HR: 1330h
AN: OS23A-10    [Abstracts]
TI: Nutrient Distribution and Benthic Foraminiferal Assemblages on the South Brazilian Shelf: Relationship With Water Masses and Freshwater Influx
AU: Eichler, B B
EM: bbeichle@usp.br
AF: Instituto Ocenografico da Universidade de São Paulo, Praca do Oceanografico, 191, Sao Paulo, SP 05508-900 Brazil
AU: Sen Gupta, B K
EM: barun@geol.lsu.edu
AF: Department of Geology and Geophysics Louisiana State University, 359 Howe-Russell Geoscience Complex, Baton Rouge, LA 70803-4101 United States
AU: * Eichler, P P
EM: patriciaeichler@gmail.com
AF: Delaware Geological Survey, Delaware Geological Survey Building University of Delaware, Newark, DE 19716-7501 United States
AU: Braga, E S
EM: edsbraga@usp.br
AF: Instituto Ocenografico da Universidade de São Paulo, Praca do Oceanografico, 191, Sao Paulo, SP 05508-900 Brazil
AU: Campos, E J
EM: edmo@io.usp.br
AF: Instituto Ocenografico da Universidade de São Paulo, Praca do Oceanografico, 191, Sao Paulo, SP 05508-900 Brazil
AU: Berbel, G G
EM: glauciaberbel@uol.com.br
AF: Instituto Ocenografico da Universidade de São Paulo, Praca do Oceanografico, 191, Sao Paulo, SP 05508-900 Brazil
AU: Rodrigues, A R
EM: andrerr@usp.br
AF: Instituto Ocenografico da Universidade de São Paulo, Praca do Oceanografico, 191, Sao Paulo, SP 05508-900 Brazil
AB: Using distributions of nutrients and benthic foraminifera, we tracked water masses on the South Brazilian Shelf and assessed the seasonality of continental runoff and the dilution of coastal waters. The data were obtained from water and sediment samples collected in winter 2003 and summer 2004. Silicate concentrations were 0.02-83.52 ŸYM in winter and 0.05-41.16 ŸYM in summer. The high silicate content in the surface water was associated with low salinity (minima 21.22 in winter and 21.96 in summer), confirming a significant input of freshwater. Phosphate values, a secondary tracer of the terrestrial input, were 0.00-3.30 ŸYM in winter and 0.03-2.26 ŸYM in summer, with the shallow-water phosphate content indicating a freshwater influence. The nitrate evidence showed the presence of new water in the region, derived from South Atlantic Central Water by upwelling processes. Nitrate, nitrite, and oxygen correlations indicated a denitrification process in bottom water, associated with the oxygen minimum layer, and with the suspended matter content. Dissolved oxygen values were 3.41-7.06 mL.L-1 in winter and 2.65 mL-6.85 mL.L-1 in summer. The lowest salinities were frequently associated with >20›XC temperatures. In winter, an inner-shelf salinity front was present, and the strong influence of freshwater led to a silicate maximum (27.79 ŸYM), increased concentrations of phosphate (2.70 ŸYM), Si(OH)4 (10.88 ŸYM), NO3 (1.01 ŸYM), and Total Dissolved Nitrogen (22.98 ŸYM), together with a high value of suspended matter (44.80 mg/L). The terrestrial input was significantly reduced with increasing distance from the coast, but high values of nutrients were maintained in subsurface waters, because of SACW upwelling. At shallow sampling stations, the influence of freshwater runoff was also demonstrated by (1) the dominance of benthic foraminifera such as lagoon-related Elphidium spp. and several agglutinated species, including Gaudryina exillis and Trochammina spp; and (2) an abundance of diatoms. The assemblage at offshore stations, in contrast, was characterized by smaller forms and higher species diversity. The foraminiferal data correlate well with nutrient distributions. Our findings suggest that during winter, when freshwater influence is stronger, the front along its plume has a strong effect on physical-chemical-biological interactions. In summer, the foraminiferal species Bulimina marginata, Cancris sacra, Cassidulina subglobosa, and Uvigerina peregrina found in the area suggest the influence of SACW nutrients brought up by upwelling processes.
DE: 4532 General circulation
DE: 4804 Benthic processes/benthos
DE: 4815 Ecosystems, structure and dynamics
DE: 4845 Nutrients and nutrient cycling
SC: Ocean Sciences [OS]
MN: 2005 Joint Assembly