Introduction:

The Magdalena River in Colombia has a high anthropic intervention, and this high load of pollutants that enter mainly in the middle and upper basin of its course are becoming increasingly noticeable in the lower basin.

Objective:

This study evaluated the environmental condition of a section of the Magdalena River in the lower basin as it passes through the Department of Atlántico, using the Ecological Quality Index (EQI) based on phytoplankton.

Methods:

Six sampling events were carried out between 2021 and 2022, in order to cover contrasting moments of the hydrological regime, at six monitoring stations on the Western bank of the river. Physicochemical and phytoplankton variables were sampled to establish reference categories based on the EQI.

Results:

Conductivity was determined as the variable that explained the greatest variation in the environmental gradient, generating reference ranges to establish categories of poor, fair and good ecological quality. 77 phytoplankton morphospecies were recorded, of which five were sensitive to the physicochemical conditions of the Magdalena River, with high optimum values and low tolerance limits: Pinnularia sp2, Diatoma sp3, Navicula sp1, Closterium limneticum, C. rostratum.

Conclusions:

In general, the Magdalena River in the Atlántico showed a fair ecological quality condition, which deteriorated even more during the dry periods of the year. The EQI showed a relationship with the proportion of diatom groups and the abundance of phytoplankton, obtaining better quality values when diatoms predominate over the cyanobacteria group and when the abundance of the phytoplankton is relatively low.