Assessing water quality in lotic ecosystems using biological indicators is essential for their conservation. This study analyzed the influence of physicochemical parameters (pH, conductivity, dissolved oxygen, total nitrogen, nitrates, total phosphorus, phosphates, alkalinity, and hardness) on the diversity, abundance, and distribution of protozoa at 15 sites in the upper Soto la Marina River basin, Tamaulipas, Mexico, including the Corona, Purificación, Pilón, and San Felipe rivers and the Vicente Guerrero Reservoir. Through bimonthly sampling over the annual cycle of 2021 (February–December), 24 protozoan morphospecies were identified, with dominance of Ciliophora (10 morphospecies) and Amoebozoa (4 morphospecies). Chi-square analyses revealed that morphospecies frequency was significantly dependent on sampling site (χ2 = 246.72, df = 69, p < 0.001) but independent of seasonality (χ2 = 0.86, df = 15, p = 1.0). Beta diversity (Sørensen–Dice index) showed low faunistic similarity among most sites (<60%), suggesting high species turnover and a local environmental filter. Using Outlying Mean Index (OMI) analysis, most morphospecies exhibited a generalist niche (low marginality, high tolerance), showing no significant relationship with the measured environmental variables. However, Vorticella sp. emerged as an exception, showing a significant association (p = 0.0329) and positive correlation with maximum pH and alkalinity values and occurring at sites with high NO3− and PO43− concentrations, suggesting its potential as a bioindicator of organic enrichment conditions in the region. The absence of a strong environmental signal in most species underscores the complexity of these systems and the need to integrate multiple levels of bioindicators. This study provides the first baseline on the protozoan community in the area and discusses its relevance for water quality monitoring in a region of high conservation value, such as the area of influence of the Sierra de Tamaulipas.
Vázquez-Ochoa et al. (Wed,) studied this question.