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[eng] Climate change has emerged as one of the greatest and most pervasive threats that our natural heritage will have to face in the coming decades. Together with other anthropogenic pressures such as pollution, overfishing or habitat degradation, climate change is causing enormous impacts on oceans, affecting all kind of marine communities and driving major losses to biodiversity. In this framework of global change, the Mediterranean Sea, which has been identified as one of the major hotspots of marine biodiversity, has also become a hotspot of climate change. Consequently, some of its most emblematic and ecologically important communities are now under threat. This is the case of the coralligenous assemblages, which are temperate benthic communities which stand out for their great structural complexity and exceptional biodiversity (they harbour approximately 10% of Mediterranean species). Most of the constituent species of these communities exhibit high longevity and slow population dynamics. Consequently, they are especially vulnerable to disturbances that increase adults mortality. In fact, thermal anomalies linked to ocean warming have impacted the coralligenous assemblages in several ways during last decades, triggering changes that go from the observed mass mortalities of benthic organisms to potential changes at the community and ecosystem levels. However, despite the increasing interest of the scientific community to conserve the coralligenous, how these benthic communities are responding to climate change at the community level is poorly understood. In this thesis, we have taken this leap by integrating responses from species to the ecosystem level, through populations and communities. In particular, we have combined field observations of ecological and environmental changes, aquaria experiments, ecological theory, and cutting-edge quantitative research to improve our understanding of the bio-ecological consequences of ocean warming for Mediterranean coralligenous assemblages. In the first three chapters, we gathered field and experimental data on sensitivity of several coralligenous species and populations to marine heatwaves (MHWs) by conducting a literature review on previous mass mortality events occurred in the Mediterranean and by carrying out scaled-up thermotolerance experiments. The results of Chapter 1 revealed contrasting patterns of sensitivity to MHWs among different coralligenous species and populations. The posterior multi-specific thermotolerance experiment (including corals, sponges and tunicates) conducted in Chapter 2 confirmed the high response diversity to warming among co-occurring species observed in the field. These results demonstrate that co-occurring species that are taxonomically and morpho- functionally similar can have contrasting thermotolerances. Moreover, the observed patterns suggest that coralligenous assemblages could have certain degree of insurance against MHWs, since the decline of sensitive species could potentially be compensated by other functionally similar species that are more resistant. The results of the multi- population thermal experiment conducted in Chapter 3 with 12 populations of the red gorgonian Paramuricea clavata (Risso, 1826) revealed that this species, which has a key habitat-forming role in coralligenous assemblages, is highly sensitive to thermal stress across its distributional range. Moreover, we found little local adaptation to average thermal regime and little acclimatization to local above-average warm temperatures prior to the experiment. These findings, which were obtained at a spatial scale never addressed before (> 2000 km) for P. clavata, question the adaptive potential of this species to climate change. In Chapter 4, we further explored the vulnerability of Mediterranean habitat- forming octocorals to climate change by exploring the long-term recovery capacity of Corallium rubrum (Linnaeus, 1758) and P. clavata from recurrent MHWs. We explored the long-term trajectories (15 years) of different populations that were firstly impacted by the 2003-MHW in terms of changes in biomass, density and size structure. We found that all impacted populations have followed collapse trajectories. These results further highlight the high vulnerability of these species to the ongoing climate change. Finally, we took the leap from species and populations to the community and ecosystem levels in Chapter 5, to explore if MHWs have driven changes in the functional community structure of coralligenous assemblages, which could imply changes for ecosystem functioning. Specifically, we conducted a multi-taxa, trait-based analysis and found that MHW are shifting the functional identity of coralligenous assemblages (i.e., dominant functional traits). Interestingly, the observed changes were driven primarily by the decline of a single functional group (habitat-forming octocorals), whose species are functionally unique. Consequently, by severely affecting habitat-forming octocorals, MHWs are leading to assemblages that are deficient in key functional traits, likely causing changes in ecosystem functioning. Overall, this thesis provides an integrated, long-term view of the interconnected responses to ocean warming found in coralligenous assemblages. The combined results of this thesis suggest that despite structural changes induced by climate change, most ecosystem functions count on certain degree of insurance against MHWs. However, some essential functions (e.g., 3D-habitat provision) could be highly compromised or even totally disappear, threatening the overall assemblage stability. Given the current climate change trends, identifying and preserving the mechanisms that maintain essential ecosystem functions and the overall ecosystem stability is critical. In this thesis, we have identified the general loss of arborescent octocorals as the greatest disruptive process for coralligenous assemblages. Therefore, the future trajectories of these emblematic Mediterranean communities will greatly depend on the design and implementation of measures that favour the conservation and adaptive management of these key habitat-forming species in the face of climate change.

