HydroMet buoy - 2019-2022 water quality 1-10 m vertical profiles

Figure 12 - Vertical profiles at each meter between 1 and 10 m depth carried out every two hours (2:00, 4:00, 6:00, ..., 22:00, 24:00; i.e. 12 daily profiles). On several occasions the sounder was trapped near or on the bottom dragged by drifting fishing nets, or for unknown reasons as the last time between 27/05 and 05/06/2020. As the missions have been forbidden since March, the causes could not be determined.

Figure 13 - Time evolution of vertical temperature profiles between 0 and 10 m depth. The data follow a parabolic curve (in relation to the seasonal variation of the height of the sun on the horizon). From September 2019, the temperature increased from 11.0 ºC to a maximum of 17.5 ºC in January-February, then decreased to 11.0 ºC in June 2020. From December onwards, it remains between 15.0 and 17.5 ºC. The daily vertical variability is low at around 1.0-1.5 ºC.

Figure 14 - Evolution of conductivity, which varies slightly between a minimum of 1480 in February and a maximum of 1540 µS/cm on January 2, 2020. The conductivity drops towards ≤ 1500 µS/cm during the rainy season (January-February)...

Figure 15 - pH evolution. We only installed a calibrated sensor on November 27th. So the data are only valid from December onwards. The average pH was 9.0 and varied very little between 8.7 and 9.2, with very little vertical variability (≤ 0.1 pH unit).

Figure 16 - Temporal evolution of the oxidation-reduction potential. Its average was 242 mV with a low fluctuation range between 189 and 273 mV, with a decrease in March, a plate from April 2020, and little vertical variability (≤ 10 mV).

Figure 17 - Temporal evolution of the dissolved oxygen saturation level. On average, the water column is very well oxygenated (98%), with an amplitude between 67 and 119%. In general, the vertical variability is reduced (≤ 10%). The minimum value (65%) occurred on January 15 at the bottom.

Figure 18 - Temporal evolution of the dissolved oxygen concentration in the water column, identical to OD%. It fluctuated between 6.6 and 12.7 mg/L with an average of 10.0 mg/L, i.e. a very good oxygenation in the whole column, taking into account the altitude. The lowest values occur in January-February, the rainy season. The vertical variability is ≤ 1 mg/L.

Figure 19 - Temporal evolution of turbidity. It fluctuated between 0.03 and 2.2 NTU. During the dry season turbidity remained ≤ 0.3 NTU. It increased during the rainy season (January-February) up to 1.2 NTU on January 15. In March-April it remained ≤ 0.3 NTU. Then in May there were episodes of high turbidity (1.49 NTU on 5/14, 1.88 NTU on 5/15, and 1.07 NTU on 5/23). On average, vertical variability was low ≤ 0.4 NTU.

igure 20 - Evolution of phytoplankton chlorophyll-a concentration (in RFU = Relative Fluorescence Unit). Chlorophyll-a remained low during the dry season being ≤ 0.7 RFU or ≤ 5.6 µg/L. It increased during the rainy season (January-February) up to 2.8 RFU or 22.4 µg/L (corresponding to a mesotrophic state, according to the open trophic classification of the OECD, 1982) on January 15, 2020, then dropped again. Vertical variability was ≤ 0.5 RFU or ≤ 4 µg/L.

Figure 21 - Evolution of the concentration of phycocyanin, an accessory photosynthetic pigment of cyanobacteria. Varying between 0.01 and 0.51 RFU, being≤ 0.2 RFU in dry season, and up to 0.38 RFU (15/01) in rainy season (January-February), with a vertical variability ≤ 0.1 RFU.

Figure 22 - Evolution of the fluorescent dissolved organic matter concentration fDOM. It varied between 0.8 and 2.7 RFU (minimum-maximum). It did not exceed 1.3 RFU in the dry season, while it reached 1.8 RFU in the rainy season (January-February). Vertical variability is ≤ 0.2 RFU.

Figure 12 - Vertical profiles at every meter between 1 and 10 m depth performed daily at every two hours (2:00, 4:00, 6:00,..., 22:00, 24:00; i.e. 12 daily profiles). In 2019 and 2020, on several occasions the sonde was dammed near or on the bottom dragged by drifting fishing nets, or for unknown causes. From 2021 onwards, this occurred very infrequently.

Figure 13 - Time evolution of vertical temperature profiles between 0 and 10 m depth. The vertical amplitude of each measurement (≤ 1 ºC) indicates the temperature difference between the surface and the bottom. The data follow a parabola curve (in relation to the seasonal variation of the sun's height above the horizon). During the 1st half of the year, the 2020 temperature was slightly higher (≤ 1ºC) relative to 2021. During the 2nd half of the year, 2019 and 2020 temperatures were quite similar. In the rainy seasons (December to March) temperatures reached 16-17 ºC, while in the dry seasons (June to August) they dropped to 12-11 ºC.

