and subantarctic waters (Garzoli and Garraffo 1989; Brandini et al. 2000; Rivas
et al. 2006). Maximum annual values of chlorophyll-a at the ACS occur in spring
and summer, with minimum values in winter (Rivas et al. 2006). This high primary
productivity supports a great number of fisheries in the Southwestern Atlantic
Ocean (Bremec et al. 2000; Schejter et al. 2002; Acha et al. 2004).
In order to characterize the different coastal regions, we will describe the
physical oceanographic features of the coastal fronts (Fig. 11.2).
The Río de la Plata estuary is an extensive and shallow coastal plain estuary at
35°–36°S that receives freshwater from two major rivers (Paraná and Uruguay).
The system is characterized by strong vertical stratification. Freshwater flows
seaward on the surface while denser shelf water intrudes along the bottom, taking
the shape of a salt wedge (Guerrero et al. 1997; Acha et al. 2004; Lucas et al.
2005). Due to lack of osmoregulation in echinoderms, this frontal zone might
represent an important geographic barrier for the distribution of coastal echinoderms. Many echinoderm species are distributed off RP estuary, but only in denser
and deeper shelf waters of the front.
The front at the coastal zone of El Rincón (39°–41°S, depth \ 40 m) encloses
an area of 10,000 km
2 , showing weak seasonality. The front is characterized by
vertical homogeneity due to tidal forcing. A coastal front separates diluted coastal
water, from Negro and Colorado rivers, and shelf waters. Salinity gradients
increase on the continental shelf due to high saline waters originated in SMG
(Guerrero and Piola 1997; Acha et al. 2004; Lucas et al. 2005).
The Península Valdés tidal mixing front is a mesoscale (100–1,000 km) thermal
front. The front is observed in spring and summer, which establishes the boundary
between stratified waters offshore and a coastal vertically mixed body of water (Acha
et al. 2004). The structure of the front is maintained until autumn when stratification
of shelf waters decays. The front begins to form in spring (Acha et al. 2004).
Rivas and Ripa (1989) analyzed the temperature and salinity of the Nuevo gulf.
They observed no stratification of temperature during winter. However, this
condition was not observed during summer when a layer of warmed water overlayed cooler, deeper water. San José gulf is influenced by winds and tidal currents.
Therefore, for a great part of the year, this gulf does not show stratification. Only a
weak vertical structure occurs during summer (Rivas 1990).
Off southern Patagonia, water masses on the northern extreme of the Drake
Passage are diluted. This dilution is caused by an excess of rainfall in the SE Pacific
and the continental discharge along the west coast of South America. The flow
towards the Atlantic, known as Cabo de Hornos Current, enters onto the continental
shelf and contributes to lowering the salinity of the Magellan Strait. Several basins
drain along the strait, collecting abundant rainfall during summer and discharging
further diluted waters into the ACS. A subsequent diluted plume is then traced 200 km
off-shore (100 m depth) and 800 km northward. The plume reaches the southern limit
of SJOG and the coastal tidal front (Krepper and Rivas 1979; Acha et al. 2004).
Near the ACS break, the Subantarctic shelf waters of the Malvinas Current
produce an important thermohaline front (Martos and Piccolo 1988; Lutz and
Carreto 1991). The shelf-break front is a permanent feature that characterizes the
11 Echinoderms from Argentina
363
et al. 2006). Maximum annual values of chlorophyll-a at the ACS occur in spring
and summer, with minimum values in winter (Rivas et al. 2006). This high primary
productivity supports a great number of fisheries in the Southwestern Atlantic
Ocean (Bremec et al. 2000; Schejter et al. 2002; Acha et al. 2004).
In order to characterize the different coastal regions, we will describe the
physical oceanographic features of the coastal fronts (Fig. 11.2).
The Río de la Plata estuary is an extensive and shallow coastal plain estuary at
35°–36°S that receives freshwater from two major rivers (Paraná and Uruguay).
The system is characterized by strong vertical stratification. Freshwater flows
seaward on the surface while denser shelf water intrudes along the bottom, taking
the shape of a salt wedge (Guerrero et al. 1997; Acha et al. 2004; Lucas et al.
2005). Due to lack of osmoregulation in echinoderms, this frontal zone might
represent an important geographic barrier for the distribution of coastal echinoderms. Many echinoderm species are distributed off RP estuary, but only in denser
and deeper shelf waters of the front.
The front at the coastal zone of El Rincón (39°–41°S, depth \ 40 m) encloses
an area of 10,000 km
2 , showing weak seasonality. The front is characterized by
vertical homogeneity due to tidal forcing. A coastal front separates diluted coastal
water, from Negro and Colorado rivers, and shelf waters. Salinity gradients
increase on the continental shelf due to high saline waters originated in SMG
(Guerrero and Piola 1997; Acha et al. 2004; Lucas et al. 2005).
The Península Valdés tidal mixing front is a mesoscale (100–1,000 km) thermal
front. The front is observed in spring and summer, which establishes the boundary
between stratified waters offshore and a coastal vertically mixed body of water (Acha
et al. 2004). The structure of the front is maintained until autumn when stratification
of shelf waters decays. The front begins to form in spring (Acha et al. 2004).
Rivas and Ripa (1989) analyzed the temperature and salinity of the Nuevo gulf.
They observed no stratification of temperature during winter. However, this
condition was not observed during summer when a layer of warmed water overlayed cooler, deeper water. San José gulf is influenced by winds and tidal currents.
Therefore, for a great part of the year, this gulf does not show stratification. Only a
weak vertical structure occurs during summer (Rivas 1990).
Off southern Patagonia, water masses on the northern extreme of the Drake
Passage are diluted. This dilution is caused by an excess of rainfall in the SE Pacific
and the continental discharge along the west coast of South America. The flow
towards the Atlantic, known as Cabo de Hornos Current, enters onto the continental
shelf and contributes to lowering the salinity of the Magellan Strait. Several basins
drain along the strait, collecting abundant rainfall during summer and discharging
further diluted waters into the ACS. A subsequent diluted plume is then traced 200 km
off-shore (100 m depth) and 800 km northward. The plume reaches the southern limit
of SJOG and the coastal tidal front (Krepper and Rivas 1979; Acha et al. 2004).
Near the ACS break, the Subantarctic shelf waters of the Malvinas Current
produce an important thermohaline front (Martos and Piccolo 1988; Lutz and
Carreto 1991). The shelf-break front is a permanent feature that characterizes the
11 Echinoderms from Argentina
363