[ca] Els principals objectius d'aquesta tesi han estat els de (a) determinar els efectes de la fertilització de nitrogen (N) i fòsfor (P) sobre el creixement dels grups algals del perífiton i del fitoplàncton, i (b) avaluar la influència d'alguns processos reguladors (p.ex. contingut de N:P, contingut de clorofil·la per biovolum) involucrats en les respostes del creixement. S'ha seguit una aproximació experimental de camp basada en la utilització de substrats difusors de nutrients en el cas del perífiton (Capítol 1), i en l'addició de nutrients a mesocosms en el cas del fitoplàncton (Capítol 2 i 3). En el primer capítol es va avaluar la influència dels factors que controlen la colonització del perífiton per part de les diatomees i dels cloròfits. La renovació de l'aigua i l'estat tròfic (DOC) van tenir un efecte positiu en la colonització algal. L'alta disponibilitat de N en relació a P va tendir a afavorir els cloròfits per sobre de les diatomees. Les addicions de nutrients (sobretot de N) van resultar perjudicials pel creixement de les diatomees en molts casos, mentre que els cloròfits van tolerar millor aquestes altes concentracions. En el segon capítol es van observar diferents pautes de resposta de la producció, l'estructura de la comunitat i la composició C:N:P del seston a diferents enriquiments amb P. La producció primària va augmentar considerablement a addicions baixes i mitjanes de P, mentre que les proporcions C:N:P del seston només van canviar de forma marcada -dirigint-se cap a "Redfield"- a altes addicions de P. Les condicions d'excés de N van reduir el creixement del fitoplàncton però no van afectar les proporcions C:N:P. Finalment es van avaluar les relacions entre disponibilitat de nutrients, creixement i contingut de clorofil·la per biovolum en el fitoplàncton. Les diatomees i els cloròfits van demostrar tenir les capacitats de creixement més altes, i això podria estar associat amb la seva habilitat d'incrementar el contingut de clorofil·la quan la disponibilitat de nutrients augmenta. Les crisofícies i les dinofícies van assolir les màximes taxes de creixement a concentracions de P comparativament baixes, indicant que estarien més ben adaptades a baixes disponibilitats de nutrients.

[en] The main objectives of this dissertation were (a) to determine the effects of nitrogen (N) and phosphorus (P) fertilization on the growth of microalgal groups of periphyton and phytoplankton, and (b) to evaluate some indicators of the regulatory processes (e.g. N:P content, chlorophyll content per biovolume) involved in such growth responses. To do so, we followed a field experimental approach of short-term nutrient enrichments (21-25 days), using nutrient-diffusing substrates in the case of epilithic periphyton (Chapter 1), and mesocosms in the case of phytoplankton (Chapter 2 and 3). These experiments were performed in lakes of the Pyrenees, a region of particular interest because the average conditions are just on the threshold between the N and P limitation of algal growth. Moreover, most of these lakes are oligotrophic, and the nutrient availability is highly conditioned by N and P inputs of atmospheric origin, which appears to be changing due to human activities and global change. In the first chapter, we evaluated the influence of the main factors controlling the algal colonization of epilithic periphyton, and compared the differences between the two major algal groups (i.e. diatoms and chlorophytes). We selected nine lakes and two streams to account for environmental gradients of trophic status and water renewal, while the influence of N and P availability was evaluated in each location using nutrient-diffusing substrata. Water renewal had a significant positive effect on the algal colonization of epilithic periphyton, although this influence was only significant for diatoms. Lake trophic status (i.e. dissolved organic carbon) was the second most influential factor in the algal colonization, and similarly affected diatoms and chlorophytes. High N:P availability tended to favour chlorophytes over diatoms. The nutrient enrichments inhibited the growth of diatoms -particularly the N enrichments-, but this adverse effects were mitigated by high water currents. Chlorophytes better tolerated high nutrient levels than diatoms. The second chapter aimed to assess the influence of nutrient availability on the productivity, the composition of major phytoplankton groups, and the C:N:P proportions in seston, and evaluate the interrelationships among them. We noticed that the response of productivity, community structure, and seston C:N:P to increasing P availability was not strictly coherent. Productivity was especially sensitive to low and intermediate P additions, while seston C:N:P was only markedly changed under high P supply. The highest P-enrichment lead the seston C:N:P toward Redfield proportions in a non-linear way. N excess conditions reduced the growth of phytoplankton and enhanced the differentiation of autotrophic community, but did not affect seston C:N:P. High ammonium availability tended to increase seston C:P and N:P ratios, as well as to stimulate productivity and alter phytoplankton structure when P stress diminished. Finally, we evaluated the relationships among nutrient availability, growth, and chlorophyll content per biovolume in phytoplankton. Specifically, we evaluated to what extent intense growth is associated with high chlorophyll content or with a major plasticity of that content. Pigments content and their plasticity were determined for all algal groups estimating power-law based models. We observed that diatoms and chlorophytes were the two groups that showed the highest growth capacity, what could be associated with their ability to increase the chlorophyll content per biovolume when nutrient supply increases. Chrysophyceans and dinoflagellates achieved their maximum growth rates at relatively low P supply, and may be better adapted to cope with low P availability.