Small phytoplankton contribution to the total primary production in the highly productive Ulleung Basin in the East/Japan Sea

The Ulleung Basin in the southwestern East/Japan Sea (hereafter East Sea) is known as a biologically productive “hot spot” but climate-associated changes in the physicochemical oceanographic conditions and some biological changes have been reported. In this study, our main objective was to determine the contribution of small phytoplankton to the total primary production, which is valuable information for detecting marine ecosystem changes in the Ulleung Basin. The small phytoplankton productivity contributions determined by Moderate-Resolution Imaging Spectroradiometer (MODIS)-derived monthly productivities using a phytoplankton community-based productivity algorithm was significantly consistent with the field-measured productivity contributions of small phytoplankton in this study. The daily primary productivity of small phytoplankton ranged from 42.7 to 418.7 mg C m−2 d−1 with an average of 172.9 mg C m−2 d−1 (S.D. = ±61.4 mg C m−2 d−1, n = 120), and the annual contribution of small phytoplankton ranged from 19.6% to 28.4% with an average of 23.6% (S.D. = ±8.1%) in the Ulleung Basin from 2003 to 2012. Overall, large phytoplankton were a major contributor to the total primary production in the Ulleung Basin (76.4 ± 8.2%) from 2003 to 2012, which indicates that the Ulleung Basin is a highly productive region. A significantly negative relationship (p < 0.05) was found between the small phytoplankton primary productivity contribution and the annual primary production in this study. This finding revealed that the recent decreasing annual primary production in the Ulleung Basin could be a consequence of the increasing contribution of small phytoplankton. The response of phytoplankton to ongoing climate change depending on different-size phytoplankton compositions should be a subject for further investigation in the Ulleung Basin as a biologically highly productive region in the East Sea.