Potential inhibition of the harmful dinoflagellate Cochlodinium (= Margalefidinium) polykrikoides by the intrusion of Changjiang diluted water into Korea coastal waters

Abstract

During the summer monsoon, a large amount of the Changjiang River discharge (CRD) extends into Korean coastal waters (KCWs) in the East China Sea, forming Changjiang diluted water (CDW). The harmful dinoflagellate Cochlodinium (=Margalefidinium)polykrikoides frequently causes massive blooms during the monsoon period, and these result in extensive economic losses in southern KCWs. We hypothesized that the scale of C. polykrikoides blooms can be varied depending on the intrusion strength of CDW into KCWs. We analyzed long-term (20 years) data (physicochemical factors and C. polykrikoides bloom area and duration) to test this hypothesis. In confirming the variability in annual salinity, the average Niño 3.4 index in the preceding winter was found to be positively correlated with the CRD in the following summer (R2 = 0.203, p < 0.05). However, the surface salinity in our study area was not significantly correlated with the CRD (p > 0.05), indicating that not all of the CDW volume enters the southern KCWs. This incomplete transfer of CDW into KCWs was related to the path of typhoons in the East Asian region; typhoons are able to moderate the influence of CDW through strong water mixing between the upper and bottom layers. We found that the scale of C. polykrikoides blooms in KCWs was negatively correlated with the strength of the CDW intrusion. Together with previous and present findings, the influx of low salinity CDW can accelerate a rise in surface temperature, and this elevated temperature condition which is unfavorable for C. polykrikoides growth may lead to reductions in the scale of this dinoflagellate blooms. Our findings suggest that intrusion strength of CDW into KCWs plays important role in regulating C. polykrikoides population dynamics by causing inhibitory conditions.