Introduction and objectives Diatoms are the most abundant group within benthic microalgae that form biofilms in aquatic systems. These communities play a key role in the functioning of shallow coastal systems, such as lagoons and estuaries, at the primary production level and in biogeochemical cycles, and are considered the basis of the food chain. They also constitute one of the first compartments that react to any environmental variation or type of impact. In the marine environment, information on distribution, ecology and taxonomy, and the use of benthic diatoms as biological indicators of environmental changes, are scarce. The present assay was carried out in the coastal Mar Menor lagoon (SE Spain), where previous floristic information practically cannot be found. The lagoon is subject to multiple anthropogenic impacts, including contributions of metals and nutrients loads. Populations may be strongly affected by the increase in temperature associated with global climate change. Therefore, it is an ideal study area to investigate the relationships of species and communities with environmental gradients, and it may help to understand the biotic and abiotic factors that structure benthic diatom communities. The general objective of this thesis is to improve knowledge about the ecology and taxonomy of the benthic diatom communities in the coastal Mar Menor lagoon and to evaluate their potential as biological indicators of the multiple stressors that affect and threaten the lagoon. Any work conducted on bioindicators is based on good floristic knowledge, but this information is lacking in Mar Menor. For this reason, the following specific objectives were identified: i) to morphologically, ecologically and phylogenetically study the genus Hyalosynedra; ii) investigate the composition and abundance of the species of the genus Licmophora in the lagoon, and its phylogenetic relationships; iii) evaluate the combined effect of temperature and the stoichiometric N:P ratio of water on populations of the stalked marine diatom Licmophora colosalis, and to test whether overproduction of stalks could serve as an indicator of nutrient contamination; iv) study the composition, structure and biological traits of the species of two benthic diatom communities influenced differently by the chronic contamination of metals and nutrients, and investigate their responses to the sporadic exposure of both factors. These objectives aim to evaluate the suitability of these communities as a tool to environmentally monitor the lagoon. Methodology Floristic works include morphological descriptions obtained by optical microscopy and scanning electron microscopy, and by molecular analyses using cell cultures, extraction of genetic material, sequencing and phylogenetic analyses of sequences. Laboratory bioassays consist of applying different temperatures and nutrient treatments to evaluate the effect on population dynamics (growth measures, cell size, mortality), stalks production (by image analyses) and carbohydrates (chemical analyses) of Licmophroa colosalis. The field experiment involves applying different treatments of metals and nutrients individually and combined.
