The study looked at diatoms in two rivers in Spain affected by human activities. The diatom communities were similar in the headwaters of both rivers, but pollution changed the species present in the middle and lower reaches. In the Ter River, pollution led to the appearance of more tolerant species like Nitzschia palea. In the Llobregat River, pollution from salt mines changed the diatom community, with species like Nitzschia frustulum becoming more abundant. Reservoirs in the Ter River improved water quality, leading to a dominant community of Fragilaria downstream.