Aspects of a unique natural limnological phenomenon in the Brazilian Pantanal

Abstract

This study compares physical and chemical parameters of water samples collected in a marginal lake (Castelo Bay) in the Paraguay River Basin in southern Pantanal during a significant natural phenomenon of hypoxia, locally called decoada, and shortly after it. Limnological parameters were analyzed from four sampling sites along the bay. Comparisons of the physical and chemical parameters between decoada and post-decoada periods were performed by Student’s t-test, principal component analysis and multivariate permutational analysis of variance (permanova). During the period of the decoada, there was a significant reduction in mean values of water transparency and concentrations of dissolved oxygen, sodium and nitrite compared to the post-decoada period. On the other hand, water temperature and conductivity, and concentrations of orthophosphate, total nitrogen and total iron have all had higher values during the decoada period. An effect of connectivity between lake and river was found to generate a gradient of water characteristics at Castelo Bay. The limnological characteristics that differed the most between decoada and post-decoada periods are those associated with the reduction of dissolved oxygen that could cause natural death of fish and the increase in nutrients during the decoada, yet they vary on temporal and spatial scales.

Keywords: anoxia, dissolved oxygen, floodplain lakes.