Chapter 1 Biogeographical Features of the Study Area
The study area lies within the geographical range
of the so-called antiboreal South American (Eckman 1953), Magellanic (Balech 1954; Schilder
1956; Briggs 1974) or Magellanean (Antezana
1981) Province which extends from 42°S to the
Antarctic Convergence along the channels of
western South America, around its tip to Peninsula Valdes and along the east coast of South
America, including the Falkland (Malvinas) Islands. The northern limits and districts of this
Province, on the Pacific side, are not well established, according to authors who have studied
the littoral (Hedgpeth 1969) and pelagic (Antezana 1981) faunas in this area. Based on the littoral
fauna, Balech (1954) distinguished two districts:
the chiloensis from 41 os to 51 os and the fuegensis
from 51 os to the tip of South America. Antezana
(op. cit.) recognized these two districts for the
pelagic fauna as well, but proposed boundaries
from 42-47°S and 47 -60°S. The break between
the two districts was given by the southern limit
of the sub-Antarctic temperate euphausiid Euphausia lucens and by the northern limit of intrusion of transition zone species such as the
euphausiids Thysanoessa gregaria and Nematoscelis megalops in coastal waters. The southern
limit, at the Antarctic Convergence, was associated with the distribution of many planktonic species such as the medusae Cosmostirella davisi
and Phialidium simplex, the salp Salpa thompsoni, the pteropods Limacina helicina and Clio sulcata, and the euphausiids Euphausia vallentini,
E. similis and E. longirostris. The northern limit,
which extends up to ca. 38°S, was associated to
the limits of most sub-Antarctic species in the
Humboldt Current System, such as Sagitta gazellae, C. davisi, E. vallentini and E. lucens. No districts within the Magellanic Province were recognized by Eckman (1953), Knox (1960), Briggs
(1974), and Brattstrăm and Johanssen (1983).
The biota of the fjords, the Straits of Magellan
included, includes, as expected, components of
the Magellanic Province, such as sub-Antarctic
copepods (Marin and Antezana 1985; Mazzocchi et al. 1995), euphausiids (Antezana 1976) and
other zooplankters (Guglielmo et al. 1991; Antezana et al. 1992), but also transition zone species
(sensu Brinton 1962; Johnson and Brinton 1963)
such as the euphausiids T. gregaria and N. megalops (Antezana 1976).
The intrusion or colonization of the pelagic
environment of the Straits by oceanic sub-Antarctic and transition zone species may be limited
by physical barriers such as the shallow shelf at
the Atlantic entrance and the shallow sill at the
Pacific entrance. This may be particularly true
for meso- and bathypelagic species, which are
often strong diel vertical migrators. Other specific adaptations to an extremely heterogeneous,
and physically and chemically modified subAntarctic environment may determine the species richness, diversity, dominance and species
ro le in the energy flow within the community. A
trend towards strong geographical patchiness of
zooplankton with the aggregation of single species (Antezana 1976), high frequency of occurrence ofred tides (Clement and Guzman 1989),
dominance of E. vallentini among the euphausiids (Antezana 1981) have been reported in the
context of pelagic adaptation to this estuarine
environment. Species differentiation with incomplete isolation may be presently occurring in
this geologically new environment.
A major source of discussion refers to the
geographical extent of the influence of Atlantic
and Pacific waters and fauna in the Straits of
Magellan. Based on temperature-salinity diagrams, Pickard (1973) suggested that the change
in water mass structure from the Atlantic to the
Pacific occurs between Carlos III Island and
The study area lies within the geographical range
of the so-called antiboreal South American (Eckman 1953), Magellanic (Balech 1954; Schilder
1956; Briggs 1974) or Magellanean (Antezana
1981) Province which extends from 42°S to the
Antarctic Convergence along the channels of
western South America, around its tip to Peninsula Valdes and along the east coast of South
America, including the Falkland (Malvinas) Islands. The northern limits and districts of this
Province, on the Pacific side, are not well established, according to authors who have studied
the littoral (Hedgpeth 1969) and pelagic (Antezana 1981) faunas in this area. Based on the littoral
fauna, Balech (1954) distinguished two districts:
the chiloensis from 41 os to 51 os and the fuegensis
from 51 os to the tip of South America. Antezana
(op. cit.) recognized these two districts for the
pelagic fauna as well, but proposed boundaries
from 42-47°S and 47 -60°S. The break between
the two districts was given by the southern limit
of the sub-Antarctic temperate euphausiid Euphausia lucens and by the northern limit of intrusion of transition zone species such as the
euphausiids Thysanoessa gregaria and Nematoscelis megalops in coastal waters. The southern
limit, at the Antarctic Convergence, was associated with the distribution of many planktonic species such as the medusae Cosmostirella davisi
and Phialidium simplex, the salp Salpa thompsoni, the pteropods Limacina helicina and Clio sulcata, and the euphausiids Euphausia vallentini,
E. similis and E. longirostris. The northern limit,
which extends up to ca. 38°S, was associated to
the limits of most sub-Antarctic species in the
Humboldt Current System, such as Sagitta gazellae, C. davisi, E. vallentini and E. lucens. No districts within the Magellanic Province were recognized by Eckman (1953), Knox (1960), Briggs
(1974), and Brattstrăm and Johanssen (1983).
The biota of the fjords, the Straits of Magellan
included, includes, as expected, components of
the Magellanic Province, such as sub-Antarctic
copepods (Marin and Antezana 1985; Mazzocchi et al. 1995), euphausiids (Antezana 1976) and
other zooplankters (Guglielmo et al. 1991; Antezana et al. 1992), but also transition zone species
(sensu Brinton 1962; Johnson and Brinton 1963)
such as the euphausiids T. gregaria and N. megalops (Antezana 1976).
The intrusion or colonization of the pelagic
environment of the Straits by oceanic sub-Antarctic and transition zone species may be limited
by physical barriers such as the shallow shelf at
the Atlantic entrance and the shallow sill at the
Pacific entrance. This may be particularly true
for meso- and bathypelagic species, which are
often strong diel vertical migrators. Other specific adaptations to an extremely heterogeneous,
and physically and chemically modified subAntarctic environment may determine the species richness, diversity, dominance and species
ro le in the energy flow within the community. A
trend towards strong geographical patchiness of
zooplankton with the aggregation of single species (Antezana 1976), high frequency of occurrence ofred tides (Clement and Guzman 1989),
dominance of E. vallentini among the euphausiids (Antezana 1981) have been reported in the
context of pelagic adaptation to this estuarine
environment. Species differentiation with incomplete isolation may be presently occurring in
this geologically new environment.
A major source of discussion refers to the
geographical extent of the influence of Atlantic
and Pacific waters and fauna in the Straits of
Magellan. Based on temperature-salinity diagrams, Pickard (1973) suggested that the change
in water mass structure from the Atlantic to the
Pacific occurs between Carlos III Island and