198
H. Mianzan eta!.
boromb6n Bay, followed by the Santa Lucia River mouth, are also the main
nursery ground for Pogonias cromis, Odonthestes spp., Mugilliza, Macrodon ancylodon, Paralichthys spp., and other fish species that inhabit the
estuary (Lasta 1995; Wells and Daborn 1997).
Zooplankton aggregations below the halocline at the head of the salt
wedge are largely represented by different life stages of Acartia tons a (up to
8,000 ind. m- 3 ; Fig. 13.2A). The upper freshwater layer, which may represent
more than 70% of the water column, is inhabited by few freshwater copepods ( <200 ind. m- 3 ; Ramirez, pers. comm.). At the thick freshwater layer,
the turbidity front, and suspended sediments constrain photosynthesis,
thus phytoplankton is poor. High concentrations of detrital biomass
(pheopigments; CARP 1989), tintinnids, and bacteria seem to constitute
the main energy supply for omnivorous species like Acartia tonsa. Where
horizontal salinity gradients reach a maximum, beds of the deposit-feeding clam Mactra isabelleana concentrate along the bottom salinity front,
probably taking advantage of high suspended matter deposition. Both
freshwater species (Doradidae) and the estuarine Micropogonias furnieri
prey on the clam.
13.4.2 Halocline and Surface Salinity Front
The widest range of salinity occurs in the mixohaline system and salinities
control the differential penetration and spatial distribution of adult fishes
in the estuary. Several species of Sciaenidae exhibit different salinity preferences like Micropogonias furnieri (1-33 ), Paralonchurus brasiliensis
(5-33), Macrodon ancylodon and Pogonias cromis (10-25), Menticirrhus
americanus (25-33), Cynoscion guatucupa (> 15), and Umbrina canosai
(about 30). At the salt wedge fish occur exclusively in the bottom layer.
However, in the outer mixohaline region, where the halocline is less pronounced, pelagic species like Trachurus lathami and Engraulis anchoita
occur in the increasingly saltier upper layer. Juveniles and small adults of E.
anchoita dominate at the surface salinity (24-33) front (Hansen and Madirolas 1996) where they probably feed on plankton. Parona signata, Paralichthys patagonicus, and Cynoscion guatucupa also spawn near this front
(Macchi 1998; Macchi and Acha 1998).
Plankton is mainly located in the lower saline layer and the highest zooplankton biomass aggregates below the halocline (Madirolas et al. 1997;
Fig. 13.2B). Where the halocline intersects the surface layer, live and dead
individuals ( 40%) of Acartia tons a concentrate in the upper layer (up to
110,000 ind. m- 3 ; Ramirez and Sorarrain, pers. comm.) and gelatinous
plankton (mainly Mnemiopsis maccradyi) aggregate at the surface salinity
H. Mianzan eta!.
boromb6n Bay, followed by the Santa Lucia River mouth, are also the main
nursery ground for Pogonias cromis, Odonthestes spp., Mugilliza, Macrodon ancylodon, Paralichthys spp., and other fish species that inhabit the
estuary (Lasta 1995; Wells and Daborn 1997).
Zooplankton aggregations below the halocline at the head of the salt
wedge are largely represented by different life stages of Acartia tons a (up to
8,000 ind. m- 3 ; Fig. 13.2A). The upper freshwater layer, which may represent
more than 70% of the water column, is inhabited by few freshwater copepods ( <200 ind. m- 3 ; Ramirez, pers. comm.). At the thick freshwater layer,
the turbidity front, and suspended sediments constrain photosynthesis,
thus phytoplankton is poor. High concentrations of detrital biomass
(pheopigments; CARP 1989), tintinnids, and bacteria seem to constitute
the main energy supply for omnivorous species like Acartia tonsa. Where
horizontal salinity gradients reach a maximum, beds of the deposit-feeding clam Mactra isabelleana concentrate along the bottom salinity front,
probably taking advantage of high suspended matter deposition. Both
freshwater species (Doradidae) and the estuarine Micropogonias furnieri
prey on the clam.
13.4.2 Halocline and Surface Salinity Front
The widest range of salinity occurs in the mixohaline system and salinities
control the differential penetration and spatial distribution of adult fishes
in the estuary. Several species of Sciaenidae exhibit different salinity preferences like Micropogonias furnieri (1-33 ), Paralonchurus brasiliensis
(5-33), Macrodon ancylodon and Pogonias cromis (10-25), Menticirrhus
americanus (25-33), Cynoscion guatucupa (> 15), and Umbrina canosai
(about 30). At the salt wedge fish occur exclusively in the bottom layer.
However, in the outer mixohaline region, where the halocline is less pronounced, pelagic species like Trachurus lathami and Engraulis anchoita
occur in the increasingly saltier upper layer. Juveniles and small adults of E.
anchoita dominate at the surface salinity (24-33) front (Hansen and Madirolas 1996) where they probably feed on plankton. Parona signata, Paralichthys patagonicus, and Cynoscion guatucupa also spawn near this front
(Macchi 1998; Macchi and Acha 1998).
Plankton is mainly located in the lower saline layer and the highest zooplankton biomass aggregates below the halocline (Madirolas et al. 1997;
Fig. 13.2B). Where the halocline intersects the surface layer, live and dead
individuals ( 40%) of Acartia tons a concentrate in the upper layer (up to
110,000 ind. m- 3 ; Ramirez and Sorarrain, pers. comm.) and gelatinous
plankton (mainly Mnemiopsis maccradyi) aggregate at the surface salinity