RICERCA ITALIANA     

Tyrrhenian Seamounts ecosystems: an Integrated Study (TySEc)

Università Politecnica delle Marche

Abstract

Seamounts are among the most important topographic features of the ocean floor. Extending over a considerable depth range from base to summit, and showing a variety of substrates and habitats, they constitute unique ecosystems for biota in the open ocean.
The proposed research activities will be conducted on a Tyrrhenian seamount, the Vercelli Ridge, situated northwest of Olbia (Sardinia) (41°05’00 N / 10°53’00 E; summit depth: 55 m). From a geologic point of view, the Vercelli seamount represents a submerged intrusion of granitic material occurred about 7 Ma ago during a extensional tectonic regime that involved the entire northern portion of the Tyrrhenian Sea. Few previous works have studied the geology of the Vercelli intrusion highlighting a different petrologic affinity in relation to the average chemism of the other batholitic intrusions of the Tyrrhenian Sea.
The high levels of species richness found on the seamounts are related to different factors: the high productivity of these environments, coexistence, in a relatively limited area, of various biocoenoses characterised by different bathymetric ranges, the role of seamounts as “stepping stones” during the dispersion of benthic organisms, and the high percentage of endemisms. Therefore seamounts are considered important habitat, nurseries, and/or refugees for numerous species of fish and invertebrates.
The aim of the proposed research project is to study the peculiar geomorphological and hydrological features around the seamount and their influence on the ecosystem, focusing on the geomagnetic and geologic characteristics of the granitic intrusion, the relationships between water column and sediments (Pelagic - benthic Coupling), and the rich and diversified communities of benthic invertebrates and fish inhabiting the slopes of the seamount.
The technologies employed and the main tasks of the three OU are manifold: first of all a geomagnetic survey in high resolution has been planned, covering both the topographic structure and its neighbourhood regions. The morphological pattern will be defined by a high resolution sampling with multibeam coverage that is useful to obtain a bathymetric dataset. The multibeam acquisition may be done during the geomagnetic survey. It is also proposed a sampling activity at the top of the intrusion using a box-core or other devices. The rock samples will be analysed chemically and physically in the laboratory. The data collected during the survey will be used as a base for a geomagnetic and geological study of the Vercelli seamount using a 2D-3D inverse algorithm of the magnetic anomaly field. The final model will be integrated in a regional-scale interpretation of the tectonics evolution of the North Tyrrhenian sea during the Miocene - Tortonian phase.
Moreover the hydrographical processes controlling circulation, mixing and exchange of fluids in the vicinity of the seamount are going to be identified and described. In fact they represent the key requirements to understand the biogeochemical processes, and are essential also to design an effective geochemical and biological sampling strategy. Another task is to assess the origin, the quantity and the dynamics of the organic material within the water column and in the superficial sediments of the seamount, in order to estimate the extent of the ridge’s influence on the biogeochemical processes, focusing on the Pelagic - benthic Coupling relationship.
Finally it has been planned the assessment of biodiversity, the biocenotic characterisation of the benthic communities and the study of the ichthyofauna of the Vercelli seamount. The biodiversity will be studied both through technical dives using mixed-gas and, for the major depths, with remote operating vehicles and oceanographic dredges. During these surveys the human impact caused by fishing on the benthonic biocoenoses of the seamount will be evaluated through the count of the number of eradicated colonies or those with broken branches.
Furthermore the isotopic composition of the calcite and organic skeletons of cnidarians with slow growths and extremely long life spans will provide useful informations concerning possible mid-terms variations of the Tyrrhenian temperatures in relation to global warming. <<<

Principal Investigator

Giorgio Bavestrello Università Politecnica delle MARCHE

Research Objectives

Due to the extraordinary interest of seamounts, to their importance in the ecosystem functioning and to their high degree of vulnerability, international agreements and research programs have been established to preserve and study these peculiar habitats. Examples are the treaty between France and Australia regarding the seamounts arising between Australia and New Caledonia or the Oasis Project of the European Community to study the seamounts of the Middle Atlantic Ridge. In 2006 Greenpeace International, in agreement with ACCOBAMS, has chosen the Thyrrenian seamounts as possible Marine Protected Areas due to their peculiar biological richness and environment.
These data clearly indicate the need of a long-term project allowing the record of lacking data on all the Mediterranean seamounts.
The main task of the TySEc Project is to conduce an integrated study on one of the most interesting and less known seamount of the Tyrrhenian area: the Vercelli seamount placed in the northern Tyrrhenian Sea (41°05’00 N / 10°53’00 E, depth of the top, 55 m).

