Regulation of freshwater filamentous green algae (Cladophora) and its impact on malodorous volatile organic sulfur compound (DMS) by biomanipulation.

Abstract

When improving the water quality of natural bodies such as lakes, the explosive growth of filamentous green alga Cladophora can limit the growth of submerged macrophytes and prevent the water from shifting to a clear state. During the decay of Cladophora, it can cause various water quality issues such as reduced dissolved oxygen, increased nutrient levels and water odor. Biomanipulation, involving the introduction of a suitable density of aquatic animals into the water, can reduce the biomass of filamentous algae. We hypothesized that stocking appropriate densities of aquatic animals could reduce filamentous algal biomass and at the same time reduce the concentration of odorants in the water. Our study investigated the impact of stocking swamp shrimp (Macrobrachium nipponense), rosy bitterling (Rhodeus ocellatus), and silver carp (Hypophthalmichthys molitrix) at low (30 g/m³), medium (60 g/m³) and high (120 g/m³) densities on water quality, biomass of primary producers (such as Cladophora, submerged macrophyte and algae) and malodorous volatile organic sulfur compound dimethyl sulfide (DMS) in the water, respectively. It was found that the swamp shrimp treatment groups and the rosy bitterling high-density groups effectively inhibited the growth of filamentous green algae cover, in which the rosy bitterling high-density group reduced the filamentous green algae mat coverage by 29.65 % compared with the control group. Additionally, the high-density swamp shrimp and rosy bitterling groups notably promoted the growth of submerged macrophytes (Vallisneria denseserrulata), and significantly reduced the concentration of the malodorous DMS in the water. Overall, stocking swamp shrimp and rosy bitterling can benefit the restoration of aquatic ecology and the maintenance of clear water. However, it is essential to consider potential changes in water quality resulting from excessive stocking density. Therefore, the appropriate density and proportion of stocking should be determined in conjunction with the specific scale of the aquatic ecological restoration project.