The present study provides information about the structure plankton in lake Horia, Tulcea County. We analyzed the structure of plankton as a tool for assessing trends in the trophic status of the aquatic ecosystem Horia winter. In November 2016 the structure of phytoplankton were identified five taxonomic groups. Between species who were identified, Cymbella cistula, Monoraphidium contortum and Pediastrum bory are predominant species in these samples. In the structure of the zooplankton were three taxonomic groups. The samples are identified predominant copepods with species Cyclops strenuus and Eucyclops albidus in various stages of development and rotifers with the species Brachionus diversicornis and Brachionus urceolaris. The small number of species evidences a reduced biodiversity of plankton during the studied period.