Hidrogeoquímica do fósforo no estuário do Jaguaribe (Ce.)

Abstract

The Jaguaribe estuary has undergone hydroc hemical changes as a function of anthropogenic activities, such as intensiv e urbanization and dams along its drainage basin that promote alterations in the quality and amount of discharges to the Atlantic Ocean. This study aims to evaluate the spatial distribution of phosphorus in the water column and surficial sediments of t he estuary and its dependence on anthropogenic influences and hydrochemical variability. Water and sediment samples were collected in 5 stations along the estuary, in 3 seasonal campaigns in the dry and rain seasons of the region. Water samples were collected at Secchi depth. The parameters (pH, OD, %OD, T, Sal) were obtained in situ with portable instrument. The extraction of total phosphorus in sediment samples followed the methodology of Berner & Rao (1994). The fractions of phosphorus in the water and sediments were determined by spectrophotometry, according to Grasshoff et al. (1999). Chlorophyll-a was estimated by Jeffrey & Humphrey (1975) and phaeopigments were measured as suggested by APHA (2001). Average temperature ranged from 29ºC±0.5ºC, which favors evaporation. Salinity varied, respectively, from 29.5- 35.1, 27.0-39.2 and 1.2-17.5, with the lowest values in the higher estuary. Average the contend of DO ranged from 6.3mg.l-1, 5.2mg.l-1 and 7.0mg.l-1 (%DO: 98.8%, 79.5% and 94.9%), with the lowest values measured in the middle estuary, related to the mixture zone and considered as saturated the supersaturated, according to the classification of Macêdo & Costa (1978). The pH was alkaline, between 7.8 and 8.2 in the entire estuary, showing a marine influence and the hydrological negative balance of the region (evaporation>precipitation). Average electric conductivity varied from 51.3mS.cm -1, 55.0mS.cm-1 and 12.6mS.cm-1, in the different campaign, favoring the flocculation of suspended matter. The SPM presented an average of 13.6mg.l-1, 17.7mg.l -1, 30.3mg.l-1, indicating strong dependence on the climatic regime of the region. Total, dissolved and particulate phosphate varied from 0.2-3.8μM, 2.0-5.3μM and 3.7-6.7μM (T-PO 4); < 0.04-2.7μM, 1.2-3.4μM and 1.2-2.5μM (D-PO4); < 0.04-1.2μM, 0.8-3.4μM and 1.3-4.5μM (Part-PO4), with the highest values associated with the inputs from anthropogenic sources and the fluvial influence. Chl-aranged from 1.2-8.2mg.m-3, 2.9-16.2mg.m-3 and 9.5-17.4mg.m-3, being the major responsible for the absorption of phosphorus in the euphotic zone and characterizing the estuary as euthrophic. The Phaeo-pig varied from 1.4-3.2mg.m-3 and 2.0-10.5mg.m-3, with the lowest values related with the zone of mixture of the estuary. Organic matter in the sediments oscillated from 0.0-0.1%, 0.0-1.0% e 0.0-0.4%, indicating an area source in the middle estuary. P-Total in the sediment ranged from 17.4-60.3μgP.g-1, 18.0-202.0μgP.g -1 and 14.3-134.3μgP.g-1, with the highest concentrations associated with the presence of sediments characteristic of mangroves and with the anthropogenic sources of phosphorus, as show n by the data of the water columnconcentrations.