CHAPTER 4 . Nutrients and Suspended Matter Behaviour in the Patos Lagoon Estuary (Brazil)
69
the estuarine region, and thus have a significant residence time within the lagoon
during which biogeochemical process may result in their consumption, consequently
reducing the inputs associated with freshwater to the estuary. In Patos Lagoon, the
residence time depends more upon meteorological conditions (wind, rainfall,
evaporation, etc.) than on tidal exchange because of the low tidal range (Moller et al.
1991). The time that water takes to travel from Guafba River to the estuarine region is
about 20 days (Herz 1977). In this area the annual rainfall exceeds annual evaporation.
Vieira and Rangel (1988), using data collected over a twenty year period (1957-1977),
calculated the annual rainfall to be 132 cm and the annual evaporation to be as ca. 90 cm.
There have been few studies of nutrients and suspended matter behaviour in this
system. Vilas Boas (1990), however, suggests that the Northern part of Patos Lagoon
may receive significant anthropogenic inputs as a result of rapid population growth
and industrialisation along the northern region, particularly during the last few decades around Porto Alegre, the fifth largest Brazilian city. Another additional input to
the estuary is that from Sao Gon~alo Channel, which has a discharge averaged over
30 year period of 700 m3 S-I (Vieira and Rangel 1988). Concentrations of nutrients and
suspended sediments in the Sao Gon~alo Channel input from Mirim Lagoon are the
result of agricultural activities in its watershed (51194 km 2 ) and untreated sewage from
Pelotas city (300000 hab.) which is located on the NE border of Sao Gon~alo Channel. Patos Lagoon Estuary also receives high nutrient inputs from Rio Grande's urban
area (200000 hab.) and associated harbour and industrial activities (Almeida et al. 1993).
In spite of the constant short-term variability of salinity, dissolved oxygen, suspended matter and nutrients (Kantin and Baumgarten 1982; Kantin 1983; Niencheski
et al. 1986; 1988; Baumgarten and Niencheski 1990) phytoplankton production and
biomass present a clear seasonal pattern (Proen~a 1990) with maximum biomass values (10.56 mg I-I) being related to the increase in light intensity, temperature and dissolved inorganic nutrients during spring and summer and low values during fall and
winter (less than 2 mg I-I) (Abreu 1992).
4.2
Materials and Methods
Water samples were collected in the southern part of the Patos Lagoon, during 8 cruises
at twenty fixed stations within the estuarine mixing zone and one fixed station in Sao
Gon~alo Channel in the main navigation channel and, during 15 cruises, covering four
stations in semi-enclosed bays surrounding Rio Grande City (Saco da Mangueira and
Saco do Martins), and three stations in the Rio Grande Channel (Fig. 4.1).The freshwater input from Patos Lagoon to the upper estuarine region was determined using
data from Station 1 samples.
To obtain a better synoptic view of variability within the mixing zone, each of the
8 cruises were conducted during one day, using two vessels. The cruise dates were:
1) June 30,1989,2) August 30, 1989,3) May 30,1990,4) June 30,1990,5) August 1, 1990,
6) August 30,1990,7) December 1, 1990, and 8) April 22, 1991. The 15 monthly cruises
were carried out from May 1990 to July 1991.
Field teams worked concurrently so that samples could be collected and returned
to the laboratory as quickly as possible. At each station, except in the semi-enclosed
bays, vertical profiles of water temperature, salinity and conductivity were taken at
69
the estuarine region, and thus have a significant residence time within the lagoon
during which biogeochemical process may result in their consumption, consequently
reducing the inputs associated with freshwater to the estuary. In Patos Lagoon, the
residence time depends more upon meteorological conditions (wind, rainfall,
evaporation, etc.) than on tidal exchange because of the low tidal range (Moller et al.
1991). The time that water takes to travel from Guafba River to the estuarine region is
about 20 days (Herz 1977). In this area the annual rainfall exceeds annual evaporation.
Vieira and Rangel (1988), using data collected over a twenty year period (1957-1977),
calculated the annual rainfall to be 132 cm and the annual evaporation to be as ca. 90 cm.
There have been few studies of nutrients and suspended matter behaviour in this
system. Vilas Boas (1990), however, suggests that the Northern part of Patos Lagoon
may receive significant anthropogenic inputs as a result of rapid population growth
and industrialisation along the northern region, particularly during the last few decades around Porto Alegre, the fifth largest Brazilian city. Another additional input to
the estuary is that from Sao Gon~alo Channel, which has a discharge averaged over
30 year period of 700 m3 S-I (Vieira and Rangel 1988). Concentrations of nutrients and
suspended sediments in the Sao Gon~alo Channel input from Mirim Lagoon are the
result of agricultural activities in its watershed (51194 km 2 ) and untreated sewage from
Pelotas city (300000 hab.) which is located on the NE border of Sao Gon~alo Channel. Patos Lagoon Estuary also receives high nutrient inputs from Rio Grande's urban
area (200000 hab.) and associated harbour and industrial activities (Almeida et al. 1993).
In spite of the constant short-term variability of salinity, dissolved oxygen, suspended matter and nutrients (Kantin and Baumgarten 1982; Kantin 1983; Niencheski
et al. 1986; 1988; Baumgarten and Niencheski 1990) phytoplankton production and
biomass present a clear seasonal pattern (Proen~a 1990) with maximum biomass values (10.56 mg I-I) being related to the increase in light intensity, temperature and dissolved inorganic nutrients during spring and summer and low values during fall and
winter (less than 2 mg I-I) (Abreu 1992).
4.2
Materials and Methods
Water samples were collected in the southern part of the Patos Lagoon, during 8 cruises
at twenty fixed stations within the estuarine mixing zone and one fixed station in Sao
Gon~alo Channel in the main navigation channel and, during 15 cruises, covering four
stations in semi-enclosed bays surrounding Rio Grande City (Saco da Mangueira and
Saco do Martins), and three stations in the Rio Grande Channel (Fig. 4.1).The freshwater input from Patos Lagoon to the upper estuarine region was determined using
data from Station 1 samples.
To obtain a better synoptic view of variability within the mixing zone, each of the
8 cruises were conducted during one day, using two vessels. The cruise dates were:
1) June 30,1989,2) August 30, 1989,3) May 30,1990,4) June 30,1990,5) August 1, 1990,
6) August 30,1990,7) December 1, 1990, and 8) April 22, 1991. The 15 monthly cruises
were carried out from May 1990 to July 1991.
Field teams worked concurrently so that samples could be collected and returned
to the laboratory as quickly as possible. At each station, except in the semi-enclosed
bays, vertical profiles of water temperature, salinity and conductivity were taken at