Seasonal and interannual variability of surface chlorophyll a concentration in the Ulleung Basin, East Sea

Abstract

We investigated the seasonal and interannual variability of surface chlorophyll a concentration responding to the changes in the mixed layer depth in the Ulleung Basin for the years 2001-2012. To address the role of the changes in mixed layer depth, we used merged satellite data from SeaWiFS/MODIS-Aqua and a zero-dimensional European Regional Seas Ecosystem Model (ERSEM). The peak magnitude of chlorophyll a concentration shows a large interannual variability and a positive relationship with maximum mixed layer depth except for 2008, whereas the annual mean of chlorophyll a concentration does not show any significant relationship with maximum mixed layer depth. Thus, according to the decreasing trend of maximum mixed layer depth, the peak magnitude of chlorophyll a concentration shows a decreasing trend in this period. We compared between 2006, the year of deepest mixed layer depth, and 2010, the year of shallowest mixed layer depth, to investigate the changes in phytoplankton in more detail. The model result shows that the deep winter mixing increased the nutrient supply to the upper layer and it led to a better growth of diatoms. On the other hands, the shallow winter mixing advanced the initiation of blooms of diatoms because of enhanced light availability in winter. But the poor nutrient supply reduced the growth of diatoms in spring. Eventually, the decrease in diatoms caused the increase in other groups. Therefoer depth, we used merged satellite data from SeaWiFS/MODIS-Aqua and a zero-dimensional European Regional Seas Ecosystem Model (ERSEM). The peak magnitude of chlorophyll a concentration shows a large interannual variability and a positive relationship with maximum mixed layer depth except for 2008, whereas the annual mean of chlorophyll a concentration does not show any significant relationship with maximum mixed layer depth. Thus, according to the decreasing trend of maximum mixed layer depth, the peak magnitude of chlorophyll a concentration shows a decreasing trend in this period. We compared between 2006, the year of deepest mixed layer depth, and 2010, the year of shallowest mixed layer depth, to investigate the changes in phytoplankton in more detail. The model result shows that the deep winter mixing increased the nutrient supply to the upper layer and it led to a better growth of diatoms. On the other hands, the shallow winter mixing advanced the initiation of blooms of diatoms because of enhanced light availability in winter. But the poor nutrient supply reduced the growth of diatoms in spring. Eventually, the decrease in diatoms caused the increase in other groups. Therefo