Seasonality and cell-size structure of diatoms in the Tsugaru Strait, northwest Pacific Ocean

Abstract

Diatoms and their consumer copepods obtained monthly through a time series of plankton tows in the Tsugaru Strait were analyzed. Counts of relative species abundance were conducted on diatom and copepod assemblages, and scores of non-metric multidimensional scaling (NMDS) were calculated for the assemblages using a matrix based on these counts. Three diatom assemblages corresponding to the seasonal blooms of Chaetoceros socialis, Chaetoceros debilis and Chaetoceros radicans; one diverse assemblage with common occurrences of Bacteriastrum hyalinum and Chaetoceros lorenzianus; and one assemblage formed mostly of large cell-size diatoms including a giant centric diatom Coscinodiscus wailesii are discerned in the NMDS. Consumers of diatoms, predominantly herbivorous copepods of Pseudocalanus and Clausocalanus species, commonly co-occurred with the last assemblage. Interpretations for the occurrence of this assemblage include a biased increase of C. wailesii in the water column and/or selective grazing by copepods on small cell-size diatoms leading to the assemblage being relatively dominated by large forms.