Fragile or Robust: Research on the Structure, Energy Flow, and Associated Environmental Factors of Nearshore Coral Reef Ecosystems in Hainan

Coral reefs are typical ecosystems of high diversity and productivity, but they are facing significant degradation in their structure and function due to the dual stresses of climate change and human activities. To uncover the trophic structure, energy flow, and environmental driving mechanisms of nearshore coral reefs in Hainan, this study constructed mass balance models for five regions, Sanya, Changjiang, Lingao, Wenchang, and Wanning, using Ecopath with Ecosim, and conducted quantitative analysis in conjunction with satellite environmental monitoring data. The results showed that the trophic levels of functional groups in different ecosystems ranged from 1.00 to 5.00, with Sanya and Changjiang having the highest trophic levels (4.505, 4.656), higher than Lingao, Wenchang, and Wanning (3.776–3.924). The system average trophic transfer efficiency ranged from 15.98% to 30.25%, generally higher than the classic Lindeman 10% rule, with Changjiang being the highest (30.25%) and Lingao the lowest (15.98%). In terms of material cycling, the energy utilization efficiency of primary trophic levels was low, with a large amount of energy retained as detritus at lower trophic levels; Sanya and Changjiang showed more complete detritus chain energy cycling, with Finn cycling indices reaching 7.69 and 2.53, respectively, indicating higher maturity. Keystone analysis identified zooplankton, corals, and medium carnivorous fish as key functional groups with a decisive impact on system stability. Environmental correlation analysis revealed that particulate inorganic carbon (Pic), light diffuse attenuation coefficient (Kd), chlorophyll-a concentration (Chl_a), and sea surface temperature (Sst) were the main regulating factors, among which Pic showed a significant peaked relationship with the total system throughput, total biomass, and total production, with an optimal range of 0.0050–0.0075 mol/m2. Overall, the material cycling and energy flow states of the Sanya and Changjiang coral reef ecosystems were stronger than those of Lingao, Wenchang, and Wanning, with lower fishing pressure in the former contributing to a more complex food web and enhanced community structural stability. This study provides the first systematic quantitative assessment of food-web structures across five distinct nearshore coral reef regions in Hainan, introducing an early-warning threshold for particulate inorganic carbon that offers direct, actionable reference for regional ecosystem-based management.