This thesis focuses on the study of the meiofaunal community in streams and rivers of the Mediterranean region. It approaches to a community level from a functional point of view, studying their response as a whole to different processes occurring in those systems. The thesis is divided into 5 chapters, in which we sampled various substrates in order to study the small-scale processes affecting them and the interactions with other organisms. We focused essentially on the main lotic ecosystem determinants: hydrology and nutrient inputs (organic and inorganic). In Chapter 1, we studied the meiofaunal community over 2 years. Its structure reflected high intra- and inter-annual variability, which coincided with the alternating periods of hydrological stability and disturbances. Since flow is a major abiotic determinant of invertebrate distribution patterns in streams and its impact on meiofauna is relatively well known (see Swan and Palmer, 2000 and references therein), the study addressed mainly the adaptive responses of organisms to spates and droughts in a Mediterranean context. These adaptations were measured in terms of advantageous traits (either structural or life strategies) that could confer resistance or resilience to organisms. Results from that study pointed out that the resilient response of meiofauna in downstream reaches of the study site had a slower recovery given that heavy rainfall events resulted in higher discharges (as a consequence of higher water run-off from catchments). In that system, previous hydrological history also appeared to modulate invertebrate response to floods, diminishing recovery response of the community in that reaches. In contrast, this effect was not observed in the most upstream reach which also showed a fast recovery after spates caused by higher habitat heterogeneity and complexity that served as a refuge to fauna. Organic matter input and its availability to consumers is one of the major processes in headwater streams. In Chapter 2, we analysed the colonisation pattern of experimentally disposed riparian leaves, including all organisms (bacteria, fungi, protozoa, meiofauna and macroinvertebrates) involved to trace the fate of carbon during the process. The experiment showed a clear energy transfer from bacterial to faunal compartments modulated by differences in temperature and discharge that naturally occurred in the stream. Eutrophication of lotic systems is one of the negative results of human activity. Therefore we paid special attention to the effects of nutrient availability to meiofauna because their small size and high turnover rates can provide information of interest about their rapid response to these kinds of disturbances. In Mediterranean systems scarcity can potentiate the negative effects of eutrophication. Therefore, we also explored the interaction of hydrology and nutrient inputs. In Chapter 3 we analysed the meiofaunal response to an experimental addition of nutrients in an almost pristine headwater stream. For this purpose, we focused on differences in community assemblages and secondary production of microcrustaceans, finding that hydrological stability and even water scarcity could potentiate the effects of the nutrient addition. In that system, hydrological disturbances were the “background noise”. In contrast, in Chapter 4 we explored a nematode community in a eutrophic system and we focused on the stability of substrate. We found a different pattern of fauna inhabiting the same biofilm depending on the mobility (attached vs. free-floating cyanobacterial mats). Nematodes were found to be an active part of this detachment though their activity and adapted their life strategies to those changes. In Chapter 5, using a laboratory experiment, we analysed the fast response of two nematode species to eutrophication and examined life strategies adopted to adapt to increased nutrient availability. We could observe a clear bottom-up effect in microcosms and also that small differences in behavioural attributes may shape the successional pattern of recolonisation after a disturbance. / L’objectiu principal d’aquesta tesi doctoral és caracteritzar la comunitat de la meiofauna en sistemes mediterranis i integrar-la com a baula dins dels processos funcionals d’ambients lòtics. Tot i que el coneixement sobre la meiofauna d’aigua dolça és encara escàs, aquesta tesi no ha pretès tenir un caire descriptiu de la comunitat des d’un punt de vista taxonòmic sinó emprar classificacions funcionals que permetin integrar la meiofauna en processos i contextos més amplis. La tesi esta articulada en 5 capítols i en cadascun s’ha mostrejat diferents substrats en funció de les interaccions entre organismes i les característiques dels processos a petita escala. En aquests experiments, s’han adreçat diferents qüestions depenent de l’hàbitat o de les característiques del lloc. Essencialment, aquesta tesi s’ha centrat en els processos lòtics més influents: la hidrologia i les entrades de nutrients (ja siguin orgànics com inorgànics). La resposta general de la meiofauna trobada al llarg d’aquesta tesi ha estat sempre molt vinculada als factors climàtics, ja fos analitzant l’efecte de les avingudes o de les sequeres sobre la comunitat de meiofauna o sobre processos de descomposició, o bé la variabilitat intranual sobre comunitats de cianobacteris i nematodes. Al llarg d’aquesta tesi doctoral, els trets adaptatius de la meiofauna en front a pertorbacions hidrològiques o d’entrada de nutrients han demostrat ser nombroses i merèixer una investigació mes exhaustiva en la recerca futura.