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.

In polar ecology, zooplankton diets and survival rates vary according to the seasonality of solar radiation and oceanographic conditions. Each zooplankton species has evolved feeding strategies to survive in the diet-limited conditions of the "polar night." Many zooplankton studies have reported seasonal adaptations in feeding activity during polar night based on their trophic niches. Nitrogen isotope analysis of amino acids has provided improved accuracy in estimates of trophic position (TP) in various marine species. In this study, field work was conducted in Kongsfjorden before (October 2017) and after polar night (April 2018). As representative zooplankton, an amphipod (Themisto abyssorum), euphausids (Meganycitiphanes norvegica and Thysanoessa sp.), a chaetognath (Parasagitta elegans), and copepods (Calanus spp. and Oithona similis) were collected. trophic position values of each taxon were estimated using the nitrogen isotope ratio of glutamic acid (δ 15 N Glu ) and phenylalanine (δ 15 N Phe ). Results showed that TP values of P. elegans were relatively constant, averaging 3.2 in both seasons, likely due to continuous feeding activity during polar night. Trophic position values were also constant for Calanus spp., ranging 2.5-3.0 in both seasons, due to their ability to utilize stored high-energy wax. In contrast, average TP values for O. similis, an omnivorous zooplankton, were 2.9 in October and 2.3 the following April. Trophic position values for O. similis before polar night can be attributed to the relatively high availability of algae during longer periods of daylight. We found that TP variation in zooplankton before and after polar night differed according to feeding activities in diet-restricted circumstances.

The Pacific Arctic Ocean receives relatively warm and nutrient-rich Pacific summer water inflow via the Bering Strait and the Chukchi Sea. The East Siberian Sea, which is productive and shallow, is situated in the western part of the Pacific Arctic Ocean (Semiletov et al., 2005). In the eastern region, the northern Chukchi Sea, which is poor surface nutrient and have deep basin, is partially located at the surrounding area of the anticyclonic Beaufort Gyre (Coupel et al., 2015). The Pacific Arctic Ocean exhibits rapid responses to environmental change. Strong stratification is caused by incoming sea ice meltwater in summer (McLaughlin et al., 2011). The nutricline and surface chlorophyll maximum (SCM) depth can be deepened by stratification, decreasing primary productivity due to limited nutrient supplies (Coupel et al., 2015; Zhuang et al., 2018). The zooplankton community is tightly tied to the hydrodynamic condition and depends on the composition of their diets. In low-nutrient conditions and a stable water column, smaller phytoplankton such as pico-and nanoplankton are usually dominant (Coupel et al., 2012; He et al., 2012; Li et al., 2009). Whereas, diatom was dominant under high sea ice concentration in nutrient-rich and vertically mixed water column (Lee et al., 2019). The composition of primary producers, the cell sizes of which vary widely,

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