Analysis of total phosphorus and chlorophyll a correlations in Cear reservoirs, Brazil

Reservoirs worldwide are prone to water quality degradation caused by increased primary production. Therefore, it is essential to comprehend the factors that influence this phenomenon as it plays a fundamental role in controlling eutrophication. The aim of this study was to analyze the correlation between total phosphorus (TP) and chlorophyll a (Chla) in 155 reservoirs located in the state of Ceara, Brazil. This analysis was conducted through a comprehensive examination of historical data (2008-2021) obtained from the water resource management company of the state, which monitors these parameters. The correlation between TP and Chla was examined through simple adjustments, utilizing the coefficient of determination (R2) as an evaluation metric. Afterward, the study investigated the potential influences on the dynamics of these adjustments based on factors such as the hydrographic basin, reservoir size, trophic state (as for Chla and TP concentrations), and the volumetric variability coefficient. In general, the adjustments yielded unsatisfactory models (R2 < 0.4) for the majority of reservoirs (n = 115). Despite their inadequacy, these models align with classic literature models, indicating that in most basins, higher availability of TP in the water column contributes to increased Chla concentration. The analysis of factors influencing the pattern and dispersion of adjustments between Chla and TP revealed that the performance of R2 is associated with various factors, such as different watersheds, volumetric variability, and Chla concentrations. The variance in R2 between reservoirs of varying sizes and trophic states based on TP concentration was considered insignificant.