The present thesis was based in the study of chrysophycean cysts, and their applicability as environmental indicators. Chrysophytes respond directly to proximal factors, such as temperature, nutrients, water chemistry, hydrodynamism, ... These factors are related with whole ecosystem dynamics and they are forced by weather and catchment processes. Therefore, there is also a relationship between chrysophyte distribution and environmental features, such as bedrock type, vegetation, lake morphology, etc, which in the present thesis are called landscape factors.

The first objective of he study was to describe the chrysophycean cysts from Pyrenean lakes, and elucidate which environmental factors explain best their distribution. The number of morphotypes identified was 210 morphotypes, of which 86 were new descriptions. The study of cysts distribution patterns and the ecology for each cysts mophotype was carried out, using a survey of 105 lakes. The lakes were chosen to provide a broad range of altitude (1,615 m.a.s.l. to 2,954 m.a.s.l), size (Major 0.7 ha to Cregueña 45.4 ha), morphology, water chemistry and trophic status.

The second objective was to develop transfer functions, to reconstruct past environmental conditions using cysts sediment records. These functions were pH, alkalinity, magnesium and nitrate, concerning chemical parameters. On the other hand, I developed a temperature transfer function using the relationships between air temperature and altitude (lapse rates), found by spatial interpolation of radiosonde air temperature data (1990-1997). The mean air temperature for each sampled lake was inferred by averaging the last ten years previous to the sediment sampling data, with a proper correction according to the altitude difference.

The third objective was to analyse the capability of chrysophycean cysts for evaluating the inter-annual variability in seasonal patterns over relative short time scales. These patterns were identified using sediment traps.

Finally, the fourth objective was the application of chrysophycean cysts in order to reconstruct past environmental sequences. The transfer functions previously developed were applied to a high-resolution core corresponding to the whole Holocene period, this core was sampled in Lake Redo (Central Pyrenees). One of the main findings was to show the large drop in air temperature around 8,200 years ago (2ºC), followed by a slowly recover until 7,000 years ago. This result confirms that the widespread cold event, caused by changes in the North Atlantic thermohaline circulation, also affected SW.