Lake stratification in the Brazilian Amazon: Types and the first description of permanent meromixia

Abstract

An investigation into the physical and chemical stratification processes of the waters of 45 lakes in Central Amazonian was carried out, based on data from 1995 to 2022. Stratification profile and behavior; partial and total circulation trends; seasonality; classification and typology of the lakes of the Amazon plain were considered. The purpose of this study was to identify the limnological, hydrological and geomorphological factors most relevant to the stratification processes. Monitoring data were obtained from daily and seasonal samplings. Among the analyzed parameters are temperature; density; transparency; euphotic zone; attenuation coefficient; incident and refracted radiation profile; dissolved oxygen, conductivity, pH, salinity, suspended material and turbidity concentrations; climatic and hydrological factors; level of protection of the shores; topography of shore and bottom; shape and geometric pattern of lakes. PCA, Cluster and Pearson's analysis and numerical models were applied to the results. Lakes with total circulation patterns, multiple layered circulations, varying degrees of polymixy, and lakes with seasonal and permanent meromixy were identified. The concentration of total solids, depth, morphometry, intensity of the refracted radiation and the action of the winds were the factors considered preponderant for the identification of the stratification processes. Hydrological models were established based on the classification and typology of the lakes studied. The simulation results allowed the following conclusions: a) coefficients t1 and t2 < 0 suggest increase; b) 0< t1 and t2 < 1 is equivalent to a curve with linear trend; c) t1 and t2 > 1 is equivalent to a curve with a tendency to stratification; d) the higher the numerical value of t1,2, means resistant stratification (permanent meromixia).