Changes in the frequency of the occurrence and abundance of ostracods (Ostracoda: Halocyprididae) in the epipelagic zone of the North Pacific

The structure of the epipelagic halocypridid ostracod fauna of the boreal zone of the northwestern Pacific has changed during the last 50–60 years. As a result, the frequency of occurrence and abundance of the traditionally most common cold-water species Discoconchoecia pseudodiscophora (Rudjakov, 1962) significantly decreased, while, in contrast, those of the previously rare and relatively warmth-loving Conchoecia sp. A increased to the level of common species. In 2009 and 2010, the position of the northern boundary of the Subarctic Front in this area in the period of biological spring already corresponded to its position at maximum water warming (biological summer) in the middle of the past century. Based on the ostracod distribution, the northern boundary of the Subarctic Front roughly corresponds to the position of a 10°C surface isotherm, and at the present time in the summer it lies further north than half a century ago.