Carbohydrate-dominant Phytoplankton and Protein-high Zooplankton in the Northern Part of the Southwestern East/Japan Sea in 2015

Abstract

The biochemical compositions of phytoplankton and zooplankton were monthly investigated in the northern part of the southwestern East/Japan Sea from May to October in 2015 except August. Among the biochemical compositions of phytoplankton, carbohydrate composition (55.5 +/- 7.3%) was most dominant during the study period. A significant negative relationship (r = -0.855, p < 0.01, n = 39) was found between the carbohydrate and lipid compositions of phytoplankton in this study, which can be due to a switch from carbohydrate into lipid synthesis under persistent stress conditions. In comparison, proteins were the predominant components (56.1 +/- 5.6%) of the zooplankton communities. These high protein contents could be resulted from dominant crustacean zooplankton because copepods have generally high protein contents. Indeed, a positive correlation was found between community composition of copepods and protein composition of zooplankton communities (r = 0.618, p < 0.05, n = 13). In contrast, a lipid content of zooplankton in the northern region (7.3 +/- 1.6% DW) was significantly (t-test, p < 0.05) higher than that reported from the southern region (5.5 +/- 0.6% DW). This might be due to a higher contribution of cold water species which have generally high lipid contents or/and a low amount of prey in this region. In conclusion, a high protein and relatively lower lipid contents of copepods-dominant zooplankton communities appeared to be regional characteristics in the southwestern East/Japan Sea, which could be a good food source for fish larvae and small fish which have generally high protein requirements.