The biogeochemical consequences of changing ventilation in the Japan/East Sea

Transient tracer observations in the Japan/East Sea are used in combination with a minimum complexity water mass model to successfully constrain the timing, nature and magnitude of recent changes in its ventilation. The data and model indicate that there has been a nearly 10-fold decrease in deep water formation rates since during the early 1960s, marked by a transition from sea ice/brine-rejection based deep water mass formation to a shallower open ocean convective renewal. The ventilation rates thus computed allow the calculation of the regional scale flux of phosphate from the abyss to the euphotic zone, and hence basin-scale new production. In addition, water column remineralization rates for phosphate and oxygen utilization rates show expected and consistent patterns as a function of depth. The reduction in abyssal ventilation of the Japan/East Sea (JES) during the latter half of the 20th century led to a 2-fold decrease in basin-scale new production. This decrease has significant implications for the strength and resilience of the marine food web in a strategically important marginal sea. The change appears consistent with trends in zooplankton biomass estimates in the western JES, and possibly with changes in the character of fish-catches. Recent observations made by others indicate a significant resurgence of brine-rejection related deep water formation during the winter of 2001, but it remains to be seen whether this will be sustained in the face of global warming trends.