The Convergence Ecosystem in the Southwest Atlantic
157
waters tend to have higher biodiversity than cold waters, the elevated number of animal plankton species suggests high biodiversity for shelf and
slope areas between 30°S and 40°S (Boltovskoy 1981). Also, diatom and silicoflagellate diversity (Shannon-Weaver index 3.4-4.6) in winter compares
favorably with highly diverse marine ecosystems elsewhere (Lange and
Mostajo 1985). The relative abundance and richness of demersal fishes off
southern Brazil is influenced by seasonal bottom temperature due to the
migration of the STC, while species richness over the shelf break seems to
be related to bottom substrate, invertebrate fauna, and distinct water layers
(Haimovici 1997b ). Indications of increased biodiversity in this warm
transition ecosystem may be partly due to long distance transport by both
the coastal branch of the Malvinas Current (Stevenson et al. 1998) and the
Brazil Current (Boltovskoy 1981).
11.5 Biological Production and Trophic Structure
The presence of distinct water masses over the SW Atlantic shelf and slope
governs biological production and trophic interactions. Internal nutrient
cycling is essential for the maintenance of biological production and trophic structure in oligotrophic TW with low chlorophyll-a ( <0.5 mg m- 3 )
and particulate primary production ( <1.0 mg C m- 3 h- 1 ; Brandini 1990;
Aidar et al. 1993; Ciotti et al. 1995) and high dissolved carbon (> 20 %; Vieira and Teixeira 1981) and regenerated production (ammonia, >60 %; Metzler et al. 1997). Annual primary production rates are less than 50 g C m- 2 •
The importance of pico-nanoplankton (autotrophic and heterotrophic
picoplankton, flagellates, and ciliates) based communities indicates that
microbial processes are fundamental in structuring trophic interactions
with long and complex food webs (Castello et al. 1997). Filter feeders, like
the cladocera Pen ilia avirostris and Thaliaceae, are abundant and consume
bacteria and picoplankton. Planktonic mollusks (pteropods) feed on small
particles, phytoplankters, protozoans, and zooplankters, which adhere to
their external mucus. Euphasiaceae present filter and raptorial feeding,
while almost all coelenterates are carnivores and prey on plankters, small
fishes, and/or nekton. Zooplanktophagous fishes (Peprilus paru, young Trichiurus lepturus, Balistes capriscus, Cynoscion jamaicensis) and the benthophagous Ctenosciaenea gracilirrhus arrive from the north, while other
ichthyophagous and/or benthic feeding species (Pomatomus saltatrix,
adult Umbrina canosai, Cynoscion guatucupa, Micropogonias furnieri,
Pagrus pagrus, and Mustelus schmitti) migrate southward. In slope regions,
dense schools of the skipjack tuna (Katsuwonus pelamis) feed heavily on
157
waters tend to have higher biodiversity than cold waters, the elevated number of animal plankton species suggests high biodiversity for shelf and
slope areas between 30°S and 40°S (Boltovskoy 1981). Also, diatom and silicoflagellate diversity (Shannon-Weaver index 3.4-4.6) in winter compares
favorably with highly diverse marine ecosystems elsewhere (Lange and
Mostajo 1985). The relative abundance and richness of demersal fishes off
southern Brazil is influenced by seasonal bottom temperature due to the
migration of the STC, while species richness over the shelf break seems to
be related to bottom substrate, invertebrate fauna, and distinct water layers
(Haimovici 1997b ). Indications of increased biodiversity in this warm
transition ecosystem may be partly due to long distance transport by both
the coastal branch of the Malvinas Current (Stevenson et al. 1998) and the
Brazil Current (Boltovskoy 1981).
11.5 Biological Production and Trophic Structure
The presence of distinct water masses over the SW Atlantic shelf and slope
governs biological production and trophic interactions. Internal nutrient
cycling is essential for the maintenance of biological production and trophic structure in oligotrophic TW with low chlorophyll-a ( <0.5 mg m- 3 )
and particulate primary production ( <1.0 mg C m- 3 h- 1 ; Brandini 1990;
Aidar et al. 1993; Ciotti et al. 1995) and high dissolved carbon (> 20 %; Vieira and Teixeira 1981) and regenerated production (ammonia, >60 %; Metzler et al. 1997). Annual primary production rates are less than 50 g C m- 2 •
The importance of pico-nanoplankton (autotrophic and heterotrophic
picoplankton, flagellates, and ciliates) based communities indicates that
microbial processes are fundamental in structuring trophic interactions
with long and complex food webs (Castello et al. 1997). Filter feeders, like
the cladocera Pen ilia avirostris and Thaliaceae, are abundant and consume
bacteria and picoplankton. Planktonic mollusks (pteropods) feed on small
particles, phytoplankters, protozoans, and zooplankters, which adhere to
their external mucus. Euphasiaceae present filter and raptorial feeding,
while almost all coelenterates are carnivores and prey on plankters, small
fishes, and/or nekton. Zooplanktophagous fishes (Peprilus paru, young Trichiurus lepturus, Balistes capriscus, Cynoscion jamaicensis) and the benthophagous Ctenosciaenea gracilirrhus arrive from the north, while other
ichthyophagous and/or benthic feeding species (Pomatomus saltatrix,
adult Umbrina canosai, Cynoscion guatucupa, Micropogonias furnieri,
Pagrus pagrus, and Mustelus schmitti) migrate southward. In slope regions,
dense schools of the skipjack tuna (Katsuwonus pelamis) feed heavily on