Succession of three dominant diatoms, Skeletonema, Thalassiosira, and Pseudo-nitzschia in the inner area of Tokyo Bay from 2003 to 2017

The composition of the phytoplankton community in Tokyo Bay was analyzed using samples collected monthly at 0 and 5 m depths from 2003 to 2017, along with environmental factors. Three diatom genera-Skeletonema, Thalassiosira, and Pseudo-nitzschia-dominated over the study period. The sum of the three genera accounted for 75% of the total phytoplankton cell density. Skeletonema was the most abundant taxon, of which the cell density reached > 10(4) cells mL(-1). A pattern of seasonal change in the community composition was detected, and the varying compositions were grouped into three seasons: a mixing season (November-February), an early stratified season (March-June), and a late stratified season (July-October). The mixing season was characterized by high nutrient content (dissolved inorganic nitrogen > 30 mu M) and low temperature (< 17 degrees C), and the late stratified season was characterized by high temperature (> 22 degrees C) and low salinity (< 28 at the surface). In a modified seasonal Mann-Kendall test, we did not find any increasing or decreasing trend of three diatom genera in the analysis for whole months. However, the test applied to each seasonal group revealed a significant increasing trend of Pseudo-nitzschia abundance in the late stratified season at 0 m depth. In contrast, Skeletonema abundance showed a decreasing trend at a 5 m depth in the same season. The succession of the phytoplankton community was more obvious when the yearly trend was analyzed for each season.

Automated summary

The composition of phytoplankton community in Tokyo Bay was studied from 2003 to 2017. Three diatom genera dominated and accounted for 75% of the total phytoplankton cell density. The community composition showed seasonal changes and could be grouped into three seasons. The abundance of Pseudo-nitzschia increased significantly in the late stratified season at 0 m depth, while Skeletonema abundance decreased at 5 m depth in the same season. The succession of the phytoplankton community was more evident when analyzing the yearly trend for each season.