Temporal and Spatial Variability of Phytoplankton Pigment Concentrations in the East/Japan

摘要

Temporal and spatial variability of phytoplankton pigment concentrations in the East/Japan Sea was described, using monthly mean composite images derived from ocean color data and surface mixed layer ecosystem model. The pigment concentrations in the East/Japan Sea show remarkable seasonal variations. The two blooms of the year appear in spring and fall. The spring bloom in the southern areas (April) occurs one month in advance in comparison with that in the northern areas (May). Fall bloom appears in November and December when mixed layer depth (MLD) becomes equal to critical depth. The fall bloom can be explained by deepening of MLD in the Japan Sea. The pigment concentrations in winter are higher than that in summer. The low pigment concentrations dominate in summer. Seasonal variability of phytoplankton was investigated with surface mixed layer ecosystem model in the East/Japan Sea. The model consisted of four compartments (phytoplankton, zooplankton, nutrient, detritus) forced by mixed layer depths, photosynthetically available radiation and nutrient concentrations. From results we estimated entrainment rate 2.5-4.0 m/day to reproduce the two annual blooms, and reproduced seasonal variation of phytoplankton at southern and northern regions by the difference of surface winter MLD using the entrainment rate value 3.0 m/day. The spring blooms in the southern and northern regions closely related to deepening of a winter surface MLD. In the southern region where MLD was shallow and phytoplankton spring bloom occurs one month in advance to the northern region where MLD was deep. The amount of light increases within the MLD during the onset of stratification and water temperature increases faster in spring in the southern region than the northern region. Decrease of phytoplankton was mainly affected by zooplankton grazing in the southern region and by nutrient exhaustion in the northern region. The fall bloom in the two regions was caused by the nutrient availability and entrainment on the phytoplankton.