2
Chapter 1 Biogeographical Features of the Study Area
Ca pe Froward in the Intermediate Passage. Artegiani et al. {1991) showed the distribution for
temperature, salinity and density in a transect
through the Straits. It seems that the major inflow of water and plankton into the Straits
comes from the Pacific entrance and from the
southern entrance of Magdalena Channel. The
influence of Atlantic inflow is restricted to the
Second Narrow of the Straits due to the extreme
depth and restriction at this point and at the
First Narrow. Also, there is a strong discontinuity in temperature as one approaches the eastern
entrance (Artegiani and Paschini 1991) and another discontinuity in temperature and salinity
at the southern entrance and through the
Magdalena Channel (Artegiani et al. 1991). Finally, there is a strong break in the plankton
community in the First and Second Narrows
(Guglielmo et al. 1991). Consistent with these
findings is the presence of the euphausiid Thysanoessa gregaria and the absence of another
species of euphausiid, T. vicina, frequently encountered in the Atlantic sector of the Straits
(Antezana and Brinton 1981), suggesting that
the inflow of Atlantic waters is mainly limited to
the First Narrow.
As already reported in the first volume of this
Atlas, there are very few studies on the zooplankton of the Straits. The first volume described the
copepod assemblages of the region during the
austral summer (February-March), 1991. Of the
other zooplankton groups, amphipods, euphausiids, mysiids, ostracods and chaetognaths were
the most plentiful in this period and are described in this volume.
With regards to the amphipods, six species of
gammariids and seven species of hyperiids are
described in Chapter 3. Other species of gammariids and caprelliids may have been present at
the time, at stations north of Pun ta Arenas near
the entrance to the Atlantic Ocean. However,
these are not mentioned in this volume, because
the specimens were either too young to be correctly identified or greatly damaged. Of the 13
species described, only the hyperiid Themisto
gaudichaudii was sampled in high numbers
( 4551 specimens, 1447 of which juveniles) at aU
stations in the Straits. Four species were very
rare ( Thryphosella schellenbergi, ]as sa falcata,
Hyperoche mediterranea and Hyperiella dilatata) with only one specimen sampled for each
species. In general, hyperiids were more commonly sampled at Pacific stations with the exception of Hyperoche medusarium, which was
sampled only in Central waters from Cape Froward to Paso Ancho, and T. gaudichaudii,
present throughout the Straits. By contrast, gammariids were more uniformly distributed from
the Pacific entrance up to the Prim era Angostura
(St. 22).
Six species of euphausiids are described in
Chapter 4, Euphausia vallentini, E. lucens, Thysanoessa gregaria, Nematoscelis megalops, Stylocheiron longicorne and S. maximum. Of these,
the most abundant by far was E. vallentini. The
number and size of individuals of this species increased from the Pacific towards the Atlantic, up
to the Paso Ancho area where the largest specimens were sampled. In this area, E. vallentini
formed dense swarms in proximity to the bottom during the day, and underwent strong vertical migrations at night. N. megalops was also
sampled throughout the Straits but in lower
numbers compared to E. vallentini. It was more
commonly found in the Pacific area, and also in
the Paso Ancho region, where the greatest number of adults was recorded. In contrast, E. lucens
was sampled mainly at the Pacific entrance, until
Tamar Island, and was only sporadically present
in the Atlantic sector (St. 26). The three other
species were very rare.
Of the four species of mysids described in
Chapter 5, Boreomysis rostrata was by far the
most abundant, accounting for more than 93 o/o
of the species sampled. The species was mainly
sampled in the Pacific and Central areas of the
Straits, where both sexes and ali developmental
stages were present. In the Pacific area, it was
sampled from Tamar Island to Carlos III Island
(Sts. 6, 7, 9, 10, 11), whereas in the Central region
it was found, in great numbers, from Cape Froward to Cape S. Isidro (Sts. 13, 14 and 15). Ofthe
three other species of mysids, only two specimens of Pseudomma magellanensis were sampled in the Paso Ancho area, and only five adult
females of Arthromysis magellanica were found
at St. 26 in Atlantic waters beyond the Straits.
Neomysis monticelli was also sampled only at St.
26, where 4 males, 2 females and ca. 100 juveniles
were collected.
