Seasonal and Long-Term Behavior of TN:TP Ratio in Lake Cajititlán and Its Environmental Implications

To this day, there is still some debate on the specific conditions that limit primary production in lakes. The discussion increases as this phenomenon is studied in temperate, subtropical, and tropical lakes. This work presents an analysis of the behavior of the total nitrogen to total phosphorus ratio (TN:TP) in Lake Cajititlán (Mexico). This subtropical lake, located within a small endorheic watershed, has presented a notable decrease in the TN:TP ratio in the last few years. Considering a TN:TP ratio <16 as the criterion indicative of nitrogen limitation for lakes located in the central-western region of Mexico, Lake Cajititlán has been predominately nitrogen limited from 2009 to 2020. From 2013 to 2020, the TN:TP ratio has declined to values below 10. This is reported in literature as a threshold value that favors algal blooms in subtropical lakes. This year also corresponds with the start of an unusually large fish die-off in the lake during August and early September. The TN:TP ratio exhibits a seasonal pattern, with the lowest mean values (TN:TP < 10) occurring during the dry season, and the highest values (TN:TP > 10) coincide with the rainy season (TN:TP > 10). The seasonal variations in the TN:TP ratio are primarily controlled by the annual precipitation regime, which increases the organic matter and nutrient loads to the lake mostly from treated and raw wastewater discharges, and from the runoff from agricultural land that extends along the entire lowlands of the lake watershed. The use of the TN:TP ratio as an environmental indicator is proposed, to guide public policies and governance practices in order to reduce the high level of eutrophication of the lake and rescue the potential uses of the lake and its watershed.