A mixing analysis of surface water in the Oyashio region: Its implications and application to variations of the spring bloom

We analyzed the near-daily hydrographic data collected from ships at a station southeast of Hokkaido Japan during April 2007. We calculated mixing ratios of Coastal Oyashio Water (COW), Oyashio Water (OYW), and modified Kuroshio Water (MKW). The COW mixing ratio tended to be high in depths shallower than 30 m during April, while the OYW ratio was high at the deeper depths. Diapycnal mixing due to the strong wind was suggested between these layers. It is suggested that stratification of low-salinity COW initiates the spring bloom in early-April, but that it is not important for temporal changes of chlorophyll at each depth during and after the bloom. Despite the intense water exchange at the station, the COW ratio correlated significantly with chlorophyll for the overall data set during April and for the data in 2-4-day blocks. Silicic acid, nitrate, and phosphate were also significantly correlated with the COW ratio for each 2-4-day block. This implies that each of these variables changed simultaneously at a constant COW ratio within the temporal and spatial scales of the phytoplankton bloom, were it not for the occurrence of vigorous vertical mixing and large-scale advective events. Using the linear regression equations for each 2-4-day block, we calculated time-series of chlorophyll and nutrients at the mean COW ratio (46%). Their rates of change in these series reveal the spring bloom dynamics in 2007 as follows: The spring bloom ended due to silicate depletion and grazing by large zooplankton in mid-April. In late-April, the nutrients increased, but chlorophyll decreased, indicating little nutrient utilization.