Identification, distribution, taxonomic and functional composition of the benthic habitats covering the seco de los olivos seamount and its application to management and conservation

Resumen

Biodiversity protection and conservation of complex ecological interactions of natural ecosystems is needed to ensure the maintenance of ecosystem services for the future society. Specifically, seamounts-like structures are isolated undersea topographical elevations that are deemed essential ecosystems for supporting and maintaining global marine biodiversity. However, deep-sea fishing activities have a negative impact on benthic communities and represent a global concern as they threaten benthic habitats long-term survival. The creation of coherent, connected and properly managed networks of marine protected areas (MPAs), focused on achieving a balance between sustainable fisheries and other human activities, and habitat conservation, has emerged in recent years as a tool to alleviate this detriment to marine ecosystems. The present thesis deal with the identification and analysis of different aspects related to benthic habitats in a seamount located in the Alborán Sea known as Seco de los Olivos Seamount. The complex geomorphological and hydrodynamic characteristics of this basin explain the comparatively high phytoplankton biomass and biological productivity of the sea compared to other areas from the Mediterranean and lead to a wide diversity of environments, allowing the establishment of communities dominated by species of Atlantic and Mediterranean origin and from northern Europe and the subtropical Northeast Africa as well as endemic species typical of this sea, hosting unique mixed communities and a vast diversity of habitats. The Seco de los Olivos Seamount (SW Mediterranean Sea), a Site of Community Importance belonging to the Spanish marine Natura 2000 Network, is recognized for housing a great diversity. By analyzing biological data collected during 55 Remoted Operated Vehicle (ROV) transects from 76 m to 700 m depth, environmental data derived from multibeam bathymetry and high resolution seismic pro?les, and commercial bottom fishing data from Vessel Monitoring System Research (VMS) and logbooks, the spatial distribution patterns of benthic habitats have been determined and characterized and different spatial management scenarios have been proposed. The approach used in this work stands out for being an appropriate methodology for a submerged area characterized by difficult accessibility that requires non-harmful technologies and reveals as a crucial method in marine mapping and monitoring strategies. Based on a combination of assemblages of habitat-forming species and the environmental characteristics supporting their distribution, thirteen epibenthic biogenic habitats (with a clear biological component) were identi?ed. Depth, type of substrate, bathymetric position index (as a measure of the seascape concavity/convexity) and slope were identi?ed as the main signi?cant factors structuring epibenthic assemblages. The high environmental variability resulting from its wide geomorphologic diversity leading to different combinations of suitable ecological conditions, together with local recruitment, feeding strategies and attachment mechanisms, may explained the high diversity and patchiness distribution of benthic habitats found in the Seco de los Olivos Seamount. Biogenic habitats recorded on this seamount are structured by fragile, sessile, slow-growing and long-lived organisms sensitive to fishing disturbance internationally recognized as Vulnerable Marine Ecosystems. This thesis assesses how the presence of the biogenic component of benthic habitats influence positively the species richness and diversity of a seamount megabenthos since they increased taxonomic species richness and enhanced ? and functional diversities, being their influence more evident in soft and mixed bottoms, where the multiplying effect on structural complexity was proportionally greater than in rocky habitats. The results of this work showed that the processes underlying ? diversity values among the variety of patchy distributed habitats on the Seco de los Olivos were clearly different. Among habitats located at similar depth and type of substrate, diversity was explained mostly by species richness difference, reflecting the gain of species potentially associated to the presence of habitats-forming species. However, among habitats located in different environments (i.e. depth and/or type of substrate), ? diversity was dominated by the turnover component and hence, diversity was linked to specific environmental requirements of species. Different functional composition of megabenthic invertebrates was obtained for the different habitats suggesting differences in ecosystem functioning. Non-biogenic habitats seem to be in earlier successional stages of maturity as suggested by a dominance of some ?opportunistic? traits (e.g. small size, short-lived species and scavengers and/or carnivorous modalities) typical of r-strategy species. However, traits associated with organisms of biogenic habitats were 'larger-sized', 'longer-lived', mainly 'sessile', 'emergent' and also 'surface position', 'filter-deposit-suspension feeder', ?planar erect? and 'erect, stalk and tree like?, resembled more stable or at advanced stages of successional maturity. The better understanding of these marine ecosystem processes responsible for the different levels of richness and biodiversity would lead managers to generate a baseline for their future monitoring. This baseline is needed to evaluate the effect of anthropogenic and natural disturbances and to determine measures focused on the sustainable management of marine environment. Vulnerability of benthic habitats was assessed in relation to the most frequently bottom gears used in the study area that have the greatest potential to cause physical damage such as otter trawl and set bottoms long-line. According to our results, around 50% of the area was disturbed by fishing and the pressure exerted by the different fishing techniques on the various habitats differed: as expected, most of the trawling effort was carried out on soft bathyal substrates, both biogenic and non-biogenic while on hard bottoms, the percentage of longlining effort carried out was relatively higher than of trawling effort. Habitat-builder species that have large and arborescent growth body forms make them particularly vulnerable to encounter with fishing lines whereas the lower sensitivity of non-biogenic habitats was reflected in the lower levels of disturbance provoked by longlining. In this context, management measures aimed at both, the conservation and the potential recovery of degraded habitats should be implemented in this MPA. Our results showed that in some biogenic habitats, characterized by higher diversities and functional redundancies, more species perform similar roles, being substitutable with little impact on ecosystem processes. Therefore, these biogenic habitats are more buffered environments, less susceptible to changes and species loss because the maintenance of some individuals or colonies may favor successful local recruitment, functional replacement and may allow habitat reversibility, i.e., full recovery to pre-disturbance levels after the effects of the disturbance. By contrast, in degraded habitats subjected to continuous disturbance the community can be driven beyond the resilience threshold and move it into an alternative state characterized by the dominance of species with low vulnerability to disturbance. In these environments, key species tend to be more tolerant to disturbance, but the recovery of the habitats as well as their progress towards ecological climax is more dependent on connectivity and re-colonization from neighboring areas after disturbance and the reversibility is more difficult and even highly unlikely, leading the system in many cases to a new alternate stable state composed by a community different than that from the pre-disturbance state. Based on the knowledge acquired during the development of this thesis, three different areas of ecological importance where a variety of management measures can be put into practice, from more to less stringent measures, are proposed. Subsequent monitoring using both operational (related to specific pressures) and surveillance (related to overall condition of habitats) ecological indicators as well as associated reference levels and targets to achieve a Good Ecological Status (GES) according to the MSFD is advisable and this work constitutes a baseline from which to develop and test the different ecological indicators. Additionally, the robustness of this methodological approach leads us to propose it as a blueprint for other areas of the Natura 2000 marine network.