Abstract:Closed shallow lakes represent unique and fragile aquatic ecosystems characterized by limited hydrological exchange, which significantly shapes their community structure and functional processes. Scientifically elucidating the structural characteristics of such ecosystems and evaluating the ecological carrying capacity for key species are fundamental prerequisites for sustainable fisheries management and aquatic ecological restoration. Huangtai Lake, as a typical closed shallow lake, has undergone significant anthropogenic influences, including long-term stocking practices. However, a systematic understanding of its food web structure, energy flow pathways, and the sustainable thresholds for stocked species has been lacking, hindering the development of science-based management strategies. This study aimed to reveal the structural characteristics of the Huangtai Lake ecosystem, particularly its trophic level distribution and energy flow patterns; quantify the ecological carrying capacity for major stocked species, including silver carp (Hypophthalmichthys molitrix), bighead carp (Aristichthys nobilis), mandarin fish (Siniperca chuatsi), oriental river prawn (Macrobrachium nipponense), and crabs (Eriocheir sinensis); and assess the interspecific resource competition, especially among filter-feeding fishes, to provide a scientific basis for optimized stocking regimes and ecosystem-based management. Based on comprehensive environmental and fishery resource survey data collected from Huangtai Lake in 2024, a mass-balanced Ecopath model was constructed. The model comprised 17 functional groups, encompassing primary producers (phytoplankton), primary consumers (zooplankton, benthic organisms), secondary and tertiary consumers (various fish species, including key stocked species), and detritus. This model was employed to analyze the trophic structure, quantify energy flows between trophic levels, calculate key ecosystem indices, and estimate the ecological carrying capacities for the target species. Scenario simulations were further conducted to assess the impacts of multi-species coexistence on carrying capacities. Trophic structure and energy flow: The trophic levels of the biological community in Huangtai Lake ranged from 1.000 to 3.287. Filter-feeding fishes, namely silver carp and bighead carp, served as critical nodes in energy transfer, highlighting the ecosystem's characteristic of being dominated by lower trophic levels and possessing relatively short food chains. The total primary production was estimated at 8726 t/(km2·a). However, the energy transfer efficiencies were notably low: only 1.57% from trophic level II to III and 3.79% from III to IV, indicating severe constraints on energy delivery to higher trophic levels. Ecosystem state indicators: The system-level indices derived from the model suggested that Huangtai Lake is at an early developmental stage. The ratio of Total Primary Production to Total Respiration (TPP/TR) was 1.856. The Finn's Cycling Index (FCI) was low at 4.506%, The calculated Connectance Index (CI) for the system was 0.263, and the System Omnivory Index (SOI) was 0.116, reflecting a weak capacity for energy recycling within the system. These metrics, combined with a relatively low System Omnivory Index (SOI), depict an ecosystem with simple structure, high dependence on primary production, and limited internal complexity. Ecological carrying capacity: Under the modeled conditions, the estimated ecological carrying capacities were 0.203 t/km2 for mandarin fish, 10.906 t/km2 for silver carp, 30.653 t/km2 for bighead carp, 2.802 t/km2 for oriental river prawn, and 0.794 t/km2 for Eriocheir sinensis. Impact of species interaction: Scenario simulations considering multi-species coexistence revealed significant resource competition between silver carp and bighead carp due to substantial dietary overlap. Their maximum sustainable biomasses decreased to 7.993 t/km2 and 12.199 t/km2, respectively, when co-existing, underscoring the necessity of considering interspecific competition in stocking plans. The Huangtai Lake ecosystem is characterized by energy concentration at lower trophic levels, inefficient energy transfer to apex consumers, and structural simplicity indicative of an immature and potentially vulnerable state. Filter-feeding fishes play a pivotal but competitive role. The management of stock enhancement, therefore, must strictly adhere to the ecological carrying capacities estimated under realistic multi-species scenarios. Particular attention should be paid to controlling the combined biomass of silver carp and bighead carp to prevent excessive grazing pressure on plankton communities, which could destabilize the entire ecosystem. This study provides the first quantitative Ecopath model for Huangtai Lake, offering a systematic and holistic analysis of its ecosystem structure and function. The estimated species-specific ecological carrying capacities and the revealed competitive dynamics offer concrete, science-based benchmarks for fisheries management. The findings advocate for a shift from traditional empirical stocking towards an ecosystem-based approach that respects ecological thresholds and species interactions. The findings deliver direct, actionable quantitative benchmarks for the management of Huangtai Lake and analogous water bodies. Precision management: The estimated ecological carrying capacities for each species, especially the adjusted capacities for silver and bighead carp considering competition, establish clear biomass ceilings for formulating science-based annual stocking plans. This facilitates a shift from traditional, experience-based stocking towards a precision management paradigm constrained by ecological carrying capacity. Optimized species composition: The study unambiguously highlights significant competition between filter-feeding fishes. This serves as a critical warning for managers to coordinately plan the stocking ratio and total biomass of silver and bighead carp. Overstocking driven by pursuit of a single-species yield must be avoided to prevent plankton resource depletion, interspecific imbalance, and diminished overall stocking efficacy. Concurrently, capacity estimates for species like mandarin fish, prawns, and crabsreveal opportunities for developing diversified, complementary eco-fishery models. Informing Ecological Restoration goals: silver carp and bighead carp are classic species used in biomanipulation to control algal blooms. This study defines their sustainable carrying capacity, implying that their algae-control function must be harnessed within ecologically safe limits to prevent ecosystem distortion. The low Finn's Cycling Index (FCI) and simple structure identified call for management strategies that enhance ecosystem complexity and resilience, such as the conservative introduction of appropriate omnivorous or benthic species to strengthen detrital pathways and internal recycling. This work serves as a crucial scientific foundation for achieving sustainable fisheries yield, maintaining water quality (through biomanipulation via filter feeders), and promoting the overall health and stability of Huangtai Lake's ecosystem. The methodology and insights are also relevant for the management of similar closed shallow lake ecosystems elsewhere.
