Condições oceanográficas, ocupação territorial e problemas ambientais na praia do Atalaia (nordeste do Pará, Brasil)

Abstract

The conservation and management of the coastal zone of the Amazon region demands special attention, given the richness of its natural resources. The aim of the present study was to evaluate the impact of natural events and human activities on Atalaia beach, situated in the NE of the Brazilian state of Pará and to develop guidelines for the implementation of coastal management programs. Data were collected between November, 2008, and November, 2010. Four sets of variables were assessed: (i) physical variables (climatology, hydrodynamics and morfodinâmica), (ii) hydrological variables (water temperature, salinity, pH, turbidity, dissolved oxygen and inorganics nutrients, chlorophyll a and thermotolerant coliform levels), (iii) urban development and (iv) spatial distribution of services and infrastructure. The results indicate that climate and hydrodynamical conditions were the main factors responsible for fluctuations in water salinity, turbidity, dissolved oxygen, inorganic dissolved nutrients, and chlorophyll a concentrations. The discharge of untreated domestic sewage was responsible for bacteriological contamination, although the rapid turnover of the high-energy hydrodynamic environment limited contamination by thermotolerant coliforms. This high hydrodynamic energy, primarily during the equinoctial spring tides, and the lack of urban planning, nevertheless generates other problems, such as coastal erosion. The study area is characterized by high rainfall rates (> 1900 mm during the rainy season), NE winds with mean speeds of up to 4.36 m/s in the dry season and 3.06 m/s in rainy season, macrotidal conditions (tidal range > 4.0 m), moderate tidal current speeds (up to 0.5 m/s), and significant wave heights up to 1.5 m. In March and June (rainiest months), ebb tide currents reached a maximal of 0.4 m/s. Tidal cycle was weakly asymmetric with the ebb tide lasting up to 6 hours 40 minutes. Wave energy was slightly modulated by the low tide due to wave attenuation on sand banks. Water temperature was relatively homogeneous (27.4ºC to 29.3ºC). Salinity varied from 5.7 (June) to 37.4 (November). The water was well oxygenated (up to 9.17 mg/L), turbid (up to 118 nephelometric turbidity units), alkaline (up to 8.68), and eutrophic (maximum of 2.36 μmol/L for nitrite, 24.34 μmol/L for nitrate, 0.6 μmol/L for phosphate, and 329.7 μmol/L for silicate), and it presented high concentrations of chlorophyll a (up to 82 mg/m³). The natural conditions observed in the present study indicate the need for a review of the hydrologic criteria used for the evaluation of beaches by national and international agencies and their adaptation to the reality of the Amazon Coast. The lack of a public sanitation system has led to bacteriologic contamination and the loss of water quality. With respect to morphodynamic state, dissipative conditions were found during high and moderate hydrodynamic energy (equinoctial and nonequinoctial condition), but in November the highest wave heights generated barred dissipative characteristic, whereas during the other months non-barred characteristics dominated. Thus, this study shows that the model proposed by Masselink & Short (1993) seems to be ideal to be applied to beaches with similar characteristics to the studied beach, where wave energy is modulated by the presence of sand banks in some stages of the tide.