The detailed tasks, regarding the different aspects of the study, can be listed as follows:

1) To conduce a geomagnetic and geologic study of the characteristics of the Vercelli seamount through a survey in high resolution covering both the topographic structure and its neighbourhood regions. The morphological pattern will be defined by a high resolution sampling with multibeam coverage that is useful to obtain a bathymetric dataset.
To conduce a sampling of the rocks at the top of the intrusion as a base for a geomagnetic and geological study of the seamount, which will use a 2D-3D inverse algorithm of the magnetic anomaly field. The final aim of these activities is the preparation of an integrated model at a regional scale of the tectonic evolution of the North Tyrrhenian Sea during the Miocene - Tortonian phase.

2) To identify and describe the physical constraints effecting seamount systems. The knowledge about hydrographic processes that control circulation, mixing and exchange of fluids in the vicinity of the seamount are the key requirements to understand the biogeochemical processes.

3) To assess the origin, the quantity and the dynamics of the organic material within the water column and the surface sediments at the seamount, in order to estimate the extent of the seamount's influence on the biogeochemical processes, focusing on the Pelagic-benthic Coupling relationship. These issues are pivotal to understand the energy supply and transfer in ecosystems potentially controlled by topographic elevations, and are linked to the study of the benthic communities.

4) To define the structure and the composition of the plankton community around the seamount, focusing on its temporal and spatial variability, to investigate whether its origin is autochthonous or allochthonous.

5) To define and model the trophic ecology of the seamount ecosystem. All the information will be synthesised in a continuously updated conceptual ecosystem model, which will provide a common base for the presentation of the preliminary scientific results of the project. Within this task a quantitative study on the different species of filter-feeding organisms, generally considered the most efficient transfers of energy from planktonic to benthic trophic chains, will be conduced.

6) To study the biodiversity and define the biocenotic characterisation of the Vercelli seamount. The main taxa that are going to be investigated are: sponges, hydrozoans, gorgonians, zoanthids, antipatharians and scleractinian corals.

7) To study the fish community of the Vercelli ridge, estimated through various samplings by means of both fishing gears (gillnets, long lines, fish traps) and visual census performed with the ROV. The composition and structure of the ichthyoplanktonic community will be studied through trawled pelagic nets.

8) To study the temperature trends of the Tyrrhenian waters through temperature-recorder organisms. We will use the isotopic composition of the carbonatic and organic (proteinaceous, chitinous) skeletal structures of long living corals. It is known that while the isotopic composition of the mineral skeletons is affected by the temperature of the surrounding waters, that of the organic skeletons is affected by the surface temperature

9) To evaluate the human impact on the biocenosis of the Vercelli seamount, with the ultimate aim of suggesting a management plan for the area. The Mediterranean seamounts, in particular the Vercelli one, are interested by a significant professional fishery, carried out with both fishing gears and trawling nets. These equipments damage or completely destroy the ramified corals, species which show an extremely slow growth rate, hence a modest recovery ability. The impact of fishing on these organisms is going to be assessed through the count, along horizontal transects, of the number of eradicated colonies or those with broken branches. <<<

First Results

The principal expected result of this project is the detailed description of the seamount Vercelli, both for what concern the abiotic factors and their influence on the ecosystem.
The resulting product of this work will be the creation of a conceptual model representing the Vercelli ecosystem’s trophodynamic, based on mass budgets, and built through a specialised software (for example ECOPATH suite). Such modelling will provide the tools for a careful and correct management of the marine environment.
Concerning the geomorphological ambit, the main result will be the marine high resolution geomagnetic survey of the Vercelli seamount, that will define more precisely the magnetic characteristics of the area. Aim of this survey is to obtain the most detailed possible sampling of the anomaly related to the granitic intrusion, so to be able to modelling it.
Another result, linked to the realisation of the magnetic model for the area and achievable in concomitance to the magnetic survey, will be the acquisition of high resolution bathymetric data.
Moreover the magnetic model will be integrated with data concerning the lithological characteristics, the age of the intrusion, the tectonic and geodynamic pattern of the investigated area, and the physical parameters of the rocks, as magnetic susceptibility and density.
These results will contribute to create an exhaustive geological knowledge on the intrusive structure and will define the specific constraints for a wide-scale interpretation of the Northern Tyrrhenian tectonic dynamics.

