Mass Occurrence of Pacific Copepods in the Southern Chukchi Sea During Summer: Implications of the High-Temperature Bering Summer Water

Abstract

The Bering Strait is the only gateway to the Chukchi Sea from the Pacific Ocean and is a major route of Pacific water inflow. We recently investigated the occurrence of Pacific copepod species along with the warming of the Chukchi Sea and sought to identify the cause of the mass occurrence these copepods through an analysis of the water masses flowing through the Bering Strait. Zooplankton and Conductivity-Temperature-Depth (CTD) data collection was conducted in the Chukchi Sea and Bering Strait from 2014 to 2016. In addition, mooring systems installed in the Bering Strait were analyzed to obtain water temperature and salinity data during summer to understand the properties of the water masses. In 2015, a high abundance of Pacific copepod species (Eucalanus bungii, Metridia pacifica, and Neocalanus spp.) was observed in Bering Summer Water (BSW), which was relatively warm compared to measurements obtained from 2014 to 2016. As further confirmation, our results were consistent with 2007, 2009, and 2012 data, which showed that the abundance of Pacific copepod species was proportional to the temperature of the BSW entering the Chukchi Sea. In conclusion, we reconfirmed that Pacific copepod species are entering the Chukchi Sea along with BSW, and we newly discovered that their high abundance coincided with the relatively warm BSW, instead of other water masses. These findings suggest that the inflow of the high-temperature BSW (>3 ？C) plays an important role in the mass occurrence of Pacific copepod species in the southern Chukchi Sea.