Sector
ysed for the
; of autumn,
hese images
| during an
done in the
10 days, the
he center of
plankton. It
tant factors
listant from
with the
uth Atlantic
ope. Recent
urce of the
this region
me cases of
ys, strong
emperatures
Cabo Frio.
an the mid
c time scale
its over the
ckwise and
t inhibiting
dinamically
tern ocean
Brazil, low
nutrient, biomass and productivity levels are typically
associated with the warm, high salinity surface water
mass, the Tropical Water (TW). The oligotrophic TW
occupies the upper 200 m of the water column, being
carried by the Brazilian Current (BC) along the 200 m
isobath and is characterized by temperatures higher than
20°C and salinities greater than 36 PSU (Signorini, 1978).
Increased plankton biomass and productivity observed in
the SBB are linked to an increase in nitrogen
concentrations caused by intrusions of SACW along the
bottom of the continental shelf and inshore areas (Valentin
et al., 1987, Brandini, 1990, Gaeta ef al., 1996).
In this paper, a set of AVHRR infrared images for the
SBB are compared with surface variations in the
mesoscale distribution of chemical (nitrate) and biological
properties (phytoplankton biomass) to show that an
observed increase in productivity was caused by the
horizontal advection of a cold water mass originating at
Cabo Frio due to a strong upwelling event.
2. THE DATA SET
2.1 Satellite Data
Daily images from the Advanced Very High Resolution
Radiometer (AVHRR) — aboard the TIROS-N/NOAA
satellites have been recorded in the High Resolution
Picture Transmission (HRPT) mode by INPE in the level
I-B NOAA format (Kidwell, K.B., 1992) since
September of 1992. The digital satellite data were
processed using the SEAPAK software (McClain et al.,
1992) to generate the Sea Surface Temperature (SST)
maps used for analysis. The digital processing of this data
set involved: a) image ingestion with the separation of the
five channel images from the interleaved original data; b)
application of one of the Multichannel Sea Surface
Temperature (MCSST) NOAA algorithms for atmospheric
correction and generation of the SST images (McClain et
al, 1985); c) remaping to a common map projection
(Mercator) and geographical gridding; and d) digital
Image enhancement to increase the contrast and to
facilitate interpretation of the thermal features present.
22 Biological and Nutrient in situ data
A total of 51 stations were occupied from January 25 to
February 04, 1993 (Fig.2). Along each of the cross shelf
transects, hydrography (CTD with rosette system),
lutrlents, and phytoplankton biomass data were obtained
at indicated stations. In addition, 3-4 of the stations
449
occupied along each transect were chosen to make in situ-
simulated diurnal experimental observations of primary
productivity. Vertical distributions of nutrients and
phytoplankton biomass were based on samples collected at
(standard) depths: 0, 25, 50, 75, 100, 125, 200, 300 m.
Nutrient concentrations were determined according to
Grasshoff (1983) and chlorophyll a concentrations were
measured by fluorescence (Yentsch and Menzel, 1963).
Primary productivity was determined by the 14¢ tracer
technique of Steemann-Nielsen (1952).
3. RESULTS AND DISCUSSION
Fig. 3a shows that by the mid part of January of 1995, a
strong upwelling event had been stablished in the Cabo
Frio area. Two upwelling centers (18°C ) can be observed,
one at Cabo Frio and other near Cabo Sáo Tomé. It is also
clear that the former surface center is being advected
southwestward, with the upwelling plume reaching the
southern vicinity of Säo Sebastiäo Island (23.8°S;
45.6°W), the northernmost part of the oceanographic
survey area (See Fig. 2). A gradual increase in
temperature with distance is observed, probably resulting
from mixing and solar heating. Figure 3b shows that the
upwelling event was sustained until the end of the
January, with even lower surface temperature at the Cabo
Frio upwelling core. Again, the upwelling plume is seen
being advected to the area near S3o Sebastiäo.
Figure 4a shows the surface nitrate concentration with two
maxima (1.5 to 2.0 uM) present on the northernmost inner
shelf and slope areas. Comparing the surface nitrate field
with the SST maps, it is clear that the very extensive
tongues of cold water advected southwestward and
reaching the center of the SSB, are spatially correlated
with the upwelled nitrate. Thus, the phytoplankton
biomass bloom observed at the surface (Fig. 4b) on the
mid-shelf (100-150 m depths) was influenced by nutrient
enriched waters from Cabo Frio 300 km distant from this
region.
Values of chlorophyll a integrated over the euphotic zone
(not shown) revealed that there is not a monotonic decline
in the seaward direction, but rather shows a trend, with the
highest values occurring in two sites: one at the shelf on
the northernmost sections and the other at the slope near
the southern corner of the sampled region. The former, as
previously stated, was enriched by cold waters from Cabo
Frio and, in addition, showed maximum chlorophyll a at
the surface. On the other hand, the later showed maximum
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996
