Abstract
Lake Cajititlán is a small, shallow, subtropical lake located in an endorheic basin in western Mexico. It is characterized by a strong seasonality of climate with pronounced wet and dry seasons and has been classified as a hypereutrophic lake. This eutrophication was driven by improperly treated sewage discharges from four municipal wastewater treatment plants (WWTPs) and by excessive agricultural activities, including the overuse of fertilizers that reach the lake through surface runoff during the rainy season. This nutrient rich runoff has caused algal blooms, which have led to anoxic or hypoxic conditions, resulting in large-scale fish deaths that have occurred during or immediately after the rainy season. This study investigated the changes in the phytoplankton community in Lake Cajititlán during the rainy season and the association between these changes and the physicochemical water quality and environmental parameters measured in the lake’s basin. Planktothrix and Cylindrospermopsis were the dominant genera of the cyanobacterial community, while the Chlorophyceae, Chrysophyceae, and Trebouxiophyceae classes dominated the microalgae community. However, the results showed a significant temporal shift in the phytoplankton communities in Lake Cajititlán induced by the rainy season. The findings of this study suggest that significant climatic variations cause high seasonal surface runoff and rapid changes in the water quality (Chlorophyll-a, DO, NH4+, and NO3–) and in variations in the composition of the phytoplankton community. Finally, an alternation between phosphorus and nitrogen limitation was observed in Lake Cajititlán during the rainy season, clearly correlating to the presence of Planktothrix when the lake was limited by phosphorus and to the presence of Cylindrospermopsis when the lake was limited by nitrogen. The evidence presented in this study supports the idea that the death of fish in Lake Cajititlán could be mainly caused by anoxia, caused by rapid changes in water quality during the rainy season. Based on our review of the literature, this is the first study on the phytoplankton community in a subtropical lake during the rainy season using high throughput 16S rRNA and 18S rRNA amplicon sequencing.
Highlights
Lake Cajititlán is a small, shallow subtropical lake located in an endorheic basin in the municipality of Tlajomulco de Zúniga in the state of Jalisco, Mexico at 1,552 m a. s. l. (LimónMacias et al, 1983)
These trends are strongly associated with the lowest ES-WQI values observed during the month of July in Lake Cajititlán (Figure 2F), as well as with the greatest variations in the total nitrogen (TN):total phosphorus (TP) ratio observed during the rainy season (Figure 2E)
This study revealed that the composition of the phytoplankton community in Lake Cajititlán changed rapidly and that the population increased throughout the study period
Summary
Lake Cajititlán is a small, shallow subtropical lake located in an endorheic basin in the municipality of Tlajomulco de Zúniga in the state of Jalisco, Mexico at 1,552 m a. s. l. (LimónMacias et al, 1983). Most of the agriculture is rainfed, which means that fertilizers are used during the rainy season, often in excessive amounts. These agricultural practices are one of the principal sources of nutrient contamination leading to cultural eutrophication in the Lake (de Anda et al, 2019a). As a result of this nutrient pollution, the Lake has been classified as hypereutrophic (de Anda et al, 2019a). This process of cultural eutrophication is exacerbated by the endorheic nature of the Lake (IIEG, 2018; de Anda et al, 2019b)
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