Submerged macrophytes as primary producers play important roles in maintaining the structure, function, and biodiversity of lake ecosystems (Nyieku et al. 2020). Submerged macrophytes secrete allelochemicals, inhibit the propagation of algae, and absorb nutrients in the overlying water and sediment, which has important ecological value in controlling lake eutrophication (Sarvala et al. 2020). The restoration, reconstruction, and transformation of aquatic plants have become important methods in the ecological regulation and endogenous pollution load control of shallow lakes (Chorus et al. 2020). However, when a large number of submerged macrophytes decompose, the residue still exists in the water and decomposes as well, which releases nitrogen (N), phosphorus (P), and other raw elements into the overlying water, thereby leading to secondary pollution (Malaviya et al. 2020). Harvesting is an important method used for the overgrowth of submerged macrophytes in lakes. Harvesting submerged macrophytes is conducive not only to the recovery and growth of plants, which increases biodiversity, but also to improving the stability of the community to continuously purify the water. It can also reduce the nutrient load in a lake by harvesting plants or transferring nutrients from the lake. Additionally, harvesting can prevent the negative effects of excessive submerged macrophyte growth. For submerged macrophytes whose biomass is mainly concentrated in the upper layer or surface of the water body, harvesting can alleviate excessive biomass concentration. Therefore, it is of great importance to study the effects of harvesting on aquatic environments.
Harvesting directly affects the N and P cycles in the water. Currently, due to excess N, P, and other nutrients, submerged macrophytes overgrow and bloom in many lakes (Franceschini et al. 2020). As the seasons change, the decomposition of many submerged macrophyte residues causes serious secondary pollution, which poses a great threat to the safety of aquatic ecosystems (Wang et al. 2018). Decomposition and death are necessary stages in the life history of submerged macrophytes. The decomposition and release of nutrients are important ecological processes that play important roles in the biogeochemical cycles of N, P, and other nutrients in lake ecosystems. In the field of lake ecology, the investigation of these processes is currently a popular research area (Shilla et al. 2006). In the process of natural succession and seasonal change, if aquatic plants are not harvested, then their litter will disperse into the water and sediment, and their decomposition products (organic matter, N, P, and other nutrients) will participate in the biogeochemical cycles of lake nutrients again (Bianchi &Findlay 1991). Therefore, the decomposition of aquatic plants is the key link between material cycling and energy flow. This process will reduce the transparency of the water body and increase the contents and ratios of organic matter, N, P, and other pollutants in the water body and sediment, which will thereby result in secondary pollution (Battle &Mihuc 2000).
The P status in lakes is an important factor of submerged macrophytes that affects eutrophication. The relationship between eutrophication and limiting nutrients (mostly P) differs between lakes with and without submerged vegetation. Therefore, it is of great importance to explore the effects of harvesting on the P cycle in the water body to assist the management of vegetative lakes (Overbeek et al. 2019).
Potamogeton crispus is a submerged macrophyte widely distributed throughout China that is often used in the ecological restoration of eutrophic lakes (Hao et al. 2018). Most accomplishments in efforts to restore eutrophic lakes have been attributed to the success of aquatic macro-vegetation (Wang et al. 2017). The viable growth temperature of P. crispus is 10–20°C, and it stops growing when temperatures are > 24°C. P. crispus germinates in autumn and grows during the winter; large reproductive growth occurs in April and May, and plants finally decompose and die in the summer. It has a rapid growth rate, can tolerate high nutrient-rich environments, and grows well in contaminated water bodies (Leoni et al. 2016).
Lake Yimeng was formed in 1997 when a rubber dam (1,135 m) was built across the Yi River, capturing 12 million m3 of water. In recent decades, the lake has exhibited dense canopy-forming populations of P. crispus, which covers nearly 90% of the lake during spring and summer (Wang et al. 2018). Different types of management strategies have been implemented to reduce P. crispus blooms, including harvesting during the summer. Almost 100% of the present P. crispus was harvested in 2017 in the Yi River part of the lake; however, in the Beng River part of the lake, P. crispus was not harvested. Therefore, we selected these two areas (harvested and non-harvested) to study the effects of harvesting on P composition in the water (Fig. 1).
The aim of this work was to determine how harvesting submerged macrophytes (P. crispus) affects the P composition and environmental factors in the water. We compared the P in the water and sediment, chlorophyll-a (Chl-a), and environmental factors between the harvested and non-harvested areas of the lake for three years. The relationship between Chl-a and P in the water and environmental factors are also discussed.