70
Luis Felipe Niencheski . Maria da Gra<;a Baumgarten· Gilberta Fillmann . Herbert L. Windom
1.0 m depth intervals. Measurements were obtained using Yellow Springs Instruments
Model 33 S-C-T meters. Each day, prior the sampling, all primary and backup instruments were intercalibrated.
In the main navigation channel, both surface and bottom water samples were collected using Van Dorn bottles (1.51) and placed into clean 1.01 plastic bottles (nutrients) or into a 300 ml BOD glass bottles (dissolved oxygen). During the 15 monthly
cruises only surface water sample were collected.
Samples for nutrient analysis were immediately filtered through cellulose acetate
0-45 mm filters and analysed immediately for ammonium. Filtered aliquots for nitrate,
nitrite, silicate and phosphate determination were placed in individually cleaned
polyethylene bottles and stored in a freezer until analysed ashore (Grasshoff et al.1983).
Dissolved oxygen was analysed using the method of Grasshoff et al. (1983). Analysis of total suspended matter, ammonium, nitrite, nitrate, phosphate and sIlicate followed the procedures described by Aminot and Chaussepied (1983).
4.2.1
Data Reduction for Determining Estuarine Behaviour of Nutrients and
Total Suspended Matter
Estuaries are often elongated and relatively shallow. The major interest in the spatial
variability of materials has generally been in their distribution along the longitudinal axis of the estuary, in relation to the salinity gradient.
Advection-diffusion models have been used by many investigators to interpret estuarine chemical data referenced to salinity (e.g., Li and Chan 1979; Kaul and Froelich
1984). The distribution of a constituent in estuarine waters can be compared to salinity to determine whether a substance is:
1. conservatively transported through the estuary,
2. removed from the water column or
3. added to the water column via local inputs.
The only assumption required is that the concentrations of the constituent in the
freshwater and oceanic end members are constant over the residence time of the estuary. For Patos Lagoon, this is assumed to be satisfied sufficiently to draw the conclusions presented in this paper.
In this study, data for dissolved nutrients and TSM were plotted against salinity for
each cruise. Comparing concentrations of a substance relative to salinity, which is
assumed to be conservative, along the salinity gradient provides a basis for judging
the estuarine behaviour of the substance. For this purpose a line connecting the mean
freshwater concentration to the concentration at highest salinity (i.e., the oceanic end
members) is used to evaluate the deviation of concentrations, along the salinity gradient, from conservative mixing.
For cruises 1, 2 and 6, conducted in the main channel, where we do not have data
from freshwater regions, we have estimated the freshwater end member (average and
standard deviation) using historical data from Patos Lagoon (Vilas Boas 1990). This
estimate is shown in figures using diamond symbols for estuarine nutrient distributions in the following sections.
Luis Felipe Niencheski . Maria da Gra<;a Baumgarten· Gilberta Fillmann . Herbert L. Windom
1.0 m depth intervals. Measurements were obtained using Yellow Springs Instruments
Model 33 S-C-T meters. Each day, prior the sampling, all primary and backup instruments were intercalibrated.
In the main navigation channel, both surface and bottom water samples were collected using Van Dorn bottles (1.51) and placed into clean 1.01 plastic bottles (nutrients) or into a 300 ml BOD glass bottles (dissolved oxygen). During the 15 monthly
cruises only surface water sample were collected.
Samples for nutrient analysis were immediately filtered through cellulose acetate
0-45 mm filters and analysed immediately for ammonium. Filtered aliquots for nitrate,
nitrite, silicate and phosphate determination were placed in individually cleaned
polyethylene bottles and stored in a freezer until analysed ashore (Grasshoff et al.1983).
Dissolved oxygen was analysed using the method of Grasshoff et al. (1983). Analysis of total suspended matter, ammonium, nitrite, nitrate, phosphate and sIlicate followed the procedures described by Aminot and Chaussepied (1983).
4.2.1
Data Reduction for Determining Estuarine Behaviour of Nutrients and
Total Suspended Matter
Estuaries are often elongated and relatively shallow. The major interest in the spatial
variability of materials has generally been in their distribution along the longitudinal axis of the estuary, in relation to the salinity gradient.
Advection-diffusion models have been used by many investigators to interpret estuarine chemical data referenced to salinity (e.g., Li and Chan 1979; Kaul and Froelich
1984). The distribution of a constituent in estuarine waters can be compared to salinity to determine whether a substance is:
1. conservatively transported through the estuary,
2. removed from the water column or
3. added to the water column via local inputs.
The only assumption required is that the concentrations of the constituent in the
freshwater and oceanic end members are constant over the residence time of the estuary. For Patos Lagoon, this is assumed to be satisfied sufficiently to draw the conclusions presented in this paper.
In this study, data for dissolved nutrients and TSM were plotted against salinity for
each cruise. Comparing concentrations of a substance relative to salinity, which is
assumed to be conservative, along the salinity gradient provides a basis for judging
the estuarine behaviour of the substance. For this purpose a line connecting the mean
freshwater concentration to the concentration at highest salinity (i.e., the oceanic end
members) is used to evaluate the deviation of concentrations, along the salinity gradient, from conservative mixing.
For cruises 1, 2 and 6, conducted in the main channel, where we do not have data
from freshwater regions, we have estimated the freshwater end member (average and
standard deviation) using historical data from Patos Lagoon (Vilas Boas 1990). This
estimate is shown in figures using diamond symbols for estuarine nutrient distributions in the following sections.