Ecosystem dynamics and hypoxia control in the East China Sea: A bottom-up and top-down perspective.

ABSTRACT

Decades of overfishing have greatly altered the community structure in the East China Sea (ECS). The decrease of top predators in the food web has weakened the control exerted from higher trophic levels. As a result, the biomass of benthic crustaceans, representing the third trophic level, has increased. This has probably led to a restriction of the second trophic level, diminishing its ability to control primary producer biomass. Consequently, the ecological pyramid of trophic levels in the ECS has been altered, reducing the top-down control on the first trophic level. This has made algal blooms more susceptible to occur under nutrient loads, temperate temperatures, and light availability. The reduced abundance of the fourth trophic levels has caused a larger portion of primary productivity to sink directly to the benthic community, bypassing the food web. This influx of sinking organic matter has resulted in organic enrichment in the bottom waters, impacting the biomass and diversity of benthic organisms. Furthermore, it has intensified anthropogenic carbon storage in the sediment. Subsequently, intense decomposition processes occur, leading to the development of anoxia and even hypoxia. The seasonal hypoxia off the Changjiang Estuary can be attributed to the combined influence of top-down control and bottom-up control related to nutrient loading, and terrestrial inputs. In order to mitigate extreme hypoxia events, it is necessary to implement comprehensive fisheries policies that prioritize the maintenance of a healthy and functional ecosystem. This approach should go beyond relying solely on watershed management strategies to regulate riverine inputs. PLAIN LANGUAGE SUMMARY: Decades of overfishing changed the food web in the East China Sea and weaken the resistance of ecosystem to hypoxia. Commercial fishing on top predators decreases the fourth trophic level while relatively increases the third trophic represented by crab and shrimp, which enhances grazing on the zooplankton. The decrease of the second trophic level fails to control the biomass of phytoplankton, thus more primary productivities directly sink to the benthic community and cause organic enrichment. The elevated flux of organic matters to the bottom waters causes the thrive of the carbs and shrimps, as well as more remineralization processes and eventually low oxygen level. Unlike the bottom-up perspective of hypoxia mechanism off the Changjiang Estuary, which is from the nutrient load, phytoplankton bloom, quick sink, effective decomposition and eventually hypoxia, the top-down control focuses on the changes of ecosystem structure and thus derived hindered energy transfer, changed community structure, enhanced carbon sink, elevated remineralization and ultimately hypoxia. These two mechanisms combine with each other and control the seasonal hypoxia off the Changjiang Estuary and even other coastal regions around the world.