[spa] La tesis versa sobre los regímenes de aludes en el Pirineo Oriental (Pirineo de la Comunidad Autónoma de Catalunya). Está estructurada en tres artículos publicados en revistas de impact factor y consta de un cuarto capítulo, como artículo en preparación. Los resultados obtenidos abarcan desde la probabilidad de ocurrencia de aludes en la zona de estudio, los marcos sinópticos desencadenantes de aludes, tanto a escalas sinóptica como de baja variabilidad temporal, regionalización de tipo de manto nivoso y modelización de las variaciones que puede sufrir al manto en escenarios de aumento de temperatura del aire de hasta 3 °C debido al calentamiento global. Se concluye que la probabilidad de ocurrencia anual de un episodio o más de grandes aludes en el Pirineo de Cataluña es del 64%. Pero la frecuencia es muy variable de una región a otra, siendo máxima en el Aran-Franja Norte de la Pallaresa con un 44% y un mínimo del 3% en el Perafita-Puigpedrós. Disminuye de oeste hacia este y de norte a sur, pero vuelve a aumentar en el extremo oriental con un 25%. Estas diferencias entre regiones se deben a que las situaciones sinópticas que desencadenan grandes aludes no tienen los mismos efectos en todas las regiones. Se identifican seis patrones sinópticos desencadenantes de grandes aludes mediante un análisis de componentes principales. La situación más frecuente es la advección del NW, le sigue el paso de vaguadas, depresión aislada en el sur de la Península Ibérica, baja fría, advección del SW y dorsal anticiclónica sobre el Mediterráneo occidental. La mayor parte de estas situaciones son propias de fases negativas de la Oscilación del Atlántico Norte (NAO). Se demuestra una correlación negativa entre un índice acumulativo de valores mensuales negativos de la NAO y la actividad de grandes avalanchas (-0.7), que es máxima en los sectores de la Pallaresa y de la Ribagorçana-Vall Fosca y en el extremo oriental. En la zona oceánica no se observa ninguna correlación con la NAO ni con la Oscilación del Mediterráneo Occidental (WeMO) y los episodios de aludes suelen coincidir con fases ligeramente positivas de la NAO. A partir de un análisis de medidas de posición de variables de la estructura del manto nivoso, se ha identificado un patrón de manto nivoso propio de la zona oceánica del Pirineo y otro de la zona mediterránea. El manto oceánico tiene una mayor proporción de granos de fusión que el mediterráneo, mientras que éste presenta unos índices de inestabilidad por capas débiles internas superior y mayor proporción de granos no persistentes que el oceánico. Mediante un análisis de modos combinados de percentiles de temperatura y precipitación, se observa que en la zona oceánica en los 10 días previos a actividad de grandes aludes y antes de un accidente aumenta notablemente el número de días fríos-húmedos, mientras que en la zona mediterránea, predominan los días fríos-secos antes de actividad de aludes. En los últimos 40 años están aumentando el número días cálidos-secos (del orden de un día por año). A partir de la simulación de balances energéticos y de masa del manto nivoso, se observa que en condiciones de un aumento de la temperatura media del aire del orden de 3 °C el espesor de nieve máximo anual disminuiría entre un 15-22% y el período de innivación se reduciría entre 15-25 días. Las orientaciones soleadas sufrirían unas condiciones de fuerte restricción de nieve respecto las umbrías. Hay una serie de conclusiones de carácter climático, como el uso de índices del manto nivoso para avanzar en la clasificación nivoclimático del Pirineo y otras de carácter aplicado en la predicción, como el uso de un catálogo de situaciones sinópticas desencadenantes de aludes y el uso de índice de inestabilidad del manto nivoso en el análisis diario del peligro de aludes., [eng] This thesis deals with avalanche regimes in the Eastern Pyrenees (Catalan Pyrenees). It is structured in three articles published in impact factor journals and consists of a fourth chapter, as an article in preparation. The results cover from the probability of avalanches occurrence, the synoptic avalanche patterns, the regionalization of snowpack structures and modeling changes in the snowpack due to the global warming. It is concluded that the