Phytoplankton biomass variation after cage aquaculture removal from the Daheiting Reservoir, China: observations from satellite data

Cage-based aquaculture has been gradually discontinued in many lakes and reservoirs in China to reduce eutrophication triggered by the feed. This study aimed to analyse variations in phytoplankton biomass in Daheiting Reservoir before and after cage removal. Cage aquaculture in this reservoir was terminated in the spring of 2017. Changes in phytoplankton biomass were analysed using multispectral satellite data. Chlorophyll a concentration increased from 2016 to 2018, but decreased from 2018 to 2021. Cyanobacterial blooms were absent from 2016 to 2017, but suddenly appeared over an area of ~ 12.76 km 2 in 2018 and gradually decreased to 0.55 km 2 in 2021. Following the removal of cage aquaculture in 2017, phytoplankton biomass of the reservoir initially rapidly increased, peaking in 2018, and then declined. The rapid, short-term increase in phytoplankton biomass after cage removal might be due to sudden lack of algae predators (silver carp, bighead carp, and sturgeon, prey on algae and plankton), as cultured fish were removed from the reservoir. Decreased nutrient input in the water and the growth of wild fish resulted in subsequent decreases in phytoplankton biomass. These findings provide a reference for research methods and decision support for the change in phytoplankton biomass after cage removal.