Switching Harmful Algal Blooms to Submerged Macrophytes in Shallow Waters Using Geo-engineering Methods: Evidence from a ¹⁵N Tracing Study

Abstract

Switching the dominance from algae to macrophytes is crucial for lake management of human-induced eutrophication. Nutrients from algal sources can be utilized in the process of transition from algal blooms to macrophytes, thereby mitigating eutrophication. However, this process rarely occurs in algal bloom dominated waters. Here, we examined the hypothesis that the transition of algal blooms to macrophytes and the transfer of nutrients from algae at different temperatures (8 and 25 °C) can be facilitated by using a geo-engineering method. The results showed that the combination of flocculation and capping treatment could not only remove Microcystis aeruginosa blooms from eutrophic waters but also facilitate algal decomposition and incorporation into a submerged macrophyte ( Potamogeton crispus) biomass. The flocculation-capping treatment could trigger algal cell lysis. As compared with the control groups, the photosynthesis and respiration rate of algae were inhibited and chlorophyll-a (Chl- a) concentrations were significantly reduced in the flocculation-capping treatment groups. The N tracing study revealed that 3.3% and 34.8% of algae-derived nitrogen could be assimilated by Potamogeton crispus at 8 and 25 °C, respectively. The study demonstrated that the flocculation-capping method can facilitate the switchover from algae- to the macrophyte-dominated state, which is crucial for restoring the aquatic ecosystem.

Modified Local Soil (MLS) Technology for Harmful Algal Bloom Control, Sediment Remediation, and Ecological Restoration

“…Microorganism-modified MLS is also effective in decomposing algae toxins, because MLS flocculation will result in the intense accumulation of microbes and algae cells under the capping layer, as compared to that in the bulk water [19]. For shallow water sediment, the MLS capping aims to provide a 'window period' (usually for a few months in shallow waters) to reach appropriate water and sediment quality levels for the restoration of submerged vegetation [21].…”

“…After the MLS flocculation and capping treatment, the decomposition of algae toxins remarkably increases in the sediment compared to bulk water treatment [19]. Subsequently, when the water clarity is achieved, the capped algal flocs and associated nutrients can be utilised in-situ as fertilisers for the growth of submerged macrophytes in shallow water systems [20,21], which is beneficial as macrophytes can support a healthier food chain than algal blooms. Therefore, MLS technology has made it possible to put algae along with the excessive nutrients into the food chain by flocculating and removing them from water into sediment, and converting them into submerged vegetation [20,21].…”

“…Subsequently, when the water clarity is achieved, the capped algal flocs and associated nutrients can be utilised in-situ as fertilisers for the growth of submerged macrophytes in shallow water systems [20,21], which is beneficial as macrophytes can support a healthier food chain than algal blooms. Therefore, MLS technology has made it possible to put algae along with the excessive nutrients into the food chain by flocculating and removing them from water into sediment, and converting them into submerged vegetation [20,21]. Recently, we have developed an interface oxygen nanobubble technology [22,23], which allows the loading of a substantial amount of oxygen into the microporous surfaces of clays.…”

Lake and River Restoration: Method, Evaluation and Management

“…Owing to rapid population growth and economic development, various human activities in industry, agricultural and transport sectors have deteriorated and globally intensified the freshwater eutrophication ( Figure 1) [2][3][4][5]. In addition to the external loading of pollutants from anthropogenic discharge, the internal loading of pollutants from sediments is expected to further increase the occurrence of HABs and deliver continuous pressure on river and lake ecosystems over the coming decades [6].…”

“…decades [6]. Thus, it is urgent to draw the attention of researchers around the world in order to make great efforts towards lake and river restoration in order to eliminate the threat of eutrophication.…”