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C. Odebrecht and J.P. Castello
100 mg m- 2 (Ciotti et al. 1995). Further south (35-39.5°S), phytoplankton
maxima off the slope are associated with cyclonic eddies and SAW upwelling (Gayoso and Podesta 1996).
11.4 The Organisms
11.4.1 Plankton
In surface waters (31-34°S; Abreu 1997) heterotrophic bacteria attached
to particles (up to 5 x 10 5 cells ml- 1 ) are more abundant than free bacteria and total bacterial biomass (30-60 mg C m- 3 ) generally follows that of
phytoplankton chlorophyll-a in coastal waters and/or under influence of
SAW. Autotrophic picoplankton ( <2jlm) comprises more than 50 o/o of low
chlorophyll-a concentrations in oceanic surface water during spring,
while highest numbers (10 8 cells 1- 1 ) occur in subsurface coastal waters
(Odebrecht and Djurfeldt 1996). Phytoplankters, principally diatoms
( Chaetoceros, Coscinodiscus, Lauderia, Leptocylindrus, Rhizosolenia, Skeletonema, Thalassiosira, Pseudo-nitzschia), autotrophic (or mixotrophic)
thecate dinoflagellates ( Ceratium, Dinophysis, Prorocentrum, Scrippsiella), and naked organisms of the Gymnodiniales are the main primary
producers (Odebrecht and Garcia 1997; Mendez et al. 1998). Potentially
harmful and/or toxic species are dinoflagellates (Dinophysis acuminata,
Alexandrium tamarensis, Noctiluca scintillans, Gymnodinium catenatum,
Gyrodinium cf. aureolum), haptophycean (Prymnesium, Chrysochromulina) and raphidophycean flagellates ( Chatonella, Fibrocapsa, Heterosigma) as well as diatoms (Pseudo-nitzschia). The autotrophic ciliate
Mesodinium rubrum is common along the southern Brazilian and Uruguayan coast. Nanoflagellates share dominance with diatoms and dinoflagellates during high phytoplankton concentration (10 5 -10 6 cells l- 1 ) in
shelf and coastal waters under the influence of continental runoff, SAW
and SACW. However, under the influence of TW of the Brazil Current
nanoflagellates generally dominate in oceanic areas. Warm water phytoplankton species extend nearshore and abundance is usually low. Summer
blooms of the cyanobacteria Trichodesmium occur throughout the area
(23-35°S; Mendez et al. 1998; Rorig et al. 1998) and coccolithophorids are
important in TW and at the wide summer shelf front south of 38°S (Brandini 1988; Podesta 1997).
Protozooplankters (heterotrophic flagellates, ciliates, and ameboids)
appear to be an important component of the pelagic shelf ecosystem
C. Odebrecht and J.P. Castello
100 mg m- 2 (Ciotti et al. 1995). Further south (35-39.5°S), phytoplankton
maxima off the slope are associated with cyclonic eddies and SAW upwelling (Gayoso and Podesta 1996).
11.4 The Organisms
11.4.1 Plankton
In surface waters (31-34°S; Abreu 1997) heterotrophic bacteria attached
to particles (up to 5 x 10 5 cells ml- 1 ) are more abundant than free bacteria and total bacterial biomass (30-60 mg C m- 3 ) generally follows that of
phytoplankton chlorophyll-a in coastal waters and/or under influence of
SAW. Autotrophic picoplankton ( <2jlm) comprises more than 50 o/o of low
chlorophyll-a concentrations in oceanic surface water during spring,
while highest numbers (10 8 cells 1- 1 ) occur in subsurface coastal waters
(Odebrecht and Djurfeldt 1996). Phytoplankters, principally diatoms
( Chaetoceros, Coscinodiscus, Lauderia, Leptocylindrus, Rhizosolenia, Skeletonema, Thalassiosira, Pseudo-nitzschia), autotrophic (or mixotrophic)
thecate dinoflagellates ( Ceratium, Dinophysis, Prorocentrum, Scrippsiella), and naked organisms of the Gymnodiniales are the main primary
producers (Odebrecht and Garcia 1997; Mendez et al. 1998). Potentially
harmful and/or toxic species are dinoflagellates (Dinophysis acuminata,
Alexandrium tamarensis, Noctiluca scintillans, Gymnodinium catenatum,
Gyrodinium cf. aureolum), haptophycean (Prymnesium, Chrysochromulina) and raphidophycean flagellates ( Chatonella, Fibrocapsa, Heterosigma) as well as diatoms (Pseudo-nitzschia). The autotrophic ciliate
Mesodinium rubrum is common along the southern Brazilian and Uruguayan coast. Nanoflagellates share dominance with diatoms and dinoflagellates during high phytoplankton concentration (10 5 -10 6 cells l- 1 ) in
shelf and coastal waters under the influence of continental runoff, SAW
and SACW. However, under the influence of TW of the Brazil Current
nanoflagellates generally dominate in oceanic areas. Warm water phytoplankton species extend nearshore and abundance is usually low. Summer
blooms of the cyanobacteria Trichodesmium occur throughout the area
(23-35°S; Mendez et al. 1998; Rorig et al. 1998) and coccolithophorids are
important in TW and at the wide summer shelf front south of 38°S (Brandini 1988; Podesta 1997).
Protozooplankters (heterotrophic flagellates, ciliates, and ameboids)
appear to be an important component of the pelagic shelf ecosystem