The Influences of Sediment Load on the Biodiversity of Algal Turfs in Coral Reef Ecosystems through Environmental DNA Analysis

Sediment input is one of the most significant threats to coral reef ecosystems globally, yet its ecological impacts on biodiversity and community structure remain underappreciated. In this study, we utilized a multimarker environmental DNA (eDNA) metabarcoding technique to investigate how sediment load affects diverse taxa within algal turfs in coral reefs of the South China Sea. Using three assays (16S rRNA, nuclear 18S rRNA, and mitochondrial COI), we identified 1,143 genera, dominated by Proteobacteria, Cyanobacteria, Rhodophyta, Dinoflagellata, Mollusca, and Arthropoda. The diversity and species compositions of prokaryotes and eukaryotes varied across algal turf types subjected to different sediment load levels. Prokaryotic and eukaryotic diversity were higher in short algal turfs, whereas cyanobacteria dominated long, sediment-laden algal turfs (up to 62.30% of 16S sequences), revealing a clear shift in the dominance of primary producers from diverse eukaryotic algae to unique photosynthetic cyanobacteria. Sediment-related variables (algal canopy height, sediment mass, and grain size) were critical, explaining 25.42% and 28.00% of the variation in prokaryotes and eukaryotes, respectively─far more than other eco-environmental variables. These findings indicate that sediment load is strongly associated with the community structure of algal turfs and may be linked to reduced diversity of primary producers.