Abstract
Algal blooms are environmental challenges confronting lakes worldwide and are significantly influenced by chlorophyll a yields per unit phosphorus (Chla/TP), or nitrogen (Chla/TN). Here, the influence of inter-annual hydrometeorological variations on Chla/TP and Chla/TN were evaluated in eutrophic shallow Lake Taihu, China. Our results demonstrated significant increases (p < 0.001) in both Chla/TN and Chla/TP from 2005 to 2017, and increased Chla yields during the winter months were mainly correlated with higher water temperature and longer sunshine hours, which may cause severer blooms in winter and spring. In remaining months from 2005 to 2017, typical associations between atmospheric stilling (or water level elevation) and higher Chla yields were observed. The results also indicate that atmospheric stilling and water level elevation significantly (p < 0.001) decreased background turbidity and promoted buoyant cyanobacterial biomass, alleviating phytoplankton light limitation. Given the subtropical location, eutrophic status, and high background turbidity of Lake Taihu, light may be the critical limiting factor for summer phytoplankton growth; thus, improved light availability would promote Chla yields until self-shading caused further light limitations. If the mechanism is general, promoting the effect of atmospheric stilling on annual peak Chla in shallow lakes may be greatly underestimated, and our finding will affect future bloom mitigation efforts in such systems.
Highlights
Nutrient enrichment and the associated algal blooms are widespread water quality issues in freshwater ecosystems [1]
Significant decreasing trends (p < 0.001) in average wind speed (AWS), total nitrogen (TN), total dissolved nitrogen (TDN), and non-algal light attenuation (NALA) were observed in the northern bays of Lake Taihu from 2005 to 2017, while significant (p < 0.001) increases in chlorophyll a (Chla), Chla/TN, and chlorophyll a yields per unit phosphorus (Chla/total phosphorus (TP)) were observed from 2005 to 2017 (Figure 3)
It is reasonable that increased AWT and longer sunshine hours (SHH) increased Chla yields in Lake Taihu during winter months because algal growth in these months was generally limited by poor light availability and low temperature [21]
Summary
Nutrient enrichment and the associated algal blooms are widespread water quality issues in freshwater ecosystems [1]. Algal blooms pose a range of ecological and socioeconomic problems, including threats to drinking water safety, access to water-based recreational activities, and detrimental effects on aquatic organisms [3,4]. A range of actions have been proposed to combat algal blooms in freshwater ecosystems, including increasing the flushing rate, reducing external nutrient loading, and restoring macrophytes [6,7]. Current knowledge suggests that reducing nutrient inputs in the watershed is the most feasible way to control algal blooms owing to its predictable outcomes, long-term effectiveness, and relatively low cost [1,8,9]. In the majority of cases, reducing external nutrient loading generally resulted in a decline in phytoplankton biomass [6], which supports the implicit assumption that lake ecosystems impacted by eutrophication can be reverted to their original conditions by reducing nutrient inputs
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