Mediterranean deep-sea ecosystems: Biodiversity, functioning and vulnerability = Ecosistemes profunds de la Mediterrània: Biodiversitat, funcionament i vulnerabilitat

Abstract

The Mediterranean Sea can be considered an excellent natural laboratory for benthic ecologists, due to its peculiar environmental conditions, its broad-scale gradients, and its deep-sea fauna. Primary production levels in the Mediterranean follow a longitudinal gradient, decreasing from west to east. The present thesis focused on the fine-mud benthos habitat of the Mediterranean Sea, on its bathyal and abyssal zones, and the compartment considered was demersal megafauna. The main objective of the present thesis was to describe and characterize the deep habitats of the Mediterranean Sea, relating their biodiversity and ecosystem functioning with the varying environmental conditions along the geographic, bathymetric, and temporal axes. The work was divided into four parts. In the first part, the patterns of distribution, biomass and abundance of bathyal and abyssal megafauna were studied, in relation with environmental variables, along longitudinal and bathymetric gradients. This was a large geographic scale study, across the three basins of the Mediterranean at depths of 1200-3000 m. Benthic biomass strongly decreased with depth and longitude (west-to-east). Results showed a significant correlation between the dynamics of surface layers, the amount of food available on the bottom, and the composition of benthos. In the second part, the seasonal fluctuations of benthic communities in the Catalan continental margin were observed. A highly replicated, multi-period study was conducted at depths of 900-1500 m. Total biomass followed an inverted U-shaped pattern, peaking at depths of 1050-1350 m. Range-related ecological forcings between shallower and deeper species may have caused this biomass accumulation at intermediate slope depths. The arrival of a new water mass from the deep basin to the slope in spring may have also driven an accumulation of biomass at 900-1050 m depth over the same period. An adjacent submarine canyon was also studied, comparing communities between the canyon and the open slope area. Analyses revealed higher diversity, but not biomass, inside the canyon than in the adjacent open slope, and a significantly different assemblage between the habitats. These results strengthen the concept of submarine canyons as hotspots of biodiversity and underline the importance of their conservation as diversity repositories. The third part is a study on trophic relations of deep-sea fauna, over the same large spatial transects considered in the first part, by means of natural-abundances stable isotopes analyses and the use of novel statistical methods to analyse heavy datasets all in once. Bulk-tissue d13C and d15N isotope ratios were analysed for benthic megafauna and associated surface and mesopelagic components from the 3 basins of the Mediterranean Sea. The trophic niche width and the amplitude of primary carbon sources were positively correlated with both primary and secondary surface production indicators. Moreover, mesopelagic organic matter utilization processes showed an intermediate position between surface and deep benthic components. Thus, both primary and secondary production processes taking place at surface layers are key drivers of deep-sea food web structuring. Finally, the fourth part integrates the whole collected dataset into the first ecotrophic model of a deep-water ecosystem in the Mediterranean Sea, evaluating the general ecosystem functioning as well as the degree of vulnerability of these ecosystems. The Ecopath with Ecosim approach was used to model the flows and biomasses of the Catalan Sea continental slope ecosystem at depths of 1000-1400 m. Network analysis identified low levels of consumer biomass cycling and low system omnivory index when compared with expected values of marine ecosystems, and higher cycling and omnivory when compared with available models of shallower areas of the Mediterranean. A simulation of the possible expansion of the red-shrimp benthic trawl fishery that currently operates at shallower depths, showed reductions in fish biomass and that the state of the deep continental slope ecosystem seems to be result of a long-term succession process, which has reached ecological stability, and is particularly vulnerable to human impact and, specifically, to fisheries exploitation.

En la presente tesis se han estudiado los patrones de biodiversidad, funcionamiento trófico y ecosistémico de los sistemas bentónicos de gran profundidad en el Mar Mediterráneo. El objetivo principal fue de describir y caracterizar los hábitats profundos, relacionando su biodiversidad y sus procesos de funcionamiento con los gradientes ambientales a lo largo de los ejes longitudinal, batimétrico y estacional. El estudio de la composición de comunidades y la respectiva recogida de muestras para los análisis tróficos se han realizado a través de arrastres profundos. Los resultados de análisis de composición de comunidades hallaron una correlación significativa entre las dinámicas de las capas oceánicas de superficie, la cantidad de alimento disponible en el fondo marino, y la composición de la megafauna estudiada. A escala reducida y considerando las variaciones estacionales, la biomasa bentónica mostró un patrón a U-invertida, con un pico a profundidades de 1000-1400 m. Una posible explicación de este fenómeno podría ser la combinación de forzajes ecológicos ligados a las diferentes distribuciones batimétricas de las especies. Para estudiar las relaciones tróficas en los ejes batimétricos y geográficos de gran escala, se utilizó el análisis de isotopos estables de las componentes bentónicas, de superficie, y de capas pelágicas intermedias. El amplitud de los nichos tróficos de las comunidades bentónicas se encontraron relacionados con indicadores de productividad de superficie, tanto primaria como secundaria. Asimismo, los procesos de utilización mesopelágica del carbono orgánico se hallaron en posición intermedia entre los de superficie y los del bentos profundo. Finalmente, un análisis ecosistémico a través de “Ecopath with Ecosim”, reveló que los ecosistemas de los taludes continentales del Mediterráneo son el resultado de un largo proceso de sucesión ecológica, han alcanzado un estado de estabilidad, y son particularmente vulnerables a factores de impacto humano, entre ellos, la explotación pesquera.