Weakened hydrological oscillation period increased the frequency of river algal blooms.

The evolution of riverine aquatic ecosystems typically exhibits notable characteristic with cumulative, enduring, and hysteresis. Exploring the non-linear response of riverine ecology to long-term hydrological fluctuations become a major challenge in contemporary interdisciplinary research. In response to the critical issue of frequent river algal blooms in the lower Han River, which is impacted by Asian largest inter-basin water diversion project. We identified the non-linear response of eco-hydrology across various time scales through the integration of Continuous Wavelet Transform (CWT) and Inverse Wavelet Transform (IWT). Our study revealed that: 1) Over the past half century, the hydrological regime in the lower Han river showed a significant downward trend, and existed three significant hydrological oscillation periods (HOPs), including the short-scale Intra-AC (180 days), the medium-scale AC (365 days, the first major period), and the long-scale Inter-AC (2500 days), the variation of Inter-AC changed most dramatically. 2) We further found that the Inter-AC variation of hydrology is more closely related to the formation of river algal blooms in the Han River, and when the hydrological Inter-AC shows steady state or downward trend, the frequency of algal blooms in the lower Han River increases significantly. 3) The river algal blooms in the lower Han River is a cumulative consequence to the long-term hydrological influences. Weakened hydrological Inter-AC is more likely to increase the frequency of river algal blooms, and 10-years Inter-AC cumulation increased the frequency by 60%. Therefore, the weaken of long-scale HOP will significantly increase the frequency of river algal blooms in the future. This study received a critical scientific insight and aimed at provide guidance for the optimization of ecological management within the framework of national large-scale water conservation.

Investigation of the variations in dissolved organic matter properties and complexations with two typical heavy metals under the influence of biodegradation: A survey of an entire lake.

Biodegradation is a key factor determining the properties and metal binding behaviour of dissolved organic matter (DOM). In this study, the contributions of biodegradation to DOM properties and metal binding behaviour in Dongping Lake were explored by using synchronous fluorescence (SF) spectroscopy, two-dimensional correlation spectroscopy (2D-COS) excitation-emission matrix and parallel factor analysis (EEM-PARAFAC). According to the ratio of the fluorescence intensity of different materials to the entire fluorescence intensity (%Fmax), protein-like substances were the main substances of DOM in this lake. The reduction of protein-like substances and the enhancement of humification could be found in the whole lake under the influence of biodegradation. 3 areas (Area A, Area B and Area C) were obtained by principal component analysis (PCA), however, PCA results suggested that DOM properties and sources had some differences in the 3 areas, and DOM bioavailability in Area C was stronger than that in the other 2 areas. With copper (Cu2+) and lead (Pb2+) addition, different substances exhibited various affinities to different metal types. The locations of crosspeaks in asynchronous maps illustrated that protein-like substances were more affiliative with Cu2+, while humic-like substances were bound to Pb2+ earlier. Biodegradation had a conspicuous impact the metal binding ability of DOM in Dongping Lake. The effective quenching constants (LogK) of protein-like substances (protein-like component 2: LogKCu = 3.85 ± 0.23, LogKPb = 3.32 ± 0.23) were higher than those of humic-like substances (humic-like component 3: LogKCu = 3.15 ± 0.02, LogKPb = 2.93 ± 0.17) for both Cu2+ and Pb2+ before biodegradation. When biodegradation was finished, binding ability of humic-like substances was enhanced from 3.15 ± 0.02 to 3.41 ± 0.10 for DOM-Cu and 2.93 ± 0.17 to 3.79 ± 0.15 for DOM-Pb. On a spatial scale, metal binding ability of DOM in Dongping Lake also changed due to the influence of biodegradation. For both DOM-Cu and DOM-Pb, binding ability in south of Dongping Lake was stronger than that in other areas with the end of biodegradation.