Concerning the study of biodiversity the main results will be:

1) List of the species collected at different depths within the hard rock benthic biocoenoses. In particular all the major filter feeding organisms, like sponges, hydrozoans, gorgonians, antipatharians, zoanthids and scleractinian corals, will be studied to the species level.
Some of these groups (sponges and hydrozoans) are still largely unknown in the Mediterranean area, especially in the deep waters. Others (Gerardia and antipatharians), are considered rare animals, and the data on their distribution in the Mediterranean Sea are quite scarce. For what concern black corals, three of the five Mediterranean species, are poorly described and are in need of a new redescription. The bathymetric distribution of the gorgonian Corallium rubrum, in particular its lower limit, is still unknown and very few studies have dealt with the growth rates of deep populations. The localisation of new reefs would be extremely important for a correct management of the resource. Concerning the other gorgonians it is well known that this group has suffered of extensive mortality events, which decimated several shallow water species in all the Northern Tyrrhenian Sea.
An important result will be the evaluation of the health condition of the existing populations on the Vercelli seamount at different depths. Healthy populations, in deep waters, may be crucial to maintain the recruitment in the superficial layers. The strategic position of the Vercelli seamount could therefore represent a centre of diffusion for all the Northern Tyrrhenian Sea.
In general the analysis of the biodiversity and of the biocoenoses of the Vercelli ridge could represent a preliminary approach to the study of all the Tyrrhenian seamounts, with the ultimate goal of evaluating, as already proposed for other areas in the world, adequate protection measures for these ecosystems.

2) Contribute to the quali-quantitative knowledge of the ichthyic communities of the Vercelli seamount, through both fishing gears and visual census. For all the specimens collected some parameters, as the size, the reproductive status and the gut content will be evaluated.
The composition and the structure of the ichthyoplanktonic and micronektonic communities observed on the seamount will be defined. These results are going to be extremely important to evaluate the impact of the professional fishing activity on the ichthyic populations, within a context of conservation and management.

3) Contribute to the assessment of the effects of professional fishery, both with fishing and trawling gears, on the benthic biocoenoses, mainly composed of arborescent organisms. The deep communities are indeed considered as important nursery habitats or refugees for many species of invertebrates and fish. The destruction of these important habitats may have crucial consequences on the diversity of the shallow communities, while the comprehension of the biodiversity and of the structure of deep environments may represent a substantial knowledge for the management of superficial waters.

4) Contribute to the evaluation of the role of filter feeders composing the hard rock biocoenoses on the energy transfer between plankton and benthos (Pelagic-benthic Coupling), through the quantitative esteem of populations living at different depths along the slopes of the Vercelli seamount.
Benthic filter feeders, especially sponges and branched cnidarians, are considered among some of the most efficient transporters of energy between planktonic and benthonic trophic chains. Sponges exploit mainly the particulate organic matter, while cnidarians are mainly predators.

5) Contribute to the evaluation of the climate changes in the superficial and deep Northern Mediterranean waters, through the study of temperature-recording organisms. In particular the isotopic composition, preceded by a temporal dating, of the carbonatic and organic skeletons of long-living organism, are going to be studied. These data will be very important for the comprehension of the global changes, that interested and still interest the Mediterranean Sea. This basin, because of its semi-closed structure, is particularly sensitive to the changes related to the global warming processes.

Concerning the ecological studies the main expected results will be:

1) Contribute to the hydrological knowledge of the water masses in relation to the topography of the bottom. The information on the hydrographical processes controlling the circulation, mixing and exchange of fluids in the vicinity of the seamount represent the key requirements to understand the biogeochemical processes. These data will allow to evaluate the hypothesis stating that seamounts represent potentially important sites for the development of biological activity in oligotrophic open areas and that hydrodynamic conditions favour the increase of concentration and fluxes of organic material towards the ocean floor.

2) Contribute to the knowledge of the mechanisms controlling the biogeochemical processes in the proximity of seamounts. The definition of the origin, the quality and the dynamics of the organic material in the water column and the superficial sediments on the seamount will allow to estimate the extent of influence of this structure on the biogeochemical processes, in particular on the Pelagic-benthic Coupling.
These data will help formulating answers concerning the quali-quantitative enrichment mechanism (or mechanisms) of organic material in the waters around the seamount, the fractioning of nutrient resources within the ecosystem and the extent to which the organic matter is trapped in the sediments. This knowledge is strategic also for the comprehension of the energy transfer within an ecosystem potentially controlled by the topography and is related also to the study of benthic communities.

