Ecology and biodiversity of the deep-sea meiobenthos from the Blanes Canyon and its adjacent slope (NW Mediterranean) = Ecología y biodiversidad del meiobentos profundo del Cañón de Blanes y su talud adyacente (NO Mediterráneo)

Abstract

Submarine canyons are major topographic structures forming deep incisions in shelfs and continental margins around the globe. Due to their topographic characteristics, canyons influence both local hydrodynamic conditions and the transfers of organic material and sediment from shelfs to deep seafloor. This leads to changes in sediment characteristics and organic enrichment that play a key role in deep-sea the functioning, and particularly affect benthic communities. Metazoan meiobenthos are the most abundant deep-sea infauna, with nematodes being dominant. Due to their small size, meiofauna is intimately related with the sediment so that the physical properties are highly influencing the assemblages harbored by a given ecosystem. Therefore, habitat heterogeneity, a particularly relevant canyons feature plays a key role in shaping meiofaunal distribution and biodiversity. Blanes canyon (NW Mediterranean) has been intensively explored during the last years, mostly in relation with the fishery activities in the area. However, as in many other canyons, the meiobenthic component had been almost neglected. Within this context and in the frame of the Spanish research project Dos Mares, the present thesis, focus on the metazoan meiofauna in the Blanes Canyon system. Samples were obtained by the multicore deployments from 500 to 2,000 m depth during four oceanographic campaigns (spring and autumn in 2012-2013). The main objective was to increase the knowledge and further understanding of the patterns and trends of meiofaunal density, diversity and community composition, as well as on its main environmental drivers, with a particular emphasis on the dominant nematode assemblages. Our results indicate that Blanes Canyon exhibits: marked sediment variability, high food availability (i.e., Organic Carbon, Chlorophyll a, Chloroplastic Pigments Equivalents) compared to the adjacent slope, together with higher density, and diversity, and more marked differences in community composition and distribution, both at higher (i.e., meiofauna) and lower (i.e., nematodes and kinorhynchs) taxon level. The observed temporal variability is only partly explained by the seasonal patterns of food input, derived from phytoplanctonic production, where major oceanographic processes such as recurrent dense shelf water cascading events; seem to play a key role. Moreover, the topographically heterogeneous environment associated to the canyon bathymetric gradient, combined with the existence of recurrent, non-seasonal food pulses, are better explaining the observed meiofaunal trends, particularly those concerning nematodes. However, among all gradients analysed, the greatest effect on nematode communities occurred at small-scale (cm), both in the canyon and on the slope, with the highest abundance at the surface sediment layer leading to a marked diversity decrease along the vertical sediment profile.

This Thesis includes the first known study on Mediterranean deep-sea kinorhynchs, which contributes to increase the current knowledge on its diversity and distribution, as seven over the nine species found in the canyon system turned to be undescribed.

Our results allowed us to avoid suggesting generalizations in the description of meiofauna patterns in the canyon, highlighting those different taxa may show different bathymetric-related responses, this being particularly relevant in the case of the rare taxa (e.g., the kinorhynchs). They also support the consideration of submarine canyons as hotspots of faunal density, biomass and diversity, confirming its key role in shaping the patterns and trends shown by benthic fauna.

Finally, the overall results of this Thesis strongly suggest that the Blanes Canyon system is not functioning as a purely natural environment. The high sedimentation rates in the canyon axis suggest that the indirect but regular effects of bottom trawling are driving the structure, and possibly the functioning, of the associated meiobenthos, all along the axis down to at least, 1,750 m depth.

Los cañones submarinos son una de las principales estructuras geomorfológicas del margen continental y un recurso de heterogeneidad del fondo marino. Su abrupta topografía influye en el régimen hidrodinámico local, interrumpiendo y amplificando las corrientes e incrementando los flujos de partículas. Juegan un papel importante en el transporte entre plataforma y océano profundo atrapando, acumulando y canalizando sedimentos y materia orgánica con importantes consecuencias para la diversidad, funcionamiento y dinámica del bentos. La meiofauna es un componente importante del bentos. Dado su pequeño tamaño, está íntimamente relacionada con el sedimento y sus posibles alteraciones (p. ej., alteración de las tasas de sedimentación, resuspensión). Esta Tesis se centra en el Cañón de Blanes (NO Mediterráneo), lugar de importante actividad pesquera. Partiendo de muestras recogidas en primavera y otoño de 2012 y 2013, el principal objetivo ha sido incrementar el conocimiento y las pautas y tendencias de las comunidades de meiofauna (densidad, biomasa, diversidad y estructura) del Cañón de Blanes, así como de su talud adyacente con especial énfasis en los nematodos. Las elevados tasas de sedimentación detectados en el cañón (hasta los 1,750 m), sugieren un efecto indirecto de la pesca de arrastre, que estaría afectando a la meiofauna, sugiriendo la parcial antropogenización de los ambientes profundos del cañón. La variabilidad temporal observada obedece a: entradas estacionales de materia orgánica (derivadas de la producción primaria), procesos de formación de aguas profunda y heterogeneidad topográfica, combinados con entradas recurrentes no estacionales de materia orgánica. Sin embargo, la variabilidad a pequeña escala (cm) causa un mayor efecto en la estructura y diversidad de la comunidad de nematodos, principalmente debido a la disminución en diversidad a lo largo del perfil vertical del sedimento. El Cañón de Blanes muestra mayor variabilidad en el sedimento y disponibilidad de materia orgánica que el talud adyacente, así como una mayor variabilidad en la composición y estructura de las comunidades meiofaunales. Nuestros resultados apoyan la relevancia del papel de los cañones submarinos en la distribución de la fauna bentónica, así como su consideración como hot spots de densidad, biomasa y diversidad.