A study of changes between years in the structure of plankton community in a newly-flooded reservoir

Abstract

Temporal changes in the plankton community and trophic relations between the metazooplankton and its potential prey (bacteria, flagellates, ciliates and algae) were studied following the flooding of a new reservoir (Sep, France). The high dissolved organic matter (DOM) concentrations favoured the development of heterotrophic and mixotrophic organisms, which dominated the unicellular plankton after flooding. The proportion of strictly autotrophic organisms increased with time, reaching 67 % of the biomass of protistan plankton at the end of the study. A small-sized chrysomonade (2-5 μm) accounted for 53 % of the abundance of the heterotrophic nanoflagellates (HNF) throughout the study. The Chrysophyceae, Chrysidalis sp. accounted for 76 % of the mean relative abundance of all pigmented flagellates combined. Some mixotrophic species such as Dinobryon cylindricum and Cryptomonas ovata mainly occurred at the start of the study. The prostomatid ciliate Urotricha furcata, which dominated in terms of numbers immediately after flooding, was later replaced by mixotrophic species, an oligotrichid Pelagohalteria viridis then a peritrichid, Vorticella sp. The rotifers Polyarthra sp., Synchaeta sp. and Hexarthra mira were the first metazooplankton organisms to colonise the pelagic environment. They were replaced in the 3 rd year by Cladocera (Daphnia longispina and Ceriodaphnia quadrangula) and Copepoda, firstly calanoids (Eudiaptomus gracilis) and then cyclopoids (Cyclops vicinus and Acanthocyclops robustus). This study emphasizes the importance of the organisms of the microbial food web in the nutrition of the metazooplankton in the period following the flooding of a newly impounded reservoir.