ANALYSIS AND MODELING OF THE RELATIONS BETWEEN NUTRIENTS AND PHYTOPLANKTON IN RESERVOIRS OF CEARÁ

Abstract

This research aimed to analyze the relationships between total phosphorus
(P), total nitrogen (N), and chlorophyll a (Cla) in a small urban reservoir and in
rural reservoirs. A field study was carried out on the urban reservoir, located at
Fortaleza, Ceará, to analyze the water quality parameters and then compare the
results with historical data of 35 rural reservoirs located in the semiarid region
of the Brazilian northeast, in the state of Ceará. Empirical correlations were
developed among these nutrients and Cla. Then, different levels of nutrient load
reduction were analyzed with the empirical correlations obtained and the water
quality complete moisture hypothesis, whose phosphorus decay coefficient
used was the one developed for semiarid regions. The results showed that P
best described the concentration of Cla in all reservoirs, using power adjustment
equations. It was also shown that the concentrations of P in the urban
reservoir were higher than those of the rural ones, with a mean of 1,76 mg/L
and a N:P correlation of approximately 1, while the rural reservoirs had mean
concentrations between 0,04 – 0,59 mg/L and N:P between 5 – 50. The analysis
of the different load scenarios showed that different levels of P load reduction
could be adopted depending on the P and Cla concentrations desired. Finally,
it was concluded that the models proposed are applicable and useful as a means
to aid decision making in the management of water resources.