probability of an annual occurrence of one episode or more of large avalanches in the Pyrenees of Catalonia is 64%. Frequency variability is very high among regions. It decreases from west to east and from north to south, but again increases in the easternmost part. Regional differences are due to the fact that the synoptic situations that trigger avalanches do not have the same effects in all regions. Six synoptic patterns of large avalanches are identified through principal component analysis. Most of these situations are typical of negative phases of the North Atlantic Oscillation (NAO). A negative correlation between a cumulative index of negative monthly values of NAO and activity of large avalanches (-0.7) is found. In the oceanic zone there is no correlation with NAO or Western Mediterranean Oscillation (WEMO) and avalanche episodes usually coincide with slightly positive phases of NAO. It has been identified an oceanic snowpack pattern and a Mediterranean one. The oceanic one has a higher proportion of melting grains than the Mediterranean, while the latter presents higher instability indexes by internal weak layers and a higher proportion of non - persistent grains than the former. In the oceanic zone, in the 10 days prior to activity of large avalanches and before an accident the number of cold-wet days increase notably, whereas in the Mediterranean zone, the cold-dry days prevail before avalanche activity. In the last 40 years, the number of warm-dry days has dramatically increased. Under conditions of an increase in the average air temperature of the order of 3 °C the maximum annual snow thickness would decrease by 15-22% and the snow on the ground period would be reduced by 15-25 days. There are conclusions both of a climatic nature and of applied uses on avalanche forecasting.

“Lake macroinvertebrates and the altitudinal gradient in the Pyrenees [Los macroinvertebrados lacustres y el gradiente altitudinal en los Pirineos]” is a thesis about freshwater macroinvertebrate communities inhabitting the littoral zone of the mountain lakes of the Pyrenees, and about the effect of altitude on the species composition, and richness, of these communities. The investigation is addressed by analysing the relative relevance of the environmental and spatial components of the altitudinal gradient on macroinvertebrate communities. This is performed by comparing different macroinvertebrate groups, which attain for different evolutionary histories and ecological idiosyncrasies, including different dispersal capabilities. The taxonomical resolution achieved in macroinvertebrate determinations is high, which allows for among-group comparisons at the species level. These comparisons can be used to test ecological theories of current usage. Mountain lakes are particularly useful to this purpose, as these ecological systems account for large environmental gradients related to altitude and, at the same time, the altitudinal ecological gradient includes spatial components that may be relevant in shaping macroinvertebrate community assemblages. The spatial component of altitude relates to the uneven distribution of lakes along altitude in mountain ranges, which implies different degrees of ecological isolation between lakes. Also, the littoral zone of mountain lakes is particularly suited for among-group comparisons because it is in the littoral zone where most freshwater macroinvertebrate biodiversity of lakes concentrates. In the Results section, the results obtained are presented first as a general overview of the distribution of main macroinvertebrate taxa across altitude. The influence of environmental variables related to altitude on distributions is explored, and the relative relevance of different environmental variables on macroinvertebrate communities is partialled out. Then, the effect of different environmental and spatial factors is analysed for each macroinvertebrate group, separately. The main taxa analysed in each of the chapters are, arranged by convenience, as follows: Oligochaeta, Chironomidae, Mollusca, Megaloptera, Trichoptera, Coleoptera, Hydracarina and Nematoda. In the Discussion section, altitudinal distributions are compared between groups, and differences observed in this regard between the main