How a coastal megacity affects marine biodiversity and ecosystem function: Impacts of reduced water quality and other anthropogenic stressors

Coastal marine ecosystems are integral in providing numerous ecosystem functions and services. However, intense and chronic anthropogenic stressors often impact marine ecosystems surrounding highly urbanized, densely populated coastal megacities. The associated reduction of water quality and biodiversity of these ecosystems potentially erodes ecosystem functions and provision of services. While there is general consensus that greater biodiversity supports a multitude of ecosystem functions, evidence from marine ecosystems in real-world settings is lacking. In this study, we evaluated the influence of water quality on benthic and fish assemblages, as well as quantified six key ecosystem functions in coastal waters of Hong Kong. Hong Kong's marine ecosystems are marred with long-term damage that is directly linked with rapid economic growth and anthropogenic activities. We found that water quality significantly affected community composition and clustered assemblages into separate groups related to strength of impact: high impact sites were generally dominated by bivalves and small, opportunistic organisms such as damselfish (Pomacentrinae) and cardinalfish (Apogonidae); low impact sites were dominated by macroalgae, bryozoa, and commercially important piscivores such as groupers (Epinephelinae) and seabreams (Sparidae). Ecosystem function tests showed that low impact sites were much more productive than high impact sites, with 15-times greater macroalgal cover and 5-times higher fish biomass. On the other hand, herbivory and predation rates were context-dependent due to underlying biodiversity effects such as selection, complementarity, and competition. Eutrophication increased decomposition and reduced carbon sequestration, but excessive nutrient input could have suppressed decomposer activities via hypoxia. Therefore, we show that in high impact sites where top-down control is removed, resilient and opportunistic organisms such as cardinalfishes and sea urchins may expand to fulfill functional roles, effectively maintaining ecosystem functions such as predation and herbivory in degraded habitats. Contrary to general consensus, this study demonstrates the merit in conserving even highly impacted marine habitats due to their persistent ecosystem functionality despite chronic water quality issues and shifts in benthic and fish assemblages.