Large quantities of seston are among the most important indicators of eutrophication in aquatic ecosystems. The present study aimed to elucidate the role seston plays in the general limnological dynamics of a cluster of eighteen tropical karstic lakes with different anthropic impacts (non-impacted, oligotrophic, clear-water lakes, and impacted, eutrophic, turbid-water lakes) of the “Lagunas de Montebello” National Park lake district. The seston concentration was measured twice, in the warm/rainy and the cold/dry season. Vertical profiles of temperature, dissolved oxygen, electrical conductivity, pH, and photosynthetic active radiation (PAR) were recorded at each lake. Water samples were taken along the water column to evaluate seston and chlorophyll a (Chl-a) concentration. Impacted lakes displayed higher seston (4.1-21.0 mg/L) and Chl-a (8.1-129.8 μg/L) concentrations, reduced euphotic zone (ZEU = 2.6-6.3 m), and superficial thermo- (gradient = 0.8 ± 0.2 °C/m) and oxyclines (gradient = 4.7 ± 2.4 mg DO/m). Non-impacted lakes had lower seston (1.0-2.1 mg/L) and Chl-a (0.4-5.2 μg/L) concentrations, wide ZEU (10.1-33.4 m), and deeper thermo- (gradient = 0.5 ± 0.1 °C/m) and oxyclines (gradient = 0.6 ± 0.4 mg DO/m). The changes reported in impacted lakes linked with the increase in the seston and Chl-a concentrations are most likely related to the eutrophication process asso¬ciated with anthropogenic activities (agriculture, urban development, land-use change) in the NW part of the area. This research highlights the fragility of the tropical karst lake ecosystems worth protecting to preserve the aquatic ecosystem’s health.
Inland aquatic ecosystems are valuable sentinels of anthropic-associated changes (e.g., agriculture and tourism). Eutrophication has become of primary importance in altering aquatic ecosystem functioning. Quantifying the CO2 emissions by inland aquatic ecosystems of different trophic statuses may provide helpful information about the role of eutrophication on greenhouse gas emissions. This study investigated diel and seasonal carbon dioxide (CO2) concentrations and emissions in three tropical karst lakes with different trophic statuses. We measured CO2 emissions using static floating chambers twice daily during the rainy/warm and dry/cold seasons while the lakes were thermally stratified and mixed, respectively. The CO2 concentration was estimated by gas chromatography and photoacoustic spectroscopy. The results showed a significant seasonal variation in the dissolved CO2 concentration (CCO2) and the CO2 flux (FCO2), with the largest values in the rainy/warm season but not along the diel cycle. The CCO2 values ranged from 13.3 to 168.6 µmol L−1 averaging 41.9 ± 35.3 µmol L−1 over the rainy/warm season and from 12.9 to 38.0 µmol L−1 with an average of 21.0 ± 7.2 µmol L−1 over the dry/cold season. The FCO2 values ranged from 0.2 to 12.1 g CO2 m−2 d−1 averaging 4.9 ± 4.0 g CO2 m−2 d−1 over the rainy/warm season and from 0.1 to 1.7 g CO2 m−2 d−1 with an average of 0.8 ± 0.5 g CO2 m−2 d−1 over the dry/cold season. During the rainy/warm season the emission was higher in the eutrophic lake San Lorenzo (9.1 ± 1.2 g CO2 m−2 d−1), and during the dry/cold the highest emission was recorded in the mesotrophic lake San José (1.42 ± 0.2 g CO2 m−2 d−1). Our results indicated that eutrophication in tropical karst lakes increased CO2 evasion rates to the atmosphere mainly due to the persistence of anoxia in most of the lake’s water column, which maintained high rates of anaerobic respiration coupled with the anaerobic oxidation of methane. Contrarily, groundwater inflows that provide rich-dissolved inorganic carbon waters sustain emissions in meso and oligotrophic karstic tropical lakes.
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