Seasonal variation in abundance and species composition of the Parmales community in the Oyashio region, western North Pacific

Abstract

Seasonal variation in abundance and species composition of the Parmales community (siliceous pico-eukaryotic marine phytoplankton) was investigated off the south coast of Hokkaido, Japan, in the western North Pacific. Growth rates under various temperatures (0 to 20°C) were also measured using 3 Parmales culture strains, Triparma laevis f. inornata, Triparma laevis f. longispina and Triparma strigata. Distribution of Parmales abundance was coupled with the occurrence of Oyashio water, which originates from the cold Oyashio Current. In March and May, the water temperature was usually low (<10°C) and the water column was vertically mixed. Parmales was often abundant (>1 × 10 2 cells ml -1) and evenly distributed from 0 down to 100 m. In contrast, when water stratification was well developed in July and October, Parmales was almost absent above the pycnocline at >15°C, but had an abundance of >1 × 10 2 cells ml -1 in the sub - surface layer of 30 to 50 m at <10°C. The seasonal variations in the vertical distributions of the 3 dominant species (Triparma laevis, Triparma strigata and Tetraparma pelagica) were similar to each other. Growth experiments revealed that Triparma laevis f. inornata and Triparma strigata, and Triparma laevis f. longispina were able to grow at 0 to 10°C and 5 to 10°C, respectively, but not at over 15°C, indicating that their vertical distribution was mainly determined by temperature. The Parmales community grew in the surface layer before becoming evenly distributed with depth in winter and spring. During summer and into autumn, the population survived under the pycnocline.