salinities of 5–18) are the truly estuarine
forms. They can live in the sea, but the estuarine waters
provide space and food resources as well as less competition with other marine species. Euryhaline marine species
constitute the majority of organisms living in estuaries
with their spatial distribution ranging from the sea up to
the central segment of estuaries. Each species has its
own range of salinity tolerance. Stenohaline is a term
describing organisms that cannot tolerate a wide fluctuation in salinity. They are often fish and crabs that spend
only part of their life cycles in estuaries, using these waters
as pathways to and from their breeding areas in rivers or
the open sea. The composition of estuarine fish assemblages is determined by a combination of biotic and abiotic factors, particularly competition for space and food,
tolerance of diel and seasonal changes in salinity and turbidity, and temperature gradients (Barletta and Blaber,
2007).
Many studies on fish assemblages in estuaries and their
relationships with habitat types and environmental gradients demonstrate that geology, geomorphology, and more
immediate environmental gradient conditions, such as
salinity and temperature, are associated with fish distribution, species richness, and fisheries catch (Mathieson
et al., 2000; Thiel et al., 2003; Barletta et al., 2005;
Barletta and Blaber, 2007; Barletta et al., 2008). For example, the relative proportions of freshwater and marine
species using estuaries may be different depending on
environmental gradients of salinity and other physicalchemical and geomorphological factors (Barletta et al.,
2000, 2003, 2005, 2008; Blaber, 2000). Many functions
of estuarine fauna, such as breeding, recruitment, nursery,
and food supply, are subject to diel and seasonal fluctuations of environmental gradients. In neotropical estuaries
(Figures 1, 2, and 3), the variation in the seasonal ecocline,
238
ENVIRONMENTAL GRADIENTS