Global changes in headwater streamseffects of hydrological and nutrient fluctuations on the quality of basal resources = Canvis globals en rius de capçalera: efectes de les fluctuacions hidrològiques i dels nutrients en la qualitat dels recursos basals

Résumé

Streams are complex and highly diverse ecosystems. Within the stream network, headwater streams (1st to 3rd order) are highly important. Headwaters represent most of the total length, influence the structure and functioning of higher order streams, generate most of the water flow and support higher rates of biogeochemical processing, with a higher rate of nutrient removal. Moreover, they offer high habitat diversity, both within and among streams. From all the drainage basin, it is the riparian zone which has the greatest influence in the structure and functioning of these streams. The in-fall of vegetation and leaching from plant litter are major sources of energy for stream food webs. All energy available to consumers originates from primary producers from either out- or in-stream and food webs are fuelled by a complex mixture of allochthonous and autochthonous resources. In headwaters, allochthonous basal resources are generally larger than autochthonous energy sources. However, both the quantity and quality of allochthonous and autochthonous resources influence the stream structure and functioning. Quality of resources determines their potential nutritional value for the invertebrates feeding on them. The nutritional quality of organic matter (OM) can be assessed by measuring the elemental or biomolecule composition (polysaccharide, protein and lipid content). Lipids are the most efficient energy-storing compounds, and within lipids, fatty acids and sterols include essential molecules for consumers. In a context of global change, streams are highly affected by multiple direct and indirect anthropic impacts, such as climate change, land use changes and water course alterations, affecting hydrology and nutrient and light availability, which in turn affect the quality and quantity of basal resources. The aim of this thesis was to determine how environmental factors influence the quantity and quality (biochemical composition) of basal resources in headwater streams. Of the multiple environmental factors affected by global change, the present study focused on changes in hydrology and nutrient and light availability, with particular reference to headwater Mediterranean streams. This study has been mainly performed in Fuirosos, a Mediterranean stream in the northeast of the Iberian Peninsula, but also in the Mayfly creek artificial channels, in the Pacific Coast Mountains of Canada. Several methods were used, such as the measure of particulate and dissolved OM fluxes, elemental composition of OM, nutrient addition; chlorophyll content, bacterial density and extracellular enzymatic activities of biofilms, and the measurement of the biochemical quality of the basal resources. (i.e., epilithic and epipsammic biofilms, leaves and, transported particulate and dissolved OM). We have demonstrated that the El Niño Southern Oscillation (ENSO) affected riparian inputs to the stream through teleconnections that alter average precipitation (during El Niño periods, average annual precipitation was lower than in other periods, causing higher riparian inputs, due to the hydric stress of riparian trees while, during La Niña periods, precipitation was higher than at other times, leading to decreased riparian inputs). Drought periods caused a bimodal distribution of annual riparian inputs and reduced the quality of basal resources, whereas accumulated drought events diminished the riparian inputs. Floods decreased the benthic OM, increased the amount of transported OM and modified its quality. The addition of nutrients led to a decrease in carbon content, and higher light availability led to a decrease in protein content of allochthonous resources. Biofilm quality was generally higher with the addition of nutrients when light availability was higher. We consider that the nutritional quality of basal resources is a complex parameter. Whilst elemental and biochemical composition can sometimes be related, knowledge on the latter enables a better understanding of the mechanism by which abiotic changes affect the quality of resources. However, the complexity of measuring elemental composition, biochemical composition and fatty acid composition gradually increases. Hence, we suggest that the choice of the indicators of quality used would depend on the goal of the study that want to be performed and on the trade-off between analysis effort and information that wants to be obtained. Because food webs are fuelled by a complex mixture of allochthonous and autochthonous resources, the quantity and quality of these resources are limiting factors for stream secondary production. Hence, the changes observed in this study, will probably affect stream food webs.