Why do phyto- and zooplankton exhibit different patterns of seasonal dynamics in the large Ob river-floodplain system (West Siberia)?

Abstract A floodplain is considered as the basis for high production and biodiversity of large rivers. To test the hypothesis of the determining role of the floodplain as a supplier of plankton to the channel of a large unregulated river, we implemented a three-year study in the Ob river-floodplain (West Siberia, Russia) and revealed that factors of seasonal dynamics and sources of phyto- and zooplankton supply to the channel differed. The riverine phytoplankton was formed mostly in the main river and small side channels. The driving factor of its seasonal dynamics in the main channel and the permanently connected floodplain was water temperature, whereas in mostly isolated floodplain lakes—it was the availability of nutrients. Zooplankton in the channel was largely formed due to “recruits” arrival from the floodplain reaches. Its abundance in the channel with connection to the river-floodplain areas depended on temperature and phytoplankton amount. This study casts a light upon some major factors of seasonal dynamics of phyto- and zooplankton in the river-floodplain systems of large lowland rivers and serves the basis for the development of the flood pulse concept and monitoring program for rivers with a long freeze-up period.