Trophic model of a deep-sea ecosystem with methane seeps in the South China Sea

Benthic megafauna in deep-sea ecosystems with and without methane seeps in the South China Sea were quantified during 2013-2016. In total, more than 190 taxa were identified. Stable isotopic analyses (delta C-13, delta N-15) on the tissues of these megafauna were used to provide complementary data to reveal their trophic relationships. Ecopath models were constructed to show the flow of matter within deep-sea ecosystems. There were four integer trophic levels in both deep-sea models. Most of the omnivory indices of the megafauna in the models were small, indicating the specialized diet niches of the megafauna. The mixed trophic impact results showed that both models were top-down controlled. Seep-associated king crabs were the keystone group in the seep model. These crabs not only transferred energy from lower trophic levels to top predators but also linked energy from seeps to neighboring deep-sea ecosystems. However, the low number of trophic connections between the seep animals and neighboring deep-sea communities indicates the uniqueness of the seep ecosystems in the deep sea. All the biomass, matter flow and trophic transfer efficiency values were higher in the seep model than in the model without seeps. The higher overhead/capacity ratio in the seep model than in the model without seeps suggests that the former model was more resilient to perturbations than the latter model. Although the net primary production and matter flow in the seep model were lower than those in the shallow-sea models, the models had comparable values of biomass excluding detritus. The relatively high system omnivory indices and low matter cycling of both deep-sea models indicate that these models were more mature than the shallow-sea models. Collectively, our deep-sea models combined with other models suggest that deep-sea ecosystems are characterized by slow dynamics and high environmental stability. This study reports the first Ecopath model for methane seep ecosystems, which may serve as a basis for potential anthropogenic impact assessment and ecosystem-based management.