Excessive anthropogenically-caused nutrient loading from both external and internal sources has promoted the growth of cyanobacteria in Lake Taihu from 2005 to 2014, suggesting increased production and release of cyanotoxins. In order to explain the spatial distribution and temporal variation of microcystins (MCs), the intracellular concentrations of MCs (MC-LR, -RR and -YR, L, R and Y are abbreviations of leucine, arginine and tyrosine) were monitored monthly from July 2013 to June 2014. Three MC variants are present simultaneously in Lake Taihu; the MC-LR and -RR variants were dominant (accounting for 40% and 39% of the total), followed by MC-YR (21%). However, MC-YR accounted for a higher proportion in colder months, especially in March. The highest concentrations of intracellular MCs were found in July and October when cyanobacteria cell density also reached the maximum. The average concentrations of MC-LR, -RR and -YR in July were 4.69, 4.23 and 2.01 μg/L, respectively. In terms of the entire lake, toxin concentrations in northern parts were significantly higher than the eastern part in summer, when MC concentrations were several times higher than the guideline value by WHO throughout much of Lake Taihu. Results from correlation and redundancy analysis (RDA) showed that total MCs, including all variants, were strongly and positively correlated with cyanobacteria cell density, water temperature, total phosphorus (TP) and pH, whereas each variant had different correlation coefficients with each of the considered environmental variables. MC-RR showed a stronger relationship with temperature, in contrast to MC-YR and -LR. Dissolved inorganic carbon (DIC) showed a negative relationship with each variant, suggesting that rising DIC concentrations may inhibit cyanobacterial growth and thereby reduce MC production in the future.

Widespread harmful cyanobacterial bloom is one of the most pressing concerns in lakes and reservoirs, resulting in a lot of negative ecological consequences and threatening public health. Ocean color instruments with low spatial resolution have been used to monitor cyanobacterial bloom in large lakes; however, they cannot be applied to small water bodies well. Here, the Multi-Spectral Instrument (MSI) onboard Sentinel-2A and -2B and the Operational Landsat Imager (OLI) onboard Landsat-8 were employed to assemble the virtual constellation and to track spatial and seasonal variations in floating algae blooms from 2016 to 2020 in a small eutrophic plateau lake: Lake Xingyun in China. The floating algae index (FAI) was calculated using Rayleigh-corrected reflectance in the red, near-infrared, and short-wave infrared bands. The MSI-derived FAI had a similar pattern to the OLI-derived FAI, with a mean absolute percentage error of 19.98% and unbiased percentage difference of 17.05%. Then, an FAI threshold, 0.0693, was determined using bimodal histograms of FAI images for floating algae extraction. The floating algae had a higher occurrence in the northern region than the southern region in this lake, whilst the occurrence of floating algae in summer and autumn was higher than that in spring and winter. Such a spatial and seasonal pattern was related to the variability in air temperature, wind speed and direction, and nutrients. The climatological annual mean occurrence of floating algae from 2016 to 2020 in Lake Xingyun exhibited a significant decrease, which was related to decreases in nutrients, resulting from efficient ecological restoration by the local government. This research highlighted the application of OLI-MSI virtual constellation on monitoring floating algae in a small lake, providing a practical and theoretical reference to monitor aquatic environments in small water bodies.

Sediment nutrients can be released to the surface water when hydraulic disturbance becomes strong in shallow lakes, which contributes to nutrient enrichment and subsequent lake eutrophication in the water column. To explore the seasonal variations and spatial distributions exhibited by nutrients in the water column, surface sediment, and pore water of Lake Yangcheng and its major tributaries, we determined the concentrations of nitrogen (N) and phosphorus (P) throughout the lake in different seasons of 2018. Total N (TN) and total P (TP) concentrations in the connected rivers were much greater than those in the lake, indicating that external loading greatly contributed to the nutrient enrichment. TN concentration in the water column was highest in the winter, whereas TP peaked in the summer. A similar temporal pattern was observed for TN and TP in the sediment with maxima in the winter and minima in the summer; however, nutrients in the pore water were highest in the summer, in contrast to the temporal variation in the sediment. Additionally, high TN values in the water column and high TP in the three compartments were distributed primarily in the west part of the lake, while high TN concentrations in the sediment and pore water were observed mainly in the east portion of the lake. According to the enrichment factor index (an indicator evaluating the nutrient enrichment by comparing the detected contents and standard values), nutrients in the lake sediment were severely enriched with TN and TP averaging 2195.8 mg/kg and 543.0 mg/kg, respectively. The vertical distribution of TN and TP generally exhibited similar decreasing patterns with an increase in sediment depth, suggesting mineralization of TN and TP by microbes and benthic organisms. More attention and research are needed to understand the seasonality of nutrient exchange across the sediment–water interface, especially in eutrophic lakes.