Dissertation Abstract

Rivers and streams throughout the world are being threatened by socioeconomic pressures that alter the land uses and climate, degrading environmental conditions and hence affecting their water quality and hydrology. Approximately 40% of the entire surface of the planet is covered with crops and pastures, and the usage of fertilizers containing phosphorous and nitrogen have increased between two and seven times respectively in the last 40 years. These changes are part of what are known as “global changes”, which definition comprises both climatic changes (such as the altering of precipitation patterns and temperature, variations in atmospheric carbon dioxide, etc.), and land use changes (such as increases in urbanization and fertilizer use, etc.). It is expected that, according to the scenarios for future climate changes, water courses are affected in different degrees by global changes, depending on the area of the planet that is analyzed. In this research the focus was in the development and characteristics of the biofilm that covers fine sediments in the streambed of nutrient-rich rivers and streams. This microcommunity is shaped by many factors, including the concentration of inorganic nutrients, light, temperature, the characteristics of the substrate, the influence of herbivores, the current velocity and the characteristics of the particles that are being transported by the water flow, among others. Therefore, the pressures generated by the global changes might have significant effects on the development of the streams biofilms, which, in turn, alter the characteristics of the entire river ecosystem.


The experiments consisted of an in situ nutrient addition in a stream, and two ex situ experiments in artificial streams in the laboratory. The nutrient addition experiment (phosphorous and nitrogen) was carried out in a nutrient-rich stream reach (labeled Impact), which was compared to a similar upstream reach left unmodified (labeled Control). Both reaches were sampled for 9 months prior to a continuous nutrient addition in the Impact reach, which lasted another 12 months. The two laboratory experiments were carried out employing artificial streams. One experiment was conducted using a biofilm that came from a stream with low anthropogenic impact and the other experiment was conducted using a biofilm that came from a stream with high anthropogenic impact. The laboratory experiments in this thesis manuscript tried to prove that the combined effect of the increase in water velocity, nutrients, turbidity and temperature favored the development and activity of the biological community. The main results evidence that the species composition of the biofilm is altered when exposed simultaneously to these four stressors associated with global changes.


According to the results obtained in these experiments, it is possible to recognize that the communities in nutrient-rich streams show a large resistance to change, that is manifested through gradual responses or small changes evidenced at different scales. These changes seem subtle, but can be bigger if both the time scale and spatial scale are considered. Global changes produce their effects in temporal and spatial scales that are much greater than the ones used in these experiments, but the studies at smaller scales can provide a better understanding of the internal dynamics in biofilms and the effects of the interactions between the variables that alter its development.