macroinvertebrate taxa are discussed. The effect of both environmental and spatial factors related to altitude is suggested. Then, species richness patterns are also compared between the main macroinvertebrate taxa. The relative contribution of environmental and spatial factors is, to some extent, disentangled. Finally, the effect of altitude on the similarity of communities is also compared between groups, de-composing this effect in environmental and spatial terms. The following conclusions are drawn from the results obtained: 1) The altitudinal distribution patterns of lake macroinvertebrates are the consequence of both the selective influence of the environment and the more stochastic nature of dispersal. 2) Environmental factors covarying with altitude are not equally relevant for all macroinvertebrate groups. 3) Mountain lakes are biogeographic islands. 4) Similarity between communities is slightly higher, and stochasticity slightly lower, at mid altitudes. 5) The stochastic influence of environmental dynamics also contributes to explain species richness patterns. 6) Large river basins do not play a relevant role in the similarity of communities, which is more dependent on the altitudinal location of lakes. 7) An altitudinal ecological threshold in community assemblages occurs at ca. 2550 m a.s.l., for all groups responding to environmental changes across altitude, but independently of their dispersal strategy. 8) The altitudinal threshold, because of its environmental nature, is expected to shift upwards under current climate warming. 9) Global changes will affect macroinvertebrate communities of mountain lakes through different components, including climate warming, habitat loss and fragmentation, and invasive species, which may also show mutual interactions., “Lake macroinvertebrates and the altitudinal gradient in the Pyrenees [Los macroinvertebrados lacustres y el gradiente altitudinal en los Pirineos]” es una tesis sobre las comunidades de macroinvertebrados acuáticos que habitan la zona litoral de los lagos de montaña de los Pirineos, y sobre el efecto de la altitud en la composición y riqueza de especies de estas comunidades. La investigación se efectúa analizando la importancia relativa de los componentes ambientales y espaciales del gradiente altitudinal sobre las comunidades de macroinvertebrados. Para ello, se comparan diferentes grupos de macroinvertebrados, de distinta idiosincrasia ecológica, incluyendo diferencias en cuanto a capacidad de dispersión. La resolución taxonómica con la que se analizan los datos es elevada, lo cual permite comparaciones entre grupos a nivel de especie, útiles para contrastar teorías ecológicas de uso actual. Las conclusiones de la tesis son las siguientes: 1) Los patrones de distribución altitudinal de los macroinvertebrados lacustres son consecuencia tanto de la influencia selectiva del ambiente, como de la dispersión, un proceso de naturaleza más estocástica. 2) Los distintos factores ambientales que covarían con la altitud no son igualmente relevantes para todos los grupos de macroinvertebrados. 3) Los lagos de montaña son islas biogeográficas. 4) La similaridad entre las comunidades es ligeramente mayor, y la estocasticidad en el ensamblaje de las mismas ligeramente menor, en altitudes intermedias. 5) La influencia de la dinámica estocástica del ambiente también contribuye a explicar los patrones de riqueza de especies. 6) Las cuencas de los ríos principales no juegan un papel relevante en la similaridad de las comunidades, que es más dependiente de la altitud de los lagos. 7) Existe un umbral ecológico para las comunidades de macro-invertebrados, situado a una altitud aproximada de 2550 m, que afecta a todos los grupos que responden a los cambios ambientales asociados a la altitud, pero con independencia de su estrategia de dispersión. 8) El umbral altitudinal, debido a su naturaleza ambiental, posiblemente se desplazará hacia arriba en altitud con el cambio climático actual. 9) El cambio global afectará a las comunidades de macroinvertebrados de los lagos de montaña a través de varios de sus componentes, incluyendo cambio climático, pérdida y fragmentación de hábitat, y especies invasoras, los cuales pueden mostrar también efectos de interacción mutua.