###### Abstract  
In this manuscript, we present the results of ecotoxicological tests conducted on the Baltic microhydrobethonos community consisting largely of cosmopolitan species showing the responses at the community (taxonomic composition and structure), population (abundance), and cellular (chloroplast shape) levels. The tests were carried out for three chemicals with different modes of action on photosynthetic organisms, i.e., copper (II) chloride (at concentrations of 2:10\({}^{-5}\) g.dm\({}^{3}\) and 2:10\({}^{-3}\) g.dm\({}^{-3}\)), glyphosate in the form of Roundup(r) (concentrations ranging from 4.2\(\cdot\)10\({}^{-2}\) to 8.5 g.dm\({}^{-3}\)), and the ionic liquid [BMIJM]Cl (at concentrations of 1.13\(\cdot\)10\({}^{-3}\) g.dm\({}^{-3}\) and 1.75\(\cdot\)10\({}^{-2}\) g.dm\({}^{-3}\)). The results of the study indicate that the responses of microphyotobenthos at each level analysed are highly variable and difficult to predict a priori based on experiments performed on single strains. In addition, it was shown that microbytobethnic communities are relatively resistant to the substances tested, which is due to taxonomic richness and the resulting potential for substitution of sensitive taxa by more resistant taxa. The results obtained in the tests also indicate that the number of cells of individual taxa can remain at a similar level or increase over short periods of time despite the deformation of chloroplasts in a numerous group of cells. ccotoxicity; toxicity tests; microbytobethnic communities; diatoms; cyanobacteria; copper chloride; glyphosate; ionic liquid +
Footnote †: journal: Applied Sciences  
whole communities makes it possible to understand the response of organisms in the context of actually existing interpopulation interactions, e.g., competition (antagonistic interaction) and symbiosis (non-antagonistic interaction). However, whole-community tests are still rare due to various challenges, including the lack of a recommended consistent methodology, the acquisition of a sizeable data set for a high number of taxa representing a variety of taxonomic groups with different life strategies, and the associated problems in interpreting the results. The microhypotbenthos, which is the main element studied in this work, is ecologically very important for aquatic ecosystems, including those of coastal zones, estuaries, or shallow seas, as it is an important primary producer associated with the bottom. Organisms forming microhypotbenthic communities, due to their short life cycles, are considered good indicators of environmental change [10], and their analysis provides important information on the state of the marine environment [11]. It is worth emphasizing that the tested Baltic microhypotbenthic community includes many cosmopolitan species, which makes it possible to assume that similar reactions to selected chemical substances will be observed in communities occurring in brackish waters in other regions of the world. The study aimed to test the effects of three compounds from different chemical groups with different levels of toxicity on natural microhypotbenthic communities. The first substance used in the tests was copper in the form of copper (II) chloride. European refined copper consumption reached around 3.78 million metric tonnes in 2021, but consumption in the region has been gradually declining over the past decade [12]. Copper (II) chloride is produced on an industrial scale by the chlorination of copper. The use of copper chloride is very wide. CuCl\({}_{2}\) is used as a wood preservative, fungicide, insecticide, and herbicide. Due to its oxidising properties, it is used as a purifying agent, e.g., in water treatment, and as a deodorising agent to prevent odour in the petroleum industry. Copper (II) chloride is an important element in the production of vinyl chloride and dichloroethane and has applications in the synthesis of organic and inorganic compounds as a catalyst. It is also used in pyrotechnics as a blue-green colouring agent [13].