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Cladophora glomerata is the main macroalgae species responsible for the appearance of large mass compact algae formations, known as mats, in the fresh water of Lake Oporzynskie and the river Nielba, in western Poland. The mats are usually created by single species. In order to achieve seasonal domination, the algae secrete to the environment the allelopathic substances, which are phenolic compounds. In the period of intensive population growth, the thread-like thalli of these algae are richly incrusted with epiphytic diatoms. The amount of the secreted phenolic compounds changes in time within the vegetation period and depends on the weather conditions. The most abundant secretion of phenolic compounds takes place in July when, with increasing concentration of phenolic compounds in water, their content in the cells decreases.The main aim of the study was to establish the effect of the phenolic compounds secreted by C. glomerata thalli on the diatoms growing on them. According to our results, polyphenolic compounds have an impact not only on the abundance of diatom communities, but also on the structure of their communities. With increasing concentration of polyphenols, the abundance of Achnathidium minutissimum, Gomphonema olivaceum, Cocconeis pediculus, Navicula tripunctata decreased, while that of Amphora ovalis, Navicula rostellata, Staurosira construens, Cymbella lange-bartelotti increased. The species Cocconeis placentula reached domination thanks to the favorable influence of the environment and the effect of polyphenols secreted by the macroalgae. The variation in the diatom communities' structure was proved to also depend on the habitat parameters, mainly: EC, Turbidity, N-NH4, N-NO3, TDS.

Nowadays, algae are becoming more and more popular as a food group rich in nutrients, cosmetic raw materials full of antioxidants or valuable dietary supplements. They are of interest for the industry because they are found almost all over the world, in all climatic zones, both in fresh and salt waters. The aim of this study was to take a broad look at green algae (Chlorophyta) and to show how large the variability of the content of active compounds may depend on the species and the place and time of sample collection. Particular attention was paid to compounds with antioxidant activity, whose simplified profiles were created on the basis of complementary, semi-quantitative methods. Additionally, time-yield extraction optimizations were performed. Three different specimens of Ulva lactuca were compared: from the coastal zone of the Baltic Sea, from the open Baltic Sea area around Bornholm and Ulva spiralis (Ulva lactuca polymorph) from the Atlantic Ocean. The studied algae of the Cladophora genera were three different species of freshwater algae from various habitats: a lake (Cladophora glomerata), a river (Cladophora rivularis) and aquarium farming (Cladophora aegagropila, syn. Aegagropila linnaei). The content of antioxidants and the extraction efficiency varied significantly depending on the species.