Water quality of the São Gonçalo channel, urban and agricultural water supply in southern Brazil

ABSTRACT São Gonçalo channel connects Patos and Mirim lagoons, in South Brazil. This ecosystem is 70 Km long, and is a major regional source of urban and agricultural water supply, mainly for the municipality of Rio Grande, with ca. 250 thousand citizens. This study aims to diagnose their water quality by measuring benthic invertebrates (Shannon-Wiener diversity index, richness and abundance), use of a BMWP biotic index, physical and chemical evaluation, and applying a trophic state index. The BMWP index showed predominance of taxa indicating altered conditions and polluted waters (Sarcodina and Hydracarina). The abiotic variables indicated eutrophicated waters, chlorophyll values characteristic of eutrophic environments (>2.9 µg. L–1), low N:P rates in sediment and water, with nitrogen limitation during most periods studied. The results of trophic state indexes indicated mesotrophic and eutrophic conditions, reaching hipereutrophic periods. The highest TSI were in Piratini river, in April 2009 and February 2011, coinciding with rice crop leaching. These results regarding nutrient concentration, nitrogen and phosphorus stoichiometry and biotic index support the need to evaluate the occupation of the surroundings of this water supply.

摘要 圣贡萨洛水道（São Gonçalo Channel）连接巴西南部的帕图斯潟湖与米林潟湖。该生态系统全长70千米，是当地重要的城市与农业供水水源，主要服务于人口约25万的里奥格兰德市。本研究通过底栖无脊椎动物调查（涵盖香农-威纳多样性指数、物种丰富度与个体丰度）、BMWP生物指数法、理化指标检测以及营养状态指数（Trophic State Index，简称TSI）法，对该水道的水质状况开展诊断。BMWP生物指数结果显示，指示生境受损与水体污染的类群占优势，包括肉足虫纲（Sarcodina）与水螨类（Hydracarina）。非生物变量监测结果表明水体呈富营养化状态：叶绿素含量符合富营养化环境特征（>2.9 μg·L⁻¹），沉积物与水体中的氮磷比值偏低，且研究期内多数时段存在氮限制现象。营养状态指数的评估结果显示水体处于中营养至富营养化状态，部分时段可达超富营养化水平。2009年4月与2011年2月，皮拉蒂尼河河段的TSI值最高，该时段与水稻种植的农田淋溶期重合。上述关于营养盐浓度、氮磷化学计量比以及生物指数的研究结果证实，需对该供水水源周边区域的土地开发利用情况进行评估。