Biodiversity exploration in autumn using environmental DNA in the South China sea

The South China Sea (SCS) is an important part of the Indo-Pacific convergence zone, with high biodiversity and abundant marine resources. Traditional methods are primarily used to monitor biodiversity. However, a few studies have used environmental DNA (eDNA) metabarcoding to research the assemblage structure of the SCS. This study used eDNA metabarcoding to survey the SCS assemblage and its relationship with environmental factors over a month-long time-series (August 30th to September 30th, 2020) of seawater samples from the central part of the SCS (9 degrees-20 degrees 86' N, 113 degrees-118 degrees 47' E). 32 stations were divided into six groups (A, B, C, D, E, F) according to longitude. We collected water samples, extracted eDNA, and amplified 185 rRNA gene V4 region (18S V4), 18S rRNA gene V9 region (18S V9), and 125 rRNA gene (12S). Krona diagrams were used to show species composition. We identified 192 phytoplankton, 104 invertebrate, and 61 fish species from 185 V4, 185 V9, and 125, respectively. Generally, the three assemblage structures exhibited an increase in species diversity with increasing longitude. Group E had the highest fish diversity. Groups F and C had the highest phytoplankton and invertebrate diversity, respectively. Canonical correspondence analysis showed that four factors (chlorophyll a, depth, salinity, and temperature) were correlated with assemblage structure. Chlorophyll a was the main environmental factor that affected fish, phytoplankton, and invertebrate assemblage structures; salinity was strongly correlated with fish and invertebrate assemblage structures; temperature was a key factor that impacted fish and invertebrate assemblage structures; and depth was strongly correlated with invertebrate assemblage structure. Our results revealed that eDNA metabarcoding is a powerful tool for improving detection rate and using multiple markers is an effective approach for monitoring biodiversity. This study provided information that can be used to enhance biodiversity protection efforts in the SCS.