Spatial and temporal changes of dissolved oxygen in waters of the Pajarales complex, Ciénaga Grande de Santa Marta: Two decades of monitoring.

Ciénaga Grande de Santa Marta lagoon complex, located in the Colombian Caribbean, is a highly degraded estuarine system, in which massive deaths of organisms have occurred since the 1990s, causing socioeconomic effects on the inhabitants, who are mostly artisanal fishermen. These deaths have been attributed to the deoxygenation of the water at night, as a result of the eutrophication of the system. To understand the variability of dissolved oxygen and its relationship with other water quality variables, the monthly time series collected between 2001 and 2019, in seven stations of the Pajarales Complex (western side of the estuarine complex), were analyzed. Analyzes showed that there are significant differences between stations, as well as between the surface and the bottom of the water, indicating that the behavior of oxygen in the system is not homogeneous, a product of the hydrodynamics of the system. Also, temporal differences were found related to the periods of rain and drought, even with larger-scale climatic events such as El Niño and La Niña, with the lowest concentrations being recorded during the rainy seasons. On the other hand, the analysis of the time series of the average surface temperature of the seven stations analyzed showed a slight tendency to increase over time. Results indicate that the system is very dynamic and its oxygenation conditions are determined by climatic factors that promote changes in water chemistry, such as variations in salinity, temperature, and pH, and biological activity, determined by the abundance of the organisms. Analysis of this information becomes a tool to propose an alert system that allows reducing the impact of deaths.