Figure 14 - Conductivity evolution. Over the 2-year period, the average was 1,515 and the median 1,511 µS/cm. During the rainy season the conductivity was surprisingly slightly higher (up to 1,560 µS/cm); the opposite would be expected due to the concentration of salts when the water level drops (~1.0-1.5 m) with the drought. In 2020, conductivity remained fairly constant throughout the year except from October to December when it increased to 1,550 µS/cm. During the 1st half of the year, 2021 conductivity was slightly higher (~1,550-1,560 µS/cm) than in 2020. During the 2nd half of the year, conductivity was slightly higher in 2020 (~1,550 µS/cm) than in 2019 (~1,520 µS/cm).

Figure 15 - Temporal evolution of the dissolved oxygen saturation level (%DO). On average, the water column is very well oxygenated (> 93%), with an amplitude between 67 and 119 %. In general, vertical variability is low (≤ 10%). Oxygenation during 2020 was higher than in 2021, except from October to December when it was lower than in 2019, around 15%.

Figure 16 - Temporal evolution of dissolved oxygen concentration (DO, mg/L) in the water column. The pattern is identical to that of the DO%. During the two years, it fluctuated between 6.6 and 12.9 mg/L with an average of 9.7 mg/L, i.e. a very good oxygenation throughout the column, taking into account the altitude. The lowest values arose in January-February (~8-9 mg/L), in rainy seasons, with maxima in dry seasons (June-August, ~10.0-12.9 mg/L). Vertical variability was low ≤ 1 mg/L. The concentration was higher in 2020 (~1.5 mg/L) compared to 2021. Except in October-December, when it was higher in 2019 relative to 2020. 

Figure 17 - Temporal evolution of turbidity. It fluctuated between 0.03 and 2.9 NTU. During the dry season, turbidity remained ≤ 0.3 NTU. It increased during the rainy season (January-February) up to ~1.5-2.0 NTU. From March to September, there were episodes (for 1-3 weeks) of higher turbidity (~1.5-2.0 NTU). On average, vertical variability was low ≤ 0.4 NTU. Turbidity in 2020 was higher than in 2019 during the 2nd half of the year, and in 2021 during the 1st half of the year.

Figure 18 - Evolution of phytoplankton chlorophyll-a concentration (RFU = Relative Fluorescence Unit; 1 RFU = 8 µg/L). Chlorophyll-a remained low during the dry season (April-November) being ≤ 0.7 RFU or ≤ 5.6 µg/L. It increased during the rainy season (January-February) to 2.8 RFU or 22.4 µg/L (corresponding to a mesotrophic state, according to the open trophic classification of the OECD, 1982). Vertical variability was ≤ 0.5-1.0 RFU or ≤ 4-8 µg/L, being higher near the bottom, with surface inhibition in the first meter where solar radiation, especially ultraviolet radiation is too intense and harmful to microalgae. During the 1st half of the year, chlorophyll-a was higher in 2021 (~+1µg/L) relative to 2020. The peak occurred in mid-January 2020 (2.7 RFU = 21.6 µg/L) and later at the end of February 2021 (3.0 RFU = 24.0 µg/L). It should be noted that in this northeastern region of Lake Minor in 1979-1980, chlorophyll-a did not exceed 3.0 µg/L, which is characteristic of an oligotrophic state (Lazzaro, 1981). Thus, although not noticeable at first sight (without the use of a probe), the phytoplankton biomass has increased up to 8 times in the last 40 years, as a consequence of the increased availability of nutrients from anthropogenic inputs (mostly from the urban region of El Alto via the Katari basin) combined with the effects of global warming, which magnify eutrophication.

Figure 19 - Evolution of the concentration of phycocyanin, an accessory photosynthetic pigment characteristic of cyanobacteria. It varied between 0.01 and 0.56 RFU (1 RFU = 1 µg/L), with an average and median of 0.16 RFU. Tended to have higher concentration during the dry season, particularly in June-July and September, with periods of 2-3 weeks with sustained higher values up to 0.45-0.55 RFU and variability of up to 0.3 RFU between surface and background. During 2020, the concentration was generally higher (~+0.1 RFU) during the 1st half 2021 relative to, and during the 2nd half relative to 2019. Two peak periods are noted, one in January-February 2020-2021 (peak rainfall) and the other in July (peak of the dry season) and end of August-September (time of peak winds). This suggests that massive nutrient inputs and their vertical mixing by winds, respectively, stimulate the increase of cyanobacteria. When comparing the phycocyanin graph with the chlorophyll-a graph, it is noted that in general the phycocyanin concentration represents 10% to 20% of the chlorophyll-a concentration. This is an incentive to keep an eye on the dynamics of cyanobacteria (phycocyanin) that have the capacity to rapidly generate blooms (= proliferations or blooms) when conditions are favorable, which can be extremely damaging and could then occur recurrently every year.

Figure 20 - Evolution of the fluorescent dissolved organic matter concentration fDOM. It varied between a minimum of 0.8 RFU and an exceptional maximum of 11.1 RFU, with mean and median of 1.3 RFU. Generally, it remained low between 1-2 RFU, stably throughout the year. Its vertical variability was ≤ 0.2 RFU. We can conclude that in this relatively deep zone (11 m) for the Bolivian sector of Lago Menor (< 5 m), the concentration of dissolved organic matter is low.