Chaetognath species-specific responses to climate regime shifts in the Tsushima Warm Current of the Japan Sea

Abstract

Expanding the work in our previous paper (Nagai et al. 2006), which showed the close linkage of chaetog- nath species to changes in water temperature and reported their occurrence characteristics, chaetognath species-specific responses to climate forcing via water temperature fluctuations were ascertained by studying inter-annual linkages among climate-ocean-ecosystem elements. Using samples collected between 1972 and 2002 in the Japan Sea, an attempt was made to understand how the three dominant chaetognaths—Sagitta minima, S. nagae and S. enflata—re- spond to inter-annual variations of atmospheric signals and oceanic effects in the Tsushima Warm Current around cli- mate regime shift years. Among ocean-atmosphere parameters, the winter monsoon was the most crucial for driving the ecosystem of the Japan Sea. No significant correlation with any of the other parameters was found with the southern os- cillation index, but the winter monsoon index had a correlation with water temperature. The climate regime shift during 1976/77 occurred with anomalously cold water induced by a strong winter monsoon. The next climate regime shift (1988/89) appeared with a warm water anomaly due to retention of warm water formed in winter by a weak winter monsoon and increased transport volume in the Tsushima Warm Current water. Chaetognath abundance and species numbers decreased during the colder regime and increased during the warmer regime. Responding to temperature in a species-specific manner, chaetognaths varied notably in abundance. Consequently, changes in species numbers and switching of dominant species occurred. Thus, chaetognath species could be a significant indicator of climate events that influence the ecosystem in the Japan Sea.