74
Luis Felipe Niencheski . Maria da Gra~a Baumgarten . Gilberto Fillmann . Herbert L. Windom
enriched in the water column. For cruises 4 (winter) and 5 (winter) the phosphate
concentration throughout the study area was dominated by the freshwater end-member and was relatively low.
During cruises 6 and 7, phosphate is generally stable throughout the estuary, increasing in the seaward direction in the cruise 6 and decreasing in a seaward direction during cruise 7. Finally, for cruise 8 (fall) phosphate was depleted in the low salinity region; increasing from virtually zero, at low salinities, toward the sea and exhibiting conservative behaviour.
As pointed out above, when Patos Lagoon is fresh, the concentration of both phosphate and TSM decrease. When the system has an established salinity gradient, the
phosphate and TSM concentrations generally rise with increasing salinity.
Baumgarten and Niencheski (1990) showed that iron is presented in high concentrations in freshwater at the upper estuarine region. Iron compounds can react with
phosphate and that are know to adsorb phosphate (Jonge and Villerius 1989). On the
other hand, Niencheski et al. (1994) presented the carbonate content of samples range
from 7-3-10% for those collected at the upper estuary and from 10-28% for those collected near the mouth of the estuary. This suggests that during transport to the upstream, some calcite formed at sea and in the estuary, dissolves and consequently some
of the phosphate desorbs (Jonge and Villerius 1989). Although, the adsorption of phosphate by other suspended matter components, such as clay minerals and iron it seems
to be more important, resulting in low concentration and consequently causing perturbation of the buffer phosphate mechanism, cited by Liss (1976). Presumably also
occurs phosphate uptake by primary producers.
The higher salinity region presents phosphate concentrations higher than at the
upper estuary, average <2 JlM, value considered normal in no impacted estuaries
(Aminot and Chaussepied 1983). This suggests that a dissolved phosphate fraction is
in equilibrium with an adsorbed fraction, with the latter acting as a buffer (Jonge and
Villerius 1989) and indicating that a buffer mechanism operates under estuarine conditions.
The phosphate concentration found in the Sao Gon<,:alo Channel, during cruise 8,
was comparatively larger than the estuarine values. It is obvious however, that at this
time the input from the Sao Gon<,:alo had little affect on phosphate concentrations in
the estuarine region. This is probably due to phosphate consumption by phytoplankton and/or to dilution of the input from Sao Gon<,:alo Channel by the much larger freshwater input from the northern part of Patos Lagoon.
On the estuarine semi-enclosed bays, highest phosphate values were observed in
Saco da Mangueira during all cruises (Fig. 4-4). Data for Saco do Martins are scattered,
but generally fit within the normal distribution observed for other stations in the
study area. These two embayments have similar geomorphology and depth and receive untreated sewage inputs (Almeida et al. 1993). This suggests that the increased
concentrations observed in Saco da Mangueira, in the South of Rio Grande City, is
not only due to natural inputs or sewage discharge but are probably due to industrial
releases, especially from fertilizer complex, one of the largest in South America (annual produc-tion approximately of ca. 1.0 x 10 6 tons), located on its border (Baumgarten et al. 1995).
Taft and Taylor (1976) cited several studies concerning phosphorus cycling in coastal
plain estuaries, where highest phosphate concentrations were observed in the sum-
Luis Felipe Niencheski . Maria da Gra~a Baumgarten . Gilberto Fillmann . Herbert L. Windom
enriched in the water column. For cruises 4 (winter) and 5 (winter) the phosphate
concentration throughout the study area was dominated by the freshwater end-member and was relatively low.
During cruises 6 and 7, phosphate is generally stable throughout the estuary, increasing in the seaward direction in the cruise 6 and decreasing in a seaward direction during cruise 7. Finally, for cruise 8 (fall) phosphate was depleted in the low salinity region; increasing from virtually zero, at low salinities, toward the sea and exhibiting conservative behaviour.
As pointed out above, when Patos Lagoon is fresh, the concentration of both phosphate and TSM decrease. When the system has an established salinity gradient, the
phosphate and TSM concentrations generally rise with increasing salinity.
Baumgarten and Niencheski (1990) showed that iron is presented in high concentrations in freshwater at the upper estuarine region. Iron compounds can react with
phosphate and that are know to adsorb phosphate (Jonge and Villerius 1989). On the
other hand, Niencheski et al. (1994) presented the carbonate content of samples range
from 7-3-10% for those collected at the upper estuary and from 10-28% for those collected near the mouth of the estuary. This suggests that during transport to the upstream, some calcite formed at sea and in the estuary, dissolves and consequently some
of the phosphate desorbs (Jonge and Villerius 1989). Although, the adsorption of phosphate by other suspended matter components, such as clay minerals and iron it seems
to be more important, resulting in low concentration and consequently causing perturbation of the buffer phosphate mechanism, cited by Liss (1976). Presumably also
occurs phosphate uptake by primary producers.
The higher salinity region presents phosphate concentrations higher than at the
upper estuary, average <2 JlM, value considered normal in no impacted estuaries
(Aminot and Chaussepied 1983). This suggests that a dissolved phosphate fraction is
in equilibrium with an adsorbed fraction, with the latter acting as a buffer (Jonge and
Villerius 1989) and indicating that a buffer mechanism operates under estuarine conditions.
The phosphate concentration found in the Sao Gon<,:alo Channel, during cruise 8,
was comparatively larger than the estuarine values. It is obvious however, that at this
time the input from the Sao Gon<,:alo had little affect on phosphate concentrations in
the estuarine region. This is probably due to phosphate consumption by phytoplankton and/or to dilution of the input from Sao Gon<,:alo Channel by the much larger freshwater input from the northern part of Patos Lagoon.
On the estuarine semi-enclosed bays, highest phosphate values were observed in
Saco da Mangueira during all cruises (Fig. 4-4). Data for Saco do Martins are scattered,
but generally fit within the normal distribution observed for other stations in the
study area. These two embayments have similar geomorphology and depth and receive untreated sewage inputs (Almeida et al. 1993). This suggests that the increased
concentrations observed in Saco da Mangueira, in the South of Rio Grande City, is
not only due to natural inputs or sewage discharge but are probably due to industrial
releases, especially from fertilizer complex, one of the largest in South America (annual produc-tion approximately of ca. 1.0 x 10 6 tons), located on its border (Baumgarten et al. 1995).
Taft and Taylor (1976) cited several studies concerning phosphorus cycling in coastal
plain estuaries, where highest phosphate concentrations were observed in the sum-