Spring Phytoplankton Bloom Variability and Implications for Carbon Flux in the East/Japan Sea

Abstract

Spring phytoplankton bloom is the most distinct feature of annual phytoplankton productivity in the pelagic ecosystem. This phytoplankton bloom affects species compositions, abundance and diversity of marine organism. Furthermore, the export of primary production during the spring bloom period to the deep ocean has a significant influence on the oceanic carbon cycles. A better understanding of spring bloom dynamics and changes in carbon export during the bloom, therefore, should provide an insight on time variability of the oceanic carbon cycles. This study estimated indices of the spring bloom such as initiation timings, peak timings, peak magnitude, and period of the bloom, in the entire East/Japan Sea (East Sea) from 1998 to 2007 using remote sensing ocean color data. By combining satellite, in situ and model data a comprehensive picture of the impact of interannual variability of meteorological factors as internal forces and Asian dust as an external force on the spring bloom has been also constructed. Furthermore, a new approach was introduced to estimate carbon export flux in the East Sea during the spring bloom. The estimated indices of the spring bloom in Ulleung Basin and Yamato Basin are characterized by early initiation, gradual development and long duration of the spring bloom. The northern Japan Basin is characterized by late initiation, rapid increase and short duration of the bloom. The spring bloom is initiated in each basin as the surface mixed layer becomes shallow after reaching to its maximum in winter, in accordance with Sverdrups critical depth hypothesis. In terms of Sverdrups critical depth hypothesis, the mixed layer depth at the initiation of the bloom corresponds to the critical depth, where integrated water column community photosynthesis and respiration are equal. The critical depth was about 68.5 m (± 17.2) in the Ulleung and Yamato Basin and 25.7 m (± 7.4) in the eastern Japan Basin. The shallow critical depth in the Japan Basin indicates lower photosynthetic rate and/or higher grazing pressure in the region. In the eastern Japan Basin, zooplankton biomass reached to its peak a month ahead of the peak timing of the spring bloom. Therefore, these aspects suggest a possibility of high grazing pressure on the bloom development in the Japan Basin. It seems that the initiation of the bloom is mainly controlled not only by light availability in the mixed layer, but also by other factors such as zooplankton grazing. The initiation timings of the spring bloom showed large spatial and interannual variations. In the Yamato Basin and eastern Japan Basin, the initiation timings were positively correlated with wind speed prior to the development of the bloom, indicating the stronger wind and the later initiation of the bloom. However, in the Ulleung Basin and western Japan Basin, the correlations were weak, suggesting importance of other factors on the interannual variability. Only in the Japan Basin, temperature was positively correlated with the initiation timings of the bloom. The positive correlation with temperature may support the possibility of importance of zooplankton grazing on the bloom development, i.e. the higher temperature, higher grazing activity, and later initiation of the bloom. The net heat flux did not show any clear correlation with the initiation timings. The peak magnitude and duration of the bloom also showed large spatial and interannual variations. The peak magnitude and duration of the spring bloom in the entire East Sea were not clearly correlated with wind speed, net heat flux and SST. These unclear correlations suggest that multiple factors affect complexly phytoplankton growth and losses during the bloom periods. Effects of Asian dust on interannual variations in the spring were investigated. The data for the TOMS aerosol index and SeaWiFS chlorophyll-a clearly indicate that an early spring bloom in the northern East Sea can be initiated during the Asian dust events in association with precipitation. Spring bloom is normally initiated in this area as the surface mixed layer becomes shallower than critical depth, in accordance with Sverdrups critical depth hypothesis. However, after the passage of Asian dust accompanied by precipitation, spring bloom was initiated about one month earlier than the bloom during non-dust event years. Deepening of critical depth coincided with the early spring bloom observed during wet-dust events. This early initiation of spring bloom indicates that the supply of bio-available nutrients such as iron through wet deposition induces deepening of the critical depth, which results in such an early initiation of the bloom in the area. These aspects strongly suggest that atmospheric dust input should be considered as an important force for the spring bloom dynamics in the East Sea. The estimated initiation timings of the spring bloom in the East Sea were used to resolve Sverdrups critical depth model. The ratio of community production (P) to total community respiration (L) was derived from the critical depth model as a new proxy to estimate the carbon export ratio during the spring bloom period. The estimated export ratios in the Ulleung and Yamato Basin were more than 2 times higher than the ratios in the Japan Basin. This distinct regional pattern is consistent with the regional differences in the bloom magnitude. The export ratios of about 0.56 in the Ulleung Basin, were comparable to the export ratios derived from the 234Th method, and indicate that more than half of the total primary production in spring sinks to the deep water in the region.