EFFECTS OF FRESHWATER DISCHARGES ON THE CLASSIFICATION OF SOUTHAMERICAN COASTAL SYSTEMS
Jorge Marcovecchio
Lab. de Química Marina, Instituto Argentino de Oceanografía (IADO), Bahía Blanca, Argentina.
The aim of this exercise is to get an approach on the influence of regional freshwater discharges -mainly through estuarine areas- to the coastal systems of South America. One of the points which must be considered in this way is that large quantities of land originated materials (i.e., organic matter, nutrients, sediments) are input to the marine coastal zone carried by rivers, streams, channels, outfalls, continental runoff, etc. Thus, characteristics which joint similar systems on the coastal zone may be modified through these inputs.
Keeping in mind this aim, my first step was to cluster similar coastal systems of the selected area considering many of their characteristic properties (i.e., temperature annual average, total precipitation, basin area, salinity average, salinity gradient) as well as other variables which could indirectly point out land pressure on coastal systems (i.e., basin runoff, basin percentage of cropland coverage or basin percentage of urbanization).
Five experiments were carried out, sequentially modifying several of the variables in order to get a better product for the classification of the selected systems.
In the first experiment (E1) all the selected variables (9) were simultaneously considered, and with the same relative weight within the corresponding cluster analysis. The obtained result has given a partially useful first approach, which although has required modifications to better characterize the analyzed coastal zone.
In the second experiment (E2) only six (6) of the originally selected variables were considered, all of them with the same relative weight within the corresponding cluster analysis. In this way an improved product was obtained considering that several little regional areas within the coast were better identified. Even though other modifications were still necessary in order to get a better characterization of the system
In the third experiment (E3) six (6) of the originally selected variables were considered, and a new one (salinity gradient index) was also included trying to better represent the impact of freshwater on the coastal ecosystem. This index was obtained dividing salinity gradient values by salinity average ones. All variables have been considered with the same relative weight within the corresponding cluster analysis. In this case a nice product was obtained, where is possible to identify most of the typical systems of South America Atlantic coast.
In the fourth experiment (E4) same variables than in E3 were used, but a higher relative weight has been given to basin runoff and basin percentage of cropland coverage (the same for both). The obtained result has well agreeded with the expected ones, but simultaneously has aggregated several noises to the distribution pattern. Thus, it has not seem to be a real improvement to the previous one.
In the fifth experiment (E5) the same strategy was applied, but giving a higher relative weight to basin percentage of cropland coverage than basin runoff. Also in this case a nice product was obtained, where is possible to identify most of the typical systems of South America Atlantic coast.
As a matter of fact, it could be concluded that the conducted exercise has preliminary shown that experiment 3 (E3) and experiment 5 (E5) were the most successful ones, and which have bring the better classification maps for systems of the selected area. Both have separated five (5) units of coastal systems along South America Atlantic littoral, which is really close to the real distribution as known from field work. The mentioned units of coastal systems are:
ü Patagonian coastal systems : seems to show a large homogeinity within its coastal area. Variables which have better characterized this unit (and which have agreeded in both experiments, E3 and E5) were low temperature and low precipitation. It is reasonable keeping in mind that involved coastal systems are semi-desertic ones. Other variables which have shown to be important for this area characterization were low basin area and low basin % cropland coverage. These systems wouldn’t receive a high organic matter and nutrients load from their associated land ecosystems.
ü Agricultural plains from Central Argentina, Uruguay, Southern and NE Brazil : these zones has particularly been characterized by high basin % cropland coverage, which fully coincide with their capability for agriculture and cattle production. Other variables which have shown to be important for this area characterization were low basin area, low population and low basin runoff. These systems would receive a high organic matter and nutrients load from their associated land ecosystems, keeping in mind not only their productivity condition as previously mentioned, but also the occurrence of several large river catchments outlet within these areas (i.e., La Plata river) ; moreover, most of these coasts include tidal plains, salt marshes, estuaries and wetlands with also would add significant inputs of OM and nutrients to the system. Finally, in the case of Argentina and Uruguay the additional input from the large oceanic Malvinas current must be considered.
ü Central and North-Northeastern Brazil, Venezuela and Caribbean areas : these zones were mainly characterized through the variable high temperature. Other important variables have shown to be medium precipitation, basin area and basin runoff. These are systems which include large areas of tropical rain forests, as well as large rivers catchments (i.e., Orinoco river) ; also large cities and industrial centers are included in this area (i.e.,Sao Paulo, Rio de Janeiro). Furthermore, many estuaries, coastal lagoons, wetlands and mangrove systems occur along this unit. The above mentioned allows to sustained that this unit would probably receive the highest organic matter and nutrients load within the studied coastal system.
ü Amazonas outlet region : this area has shown the same behavior under different analysis conditions, and the variable which have always better characterized it was high salinity gradient index. Othe important variables for the representation of this unit were basin runoff, basin % cropland coverage, low population. Considering the conditions of the area, as well as the occurrence of Amazonas catchment outlet, as well as that of numerous wetlands and mangrove areas, it would be expected that a high organic matter and nutrients load was input into the corresponding coastal area.
Most details are available in the brief particular analysis and comments for each experiments (attach files). Moreover, Figure 1 shows a comparative scheme for the five obtained distribution maps of South American Atlantic coastal systems.