Weekly Variations of Prokaryotes and Phytoplankton Communities in a Temperate Coastal Area

Abstract

Phytoplankton play an important role in the marine food web and energy transfer by fixing carbon dioxide and producing organic matter in the ocean. Prokaryotes play an important role in the biogeochemical cycle of various elements through the metabolism of organic and inorganic matter as well as in energy transfer in the ocean through the microbial food web. To understand the temporal variation in microbial communities in temperate coastal areas in Korea, prokaryotic and phytoplankton community compositions were examined weekly for one year at a coastal monitoring station located in the southwestern East Sea. Prokaryotic abundance varied 10 times, ranging from 2.0 to 20.1 × 105 cells mL-1, and showed an increasing trend in spring during phytoplankton bloom. The titrated thymidine incorporation (TTI) rate was low in winter, ranging from 0.2 to 11.5 pmol L-1 h-1. However, in summer, the TTI increased, ranging from 8.3 to 17.4 pmol L-1 h-1, and showed a weak correlation with seawater temperature. Alphaproteobacteria was the most dominant class for most of the year, followed by Flavobacteria. Although the seasonal variation in prokaryote composition was not apparent at the class level, several prokaryotic species showed distinct temporal or seasonal variation throughout the year. In this study, seawater temperature and chlorophyll-a, which are well-known environmental factors that regulate prokaryotic growth, showed weak or no correlation with prokaryote biomass and activity, suggesting that the study area might be affected by various sources of organic matter in addition to these environmental factors. Among phytoplankton larger than 3 µm, Bacillariophyta was the most dominant throughout the year, especially in summer. Contrarily, Cryptophyceae was dominant in autumn. Among picophytoplankton less than 3 µm, Mamiellophyceae was the most dominant throughout the year except in summer, during which Primnesiophyceae was dominant. Additionally, Cryptophyceae abundance was significant throughout the year, while Dinophyceae appeared opportunistically among the picophytoplankton community. The weekly monitoring of the coastal area for one year enhanced our understanding of the temporal variations in prokaryote and phytoplankton communities in the study area. A consistent accumulation of data in monitoring sites for a long period would be necessary in understanding the dynamics of prokaryotes and phytoplankton in temperate coastal waters and in predicting the future impacts of climate change.