CHAPTER 4 . Nutrients and Suspended Matter Behaviour in the Patos Lagoon Estuary (Brazil)
71
4.3
Results and Discussions
4.3.1
Salinity
The penetration of seawater through the channel, responsible for the horizontal and
vertical salinity distribution within the estuary, is more dependent on meteorological
factors such as wind and precipitation than on tidal effects, which dominate many
estuarine systems. Kjerfve (1986) attributes this feature to the large area of the estuary and the low elevation of the surroundings dune-oriented topography. Wind is thus
the major factor controlling circulation and dispersion.
During three of the cruises (1,3 and 7) the upper freshwater region of the estuar:y was
well mixed (Fig. 4.2). From the middle of the estuary to the mouth, the water column was
stratified. When rainfall is low and freshwater discharge is less important, as was the case
during these cruises, water surface elevation at the head of Patos Lagoon is lower. This
along with SW winds favours saltwater inflow, resulting in the observed stratification.
During cruise 2 (August, '89), the sampling region had higher salinity and was well
mixed. Cruises 4 and 5 were carried out during periods of high rainfall when the estuary was completely fresh. Hartmann (1988) showed that the discharge from Patos
Lagoon Estuary, during the winter, forms a low salinity plume that can penetrate 40 km
across the shelf. The annual peak winter fresh water input exceeds 1500 m3 S-l with
extremes reaching 25000 m3 S-l (Herz 1977). During cruises 6 and 8, the estuary was
stratified except at either end (stations 1 and 20).
The stratification observed in cruises 1, 3, 6, 7 and 8 results in a modification of
transport of material through Patos Lagoon Estuary which coupled with climatologic
conditions favours removal and storage within the estuarine region.
4.3.2
Dissolved Oxygen
Dissolved oxygen in the estuary was close to 100% saturation during all cruises. Values
decreased to 70% rarely. Because Patos Lagoon Estuary is shallow (approx. 50% of its area
is less than 1 m depth) and windy (mean monthly wind velocity is 5.12 m S-l averaged over
30 years (Vieira and Rangel 1988), it is well oxygenated. Also, because of these features,
it is unusual to find thermal stratification, except in the navigation channel (12 m depth).
4.3.3
Total Suspended Matter
During all cruises within the main navigation channel, observed total suspended
matter (TSM) concentrations increased towards the sea. Similar patterns were found
by Brockmann (1990) for the Elbe and Shannon estuaries. In these estuaries the cause
of turbidity maxima is estuarine circulation and in Patos Lagoon Estuary the circulation is likely to be more dependent on meteorological conditions since wind than tidal
events. There is no tide in Patos Lagoon Estuary because it's located close to an amphidromic point (region without tides) (200 km South) (Moller et al. 1991).
71
4.3
Results and Discussions
4.3.1
Salinity
The penetration of seawater through the channel, responsible for the horizontal and
vertical salinity distribution within the estuary, is more dependent on meteorological
factors such as wind and precipitation than on tidal effects, which dominate many
estuarine systems. Kjerfve (1986) attributes this feature to the large area of the estuary and the low elevation of the surroundings dune-oriented topography. Wind is thus
the major factor controlling circulation and dispersion.
During three of the cruises (1,3 and 7) the upper freshwater region of the estuar:y was
well mixed (Fig. 4.2). From the middle of the estuary to the mouth, the water column was
stratified. When rainfall is low and freshwater discharge is less important, as was the case
during these cruises, water surface elevation at the head of Patos Lagoon is lower. This
along with SW winds favours saltwater inflow, resulting in the observed stratification.
During cruise 2 (August, '89), the sampling region had higher salinity and was well
mixed. Cruises 4 and 5 were carried out during periods of high rainfall when the estuary was completely fresh. Hartmann (1988) showed that the discharge from Patos
Lagoon Estuary, during the winter, forms a low salinity plume that can penetrate 40 km
across the shelf. The annual peak winter fresh water input exceeds 1500 m3 S-l with
extremes reaching 25000 m3 S-l (Herz 1977). During cruises 6 and 8, the estuary was
stratified except at either end (stations 1 and 20).
The stratification observed in cruises 1, 3, 6, 7 and 8 results in a modification of
transport of material through Patos Lagoon Estuary which coupled with climatologic
conditions favours removal and storage within the estuarine region.
4.3.2
Dissolved Oxygen
Dissolved oxygen in the estuary was close to 100% saturation during all cruises. Values
decreased to 70% rarely. Because Patos Lagoon Estuary is shallow (approx. 50% of its area
is less than 1 m depth) and windy (mean monthly wind velocity is 5.12 m S-l averaged over
30 years (Vieira and Rangel 1988), it is well oxygenated. Also, because of these features,
it is unusual to find thermal stratification, except in the navigation channel (12 m depth).
4.3.3
Total Suspended Matter
During all cruises within the main navigation channel, observed total suspended
matter (TSM) concentrations increased towards the sea. Similar patterns were found
by Brockmann (1990) for the Elbe and Shannon estuaries. In these estuaries the cause
of turbidity maxima is estuarine circulation and in Patos Lagoon Estuary the circulation is likely to be more dependent on meteorological conditions since wind than tidal
events. There is no tide in Patos Lagoon Estuary because it's located close to an amphidromic point (region without tides) (200 km South) (Moller et al. 1991).