Distribution, sources, and controlling factors in surface sedimentary organic matter of a typical degraded coral reef...

Global coral reef degradation alters coastal sedimentary organic matter (SOM) distribution patterns. This study examined the seasonal dynamics and influencing factors of surface SOM in the Yangmeikeng Sea (YMKS), a typical degrading coral reef ecosystem in China. Results showed that the total organic carbon (TOC, 0.54 ± 0.57 %) and total nitrogen (TN, 0.056 ± 0.059 %) contents in reef sediments were significantly lower than those in non-reef areas (TOC: 1.23 ± 0.23 %; TN: 0.131 ± 0.032 %). The sedimentary TOC levels suggested potential degradation of some coral reefs in the YMKS. The Bayesian mixing model revealed that SOM in the coral reef area was mainly derived from marine phytoplankton and coral reef-derived organic matter (OM) (contributing 47.0 ± 13.85 % and 38.3 ± 22.0 %, respectively), whereas the non-coral reef area was dominated by marine phytoplankton and terrestrial inputs (contributing 60.7 ± 4.9 % and 29.8 ± 6.4 %, respectively). These patterns indicated that coral reef ecosystems may reduce the proportional contribution of phytoplankton-derived OM to the SOM pool, while enhancing the share of coral reef-derived OM. SOM exhibited distinct spatiotemporal variations closely linked to OM sources and environmental factors. Specifically, phytoplankton-derived OM distribution in reef areas showed strong association with sediment grain-size composition, whereas in non-reef areas, its dynamics appeared influenced by multiple factors including phytoplankton primary production, sediment grain size, and redox conditions. Terrestrial OM dynamics were influenced seasonally by hydrodynamics, while coral-derived OM was co-regulated by reef-dwelling organisms and grain-size composition.