190
H. Mianzan et a!.
are constant components of the soft bottom malacofauna, the last three
species having been introduced (Darrigran 1993). Oligochaetes (Limnodrilus claparedeianus, L. hoffmeisteri, Tubificidae, Haplotaxidae, Naididae
and Narapidae), nematodes, Hirudinea, Chironomidae, and Harpacticoidea are associated with organic matter-rich sediments (Rodrigues Capitulo et al.1997).
All of the approximately 120 freshwater fish species (mainly Cypriniformes and Siluriformes) belong to the Paranoplatense ichtyofauna and
together they constitute about 65 o/o of fish species in the estuary. Freshwater species sharply decrease in the mixohaline regime, where they represent about 7% of the total species number (Cousseau 1985). Leporinus
obtusidens and Eigenmania virescens frequently occur in freshwaters adjacent to the salt wedge. Catfishes like Parapimelodus valenciennesi, Luciopimelodus pati, Pimelodus clarias, P. maculatus, and P. albicans are also
common species in low salinity mixohaline waters close to the salinity
front. Pelagic fishes are mainly Atheriniformes (i.e., Odonthestes bonariensis argentinensis). Introduced species like silver carp (Hypophthalmichthys
molitrix, Cyprinidae; Garcia Romero et al. 1998), Acipenser, Acipenseriformes (Azpelicueta and Almiron in press), and Cyprinus carpio (Cyprinidae)
have been recorded.
13.3.2 Mixohaline Environment
13.3.2.1 Plankton
Mixohaline waters with salinity between 13 and 31 and high silicate concentrations are characterized by the presence of warm-water dinoflagellates
(Ceratium candelabrum, Dinophysis caudata) and the diatom Thalassiosira
puntigera (Negri et al. 1988). Other marine and euryhaline diatoms include
Ditylum brightwelli, Skeletonema costatum, Asterionella glacialis, and species of Coscinodiscus and Chaetoceros (CARP 1989). Owing to strong wave
action, benthic diatoms (Achnanthes brevipes, A. longipes, Melosira moniliforme, and Navicula grevillei) are often suspended into the plankton community. The toxic dinoflagellate Gymnodinium catenatum is associated with
the offshore limit of the surface salinity front ( Carreto and Akselman 1996).
Ctenophores (Mnemiopsis maccradyi and Pleurobrachia pileus) comprised 81 o/o of total organic carbon at the external limit of the surface
salinity (28-33) front. Mnemiopsis is most common in areas of thermal
and saline stratification, while Pleurobrachia dominates less stratified
areas. Gelatinous plankton tends to aggregate in areas where isolines out-
H. Mianzan et a!.
are constant components of the soft bottom malacofauna, the last three
species having been introduced (Darrigran 1993). Oligochaetes (Limnodrilus claparedeianus, L. hoffmeisteri, Tubificidae, Haplotaxidae, Naididae
and Narapidae), nematodes, Hirudinea, Chironomidae, and Harpacticoidea are associated with organic matter-rich sediments (Rodrigues Capitulo et al.1997).
All of the approximately 120 freshwater fish species (mainly Cypriniformes and Siluriformes) belong to the Paranoplatense ichtyofauna and
together they constitute about 65 o/o of fish species in the estuary. Freshwater species sharply decrease in the mixohaline regime, where they represent about 7% of the total species number (Cousseau 1985). Leporinus
obtusidens and Eigenmania virescens frequently occur in freshwaters adjacent to the salt wedge. Catfishes like Parapimelodus valenciennesi, Luciopimelodus pati, Pimelodus clarias, P. maculatus, and P. albicans are also
common species in low salinity mixohaline waters close to the salinity
front. Pelagic fishes are mainly Atheriniformes (i.e., Odonthestes bonariensis argentinensis). Introduced species like silver carp (Hypophthalmichthys
molitrix, Cyprinidae; Garcia Romero et al. 1998), Acipenser, Acipenseriformes (Azpelicueta and Almiron in press), and Cyprinus carpio (Cyprinidae)
have been recorded.
13.3.2 Mixohaline Environment
13.3.2.1 Plankton
Mixohaline waters with salinity between 13 and 31 and high silicate concentrations are characterized by the presence of warm-water dinoflagellates
(Ceratium candelabrum, Dinophysis caudata) and the diatom Thalassiosira
puntigera (Negri et al. 1988). Other marine and euryhaline diatoms include
Ditylum brightwelli, Skeletonema costatum, Asterionella glacialis, and species of Coscinodiscus and Chaetoceros (CARP 1989). Owing to strong wave
action, benthic diatoms (Achnanthes brevipes, A. longipes, Melosira moniliforme, and Navicula grevillei) are often suspended into the plankton community. The toxic dinoflagellate Gymnodinium catenatum is associated with
the offshore limit of the surface salinity front ( Carreto and Akselman 1996).
Ctenophores (Mnemiopsis maccradyi and Pleurobrachia pileus) comprised 81 o/o of total organic carbon at the external limit of the surface
salinity (28-33) front. Mnemiopsis is most common in areas of thermal
and saline stratification, while Pleurobrachia dominates less stratified
areas. Gelatinous plankton tends to aggregate in areas where isolines out-