Differences in subtidal macrobenthic community structures and influencing factors between Jindo and Jeju Islands in South Korea

Although islands in South Korea have been significantly impacted by human activities, marine ecological studies focusing on island coasts have been limited. Due to their distinct characteristics, Macrobenthos are extensively utilized to assess the impact of anthropogenic influences on the marine environment. In August 2010, August 2011, and September 2012, samples of macrobenthic communities, bottom water, and sediment were collected from the subtidal zones around southern Jindo and northern Jeju Islands. Macrobenthos were identified to the species level using a stereomicroscope. Bottom seawater quality was evaluated, with a focus on dissolved heavy metal concentrations (As, Cr, Cd, Cu, Pb, and Zn). Additionally, we measured the organic matter content and mean grain size of the sediment. There were marked differences in macrobenthic community structures between the two islands, including the number of species, species abundance, species richness index, and Pielou’s evenness index (p<0.05). Cluster analysis and Non-metric analysis revealed variations in macrobenthic communities between the two islands and over the years 2010, 2011, and 2012. According to the Biota-Environment Matching (BIO-ENV), distance-based redundancy analysis (dbRDA), and Distance-based Linear Model analyses, the principal environmental variables influencing the distribution of macrobenthic communities are Cd and As. These variations likely result from different levels of human activity on each island. Moreover, interannual variations in macrobenthic communities, especially in 2012, were predominantly influenced by Pb and Cr, likely due to alterations in the influence of the Changjiang diluted water.

Methods
Sampling Collection and Processing

Sample collection occurred in August 2010, August 2011, and September 2012, establishing fourteen sampling sites in Jindo (S1-S7) and Jeju Islands (S8-S14) for each collection period. At each site, sediment and macrobenthic samples were gathered using a 0.1 m² Van Veen grab; the former covering an area of 0.1 m² and the latter 0.2 m². Additionally, bottom water samples were collected using a Rosette sampler, and parameters such as temperature, salinity, pH, and dissolved oxygen (DO) were measured using a YSI 6920 multiparameter sonde (YSI Inc., Yellow Springs, OH, USA). Each site's macrobenthic samples were sieved through a 0.5 mm mesh and preserved in 10% neutral formalin solution, while sediment samples were stored in a deep freezer for subsequent analysis. A 2 L sample of bottom water was stored directly in the freezer, and an additional 2 L sample was filtered using glass-fiber filter papers. Both the filtered water and the filter paper were kept in the deep freezer for future analysis.

 In the laboratory, macrobenthic samples were identified to the species level and quantified using a dissecting microscope (SMZ-168, Motic Ltd., China). Dissolved heavy metals (As, Cr, Cd, Cu, Pb, and Zn) in the bottom water were measured by placing 400 ml of seawater into a 500 mL Teflon separatory funnel, adjusting the pH to approximately 4.5 using an ammonium acetate buffer. A 1 mL mixture of organic chelating agents (APDC/DDDC) was then added and agitated for 10 seconds. Subsequently, 10 mL of chloroform was incorporated and stirred at 300 rpm for 5 minutes. After resting for 10 minutes, the organic solvent layer was separated and collected in a Teflon beaker. The solvent was evaporated by heating at 80°C until dry, and the residue was decomposed with concentrated nitric acid. Finally, 1-2 mL of diluted nitric acid was added, and the heavy metal content was analyzed using Inductively Coupled Plasma Mass Spectrometry (Agilent 7700, Agilent Inc, USA). Suspended solids were quantified by filtering and drying 200 mL of bottom seawater with glass fiber filter paper. Total Nitrogen (TN) and Total Phosphorus (TP) levels in bottom seawater were determined using colorimetric and spectrophotometric methods with an automated analyzer (QuAAtro SFA, Seal Analytical Ltd., UK). A 50 g sediment sample was dried at 80°C for 48 hours and subsequently heated in a muffle furnace at 550°C for 4 hours to measure ignition loss (IL). The concentrations of acid volatile sulfide (AVS) and chemical oxygen demand (COD) in the sediment were measured using COD titration and AVS detection tube methods. The wet sieving method was applied to analyze the mean grain size of the sediment. All analyses of seawater and sediment samples complied with the Notification of Marine Environmental Process Test Standards (National Institute of Fisheries Science, 2010), and each test was performed in triplicate to ensure accuracy.