196
H. Mianzan eta!.
13.4 Biological Significance of the Salt Wedge Regime
Estuarine processes are inherently three-dimensional. As a main characteristic of the Rio de la Plata estuary, the salt wedge regime plays an important
role for reproductive processes of fishes and, owing to density discontinuity between two different water masses, by generating border systems,
which are the surface for several ecological processes. The bottom salinity
front near the head of the salt wedge is typical of pronounced density discontinuities and the retention of particulate matter and plankton coincides
with the occurrence of turbidity maxima. The tip of the salt wedge represents an ecotone between the river and the estuary that few species (i.e.,
the anchovy Lycengraulis grossidens) seem to be able to cross. Density discontinuities extend for up to 200 km of the mixohaline regime along the
halocline and terminate at the surface salinity front. This front represents
the highly dynamic part of the salt wedge regime with a less defined salinity gradient, which delineates the boundary between estuary and continental shelf waters (Fig. 13.2 ).
13.4.1 Head of the Salt Wedge
In general, the upper freshwater layer at the head of the salt wedge is
poorer in fish and plankton species than the bottom saline layer. During
spring and summer, the whitemouth croaker (Micropogonias furnieri) and
Brazilian menhaden (Brevoortia aurea) aggregate close to the bottom salinity front (Macchi et al. 1996; Acha 1999) and spawn pelagic eggs which
remain below the halocline taking advantage of the retention properties at
the head of the salt wedge (Fig. 13.2A). In spite of not being a common
event in estuaries elsewhere, the large-scale salt wedge regime in the Rio de
la Plata estuary favors the spawning of pelagic eggs. The retention of eggs
and larvae in the convergence zone is not complete and some are transported seaward by residual currents; however, larvae appear to have
enough time to develop into a stage which permits control of their vertical
position in the water column (Acha 1999; Acha et al. 1999). Vertical migrations enable the larvae to take advantage of the seaward flowing surface
layer and the landward moving bottom layer, thus contributing to their
permanence in the system. Although adults of M. furnieri are also common
in continental shelf waters, the entire spawning cycle seems to occur within
the estuary (Fig. 13.3). Juveniles (about 2 em long) then migrate towards
the protected waters of Samboromb6n Bay, which they abandon after
2-3 years. Apart from Micropogonias furnieri and Brevoortia aurea, Sam-
H. Mianzan eta!.
13.4 Biological Significance of the Salt Wedge Regime
Estuarine processes are inherently three-dimensional. As a main characteristic of the Rio de la Plata estuary, the salt wedge regime plays an important
role for reproductive processes of fishes and, owing to density discontinuity between two different water masses, by generating border systems,
which are the surface for several ecological processes. The bottom salinity
front near the head of the salt wedge is typical of pronounced density discontinuities and the retention of particulate matter and plankton coincides
with the occurrence of turbidity maxima. The tip of the salt wedge represents an ecotone between the river and the estuary that few species (i.e.,
the anchovy Lycengraulis grossidens) seem to be able to cross. Density discontinuities extend for up to 200 km of the mixohaline regime along the
halocline and terminate at the surface salinity front. This front represents
the highly dynamic part of the salt wedge regime with a less defined salinity gradient, which delineates the boundary between estuary and continental shelf waters (Fig. 13.2 ).
13.4.1 Head of the Salt Wedge
In general, the upper freshwater layer at the head of the salt wedge is
poorer in fish and plankton species than the bottom saline layer. During
spring and summer, the whitemouth croaker (Micropogonias furnieri) and
Brazilian menhaden (Brevoortia aurea) aggregate close to the bottom salinity front (Macchi et al. 1996; Acha 1999) and spawn pelagic eggs which
remain below the halocline taking advantage of the retention properties at
the head of the salt wedge (Fig. 13.2A). In spite of not being a common
event in estuaries elsewhere, the large-scale salt wedge regime in the Rio de
la Plata estuary favors the spawning of pelagic eggs. The retention of eggs
and larvae in the convergence zone is not complete and some are transported seaward by residual currents; however, larvae appear to have
enough time to develop into a stage which permits control of their vertical
position in the water column (Acha 1999; Acha et al. 1999). Vertical migrations enable the larvae to take advantage of the seaward flowing surface
layer and the landward moving bottom layer, thus contributing to their
permanence in the system. Although adults of M. furnieri are also common
in continental shelf waters, the entire spawning cycle seems to occur within
the estuary (Fig. 13.3). Juveniles (about 2 em long) then migrate towards
the protected waters of Samboromb6n Bay, which they abandon after
2-3 years. Apart from Micropogonias furnieri and Brevoortia aurea, Sam-