Chapter 1 Biogeographical Features of the Study Area
Ca pe Froward in the Intermediate Passage. Artegiani et al. {1991) showed the distribution for
temperature, salinity and density in a transect
through the Straits. It seems that the major inflow of water and plankton into the Straits
comes from the Pacific entrance and from the
southern entrance of Magdalena Channel. The
influence of Atlantic inflow is restricted to the
Second Narrow of the Straits due to the extreme
depth and restriction at this point and at the
First Narrow. Also, there is a strong discontinuity in temperature as one approaches the eastern
entrance (Artegiani and Paschini 1991) and another discontinuity in temperature and salinity
at the southern entrance and through the
Magdalena Channel (Artegiani et al. 1991). Finally, there is a strong break in the plankton
community in the First and Second Narrows
(Guglielmo et al. 1991). Consistent with these
findings is the presence of the euphausiid Thysanoessa gregaria and the absence of another
species of euphausiid, T. vicina, frequently encountered in the Atlantic sector of the Straits
(Antezana and Brinton 1981), suggesting that
the inflow of Atlantic waters is mainly limited to
the First Narrow.
As already reported in the first volume of this
Atlas, there are very few studies on the zooplankton of the Straits. The first volume described the
copepod assemblages of the region during the
austral summer (February-March), 1991. Of the
other zooplankton groups, amphipods, euphausiids, mysiids, ostracods and chaetognaths were
the most plentiful in this period and are described in this volume.
With regards to the amphipods, six species of
gammariids and seven species of hyperiids are
described in Chapter 3. Other species of gammariids and caprelliids may have been present at
the time, at stations north of Pun ta Arenas near
the entrance to the Atlantic Ocean. However,
these are not mentioned in this volume, because
the specimens were either too young to be correctly identified or greatly damaged. Of the 13
species described, only the hyperiid Themisto
gaudichaudii was sampled in high numbers
( 4551 specimens, 1447 of which juveniles) at aU
stations in the Straits. Four species were very
rare ( Thryphosella schellenbergi, ]as sa falcata,
Hyperoche mediterranea and Hyperiella dilatata) with only one specimen sampled for each
species. In general, hyperiids were more commonly sampled at Pacific stations with the exception of Hyperoche medusarium, which was
sampled only in Central waters from Cape Froward to Paso Ancho, and T. gaudichaudii,
present throughout the Straits. By contrast, gammariids were more uniformly distributed from
the Pacific entrance up to the Prim era Angostura
(St. 22).
Six species of euphausiids are described in
Chapter 4, Euphausia vallentini, E. lucens, Thysanoessa gregaria, Nematoscelis megalops, Stylocheiron longicorne and S. maximum. Of these,
the most abundant by far was E. vallentini. The
number and size of individuals of this species increased from the Pacific towards the Atlantic, up
to the Paso Ancho area where the largest specimens were sampled. In this area, E. vallentini
formed dense swarms in proximity to the bottom during the day, and underwent strong vertical migrations at night. N. megalops was also
sampled throughout the Straits but in lower
numbers compared to E. vallentini. It was more
commonly found in the Pacific area, and also in
the Paso Ancho region, where the greatest number of adults was recorded. In contrast, E. lucens
was sampled mainly at the Pacific entrance, until
Tamar Island, and was only sporadically present
in the Atlantic sector (St. 26). The three other
species were very rare.
Of the four species of mysids described in
Chapter 5, Boreomysis rostrata was by far the
most abundant, accounting for more than 93 o/o
of the species sampled. The species was mainly
sampled in the Pacific and Central areas of the
Straits, where both sexes and ali developmental
stages were present. In the Pacific area, it was
sampled from Tamar Island to Carlos III Island
(Sts. 6, 7, 9, 10, 11), whereas in the Central region
it was found, in great numbers, from Cape Froward to Cape S. Isidro (Sts. 13, 14 and 15). Ofthe
three other species of mysids, only two specimens of Pseudomma magellanensis were sampled in the Paso Ancho area, and only five adult
females of Arthromysis magellanica were found
at St. 26 in Atlantic waters beyond the Straits.
Neomysis monticelli was also sampled only at St.
26, where 4 males, 2 females and ca. 100 juveniles
were collected.