# Evaluation of Algal Control Measures in Eutrophic Reservoir Based on Aquatic Ecosystem Models
## 3 Results and Discussions
### Ecological Fish Farming  
Regarding the reservoir's ecological fish farming scheme, the results for both the reservoir center (Figure 10) and the overall reservoir (Figures 11 and 12) are presented.  
Figure 10: **(a)** Comparison of carbon content of blue-green algae (BC) in different farming schemes; **(b)** comparison of total nitrogen (TN) concentration in different farming scenarios; **(c)** comparison of total phosphorus (TP) concentration in different farming scenarios.  
The results (Figure 10) of the reservoir area simulation indicate the following conclusion:  
(1) An analysis of the ecological stocking from July to October indicated that the stocking of 200,000 fish had a significant impact on reducing algae biomass. Following the introduction of carp and silver carp, there was an average decrease of approximately 35% in blue-green algae biomass compared to pre-stocking levels. This decrease had a discernible effect on the peak outbreak, leading to a flattening of the overall algae biomass curve. A comparison with the ecological stocking of 300,000 fish by our institute in 2009 yielded similar results.  
Figure 11: After one day of release, the fish fry dispersed and swam away.  
Figure 12: Comparison of carbon content of blue-green algae (**a**,**e**), chlorophyll concentration (**b**,**f**), total nitrogen concentration (**c**,**g**), and total phosphorus concentration (**d**,**h**) in non-ecological stocking (**a**–**d**) and ecological stocking (20,000 individuals) (**e**–**h**) scenarios.  
(2) Following ecological stocking, there was a marginal decrease in the concentration of total nitrogen and total phosphorus in the deep-water area represented by the reservoir center, with a reduction of approximately 1% for total nitrogen and 0.4% for total phosphorus. There are two main reasons for the poor effect of nitrogen and phosphorus reduction. Firstly, in this model, fish only feed on phytoplankton, and the proportion of nitrogen and phosphorus content in algae to total nitrogen in the reservoir is very small. Secondly, In this simulation process, only the predation on phytoplankton was considered, while the situation of fish feeding on organic debris and zooplankton ingestion was ignored. Therefore, the assessment of the reduction ability of fish on total nitrogen and total phosphorus nutrients is relatively conservative, and there is room for improvement in actual effectiveness.  
The results (Figures 11 and 12) of the reservoir area simulation indicate the following conclusion:  
(1) Following ecological fish farming, the overall distribution of algae biomass became more evenly spread in the reservoir area due to the feeding activity of fish. Regions with previously high levels of algal biomass, such as local bays and shallow waters near the dock, exhibited a significant decrease in biomass after fish stocking.  
(2) Although the overall distribution of nutrients decreased in the area, there were instances where nutrient levels increased after ecological stocking, mainly concentrated near the dock or in shallow waters in front of small islands in the reservoir, where algae reproduction was significant.  
(3) An analysis of the nitrogen and phosphorus conversion details in the various ecological circulation processes simulated by the model revealed that, primarily due to the large-scale reproduction of algae resulting from ecological fish farming, nitrogen and phosphorus were brought to the surface after stocking, contributing to an \"anomalous phenomenon\" of increased levels without offsetting the amount brought into the sediment through the dead biomass of algae [59].  
Therefore, acknowledging the significant role of planktonic algae in the aquatic ecosystem is paramount; the controlled reproduction of algae acts as an absorber and converter of nitrogen and phosphorus, playing a crucial role in reducing and buffering nutrient concentrations in reservoirs. Pursuing the complete eradication of algae may have detrimental effects on the ecological cycle of reservoirs.