Long-term changes in summer mesozooplankton communities around Dokdo in East Sea of Korea during 2009-2020

Abstract

The waters around Dokdo island in the East sea have experienced increasing surface temperature over the last decade especially during the summertime. The mesozooplankton community may be used as indicator of climate change, so we evaluated long-term dynamics of mesozooplankton community in relation to physical features around Dokdo during summer of 2009–2020. The mesozooplankton samples were collected in the upper 100 m by vertical hauls using standard type net(mesh size: 200μm). The sea surface temperature(SST) ranged 19.3–28.0°C(mean: 25.1°C) and SST has risen by 0.7°C in 2020 compared to 2009. SST anomalies had two negative(2009–2011 and 2014) and two positive(2012–2013 and 2015-2020) phases. The surface chlorophyll-a(chl-a) concentrations showed slightly increasing trend and highly positive anomaly in 2020. The average abundance of mesozooplankton ranged 2,906–40,558 inds./m³ (mean: 10,129 inds./m³) and showed slightly increasing long-term trend(r²=0.45, p<0.1). The abundance of mesozooplankton anomalies coincided with SST anomalies except for years 2014, 2017-2018, and also coincided with chl-a concentration except for years 2014 and 2015. Cluster analysis was carried out to identify the mesozooplankton groups that contributed and had led significant changes in the long-term trend. Noctiluca scintillans contributed to the increasing trend in 2009, 2012–2015 and 2017, and immature copepods contributed to higher abundance during 2010–2011 and 2018, and abundance of Pyrocystis noctiluca contributed to rapid increase of mesozooplankton abundance in 2020. The correlation and redundancy analysis showed that the relationship between abundance of mesozooplankton and chl-a concentrations was significantly positive(r=0.81, p<0.05) in which P. noctiluca were specifically related to chl-a concentrations(r=0.74, p<0.05) and N. scintillans had positive relationship with surface temperature(r=0.29). We suggest that long-term increase of mesozooplankton, especially N. scintillans and P. noctiluca, are related to the gradual increasing trend of SST and chl-a concentration, respectively around Dokdo, and there may be shifts of zooplankton community in the future.