3) Validation of a scientific-logistic protocol for the study of seamounts in the Mediterranean sea. <<<

Timescale

24 months

National and international background

Seamounts are major topographic features of the ocean floor (Smith and Jordan 1988, Epp and Smoot 1989). Extending over a considerable depth range from base to summit, and according a variety of substrates and habitats, they constitute unique ecosystems in the open ocean (Boehlert and Genin 1987).
Considering its geographical position and its geological evolution the Vercelli seamount may be associated to the the Northern Tyrrhenian domain northwestward of the Sardinian coast. This morphologic element shows a conic shape running along a SW-NE direction, its bottom lays at 1800 m, while its top is at 55 m below the sea level. From the morphological and bathymetrical point of view, the northern portion of the Tyrrhenian sea is characterized by a sequence of structural highs running along a N-S and NNE-SSE direction with an association of flat sedimentary basins: from West to East there are several seamounts like Etruschi, Cialdi and Tiberino (Marani and Gamberi, 2004). Observing the general eastward trend of distribution of the structural highs of the North Tyrrhenian sea, the Vercelli seamount is located southerly of the other basin structures near to the discontinuity of the 41st parallel zone.
From a geological point of view, the Vercelli seamount is a granitic intrusion occurred during the Late Miocene (Barbieri et al., 1989). The chronological evolution of the magmatic products of the Tyrrhenian Sea is connected to an eastward migration of the subduction system between the African and European plates. In the northernmost portion of the Tyrrhenian Sea the evolution occurred during the Miocene time with a first evidence of a calc-alkaline magmatism around Corsica. These magmatism phase comes to its end in the Tortonian with the formation of a magmatic/volcanic center near the Apenninic system. The eastward flexural migration of the subduction system has produced a sequence of magmatic centers, that become younger from West to East. The emplacement of the Vercelli intrusive structure occurred in the Tortonian (7.8 Ma), as it happened also for the batholitic structures of Montecristo, Elba and Capraia (Barberi et al., 1978; Barbieri et al., 1989). The average composition of the rocks of the Vercelli seamount is granitic, fact that mismatches with the granodioritic affinity of the other intrusive structures of the Northern Tyrrhenian sea. In particular the granitic chemism of the Vercelli seamount has a calc-alkaline magmatic affinity with high value of K (HK-CA series from Peccerillo and Taylor (1976)) and high anomalies of Ba, Th, U. This geologic and geochemical pattern may be indicative of a peculiar magmatic source. Since the calc-alkaline affinity is connected to a magmatic process linked to a subduction system and the high anomalies of incompatible elements are directly related to a deep asthenospheric material, the Vercelli intrusion derives from the combination of the compressive tectonics with a rifting phase. During the Tortonian time, the 41° parallel zone has been affected by a E-W movement along a probable discontinuity structure, resulting in the emplacement of intrusive bodies. The region of interest may be considered as the transition point between two geodynamic phases.
The magnetic anomaly field of the North Tyrrhenian Sea gives features directly connected to its structural tectonic pattern. In this portion of sea, the tectonic movement has worked with a low rate of extension and a slight deformation of the thickened crustal portion (Patacca et al., 1990; Royden, 1988). The magnetic anomaly field is composed mainly by a contribution of low frequency linked to the deep crustal portion as the geologic basement structure. Convoluted to the low frequency components, there are many high frequency anomalies located in correspondence of the structural highs and the buried magmatic structures. The magnetic anomaly associated to the Vercelli seamount shows low amplitude with a value of 50/75 nT (Caratori Tontini et al., 2004), with a maximum positive centered on the bathymetric edifice and a main development running SW-NE like the morphologic trend of the seamount. Previous paleomagnetic studies suggested a very low magnetization for the granitic rocks of Vercelli. Instead the uppermost portion of the intrusive is affected by a thickened crust of alteration (Barbieri et al., 1989). Since the magnetic susceptibility tenure of the Vercelli’s rocks is very low, the magnetic anomaly pattern may be generated by a deep and very large source, like a batholitic structure.
Seamounts provide an increased bottom boundary layer where enhanced vertical mixing may take place. This is particularly significant in relation to the discovery of many more seamounts in the oceans than previously thought. These results indicate the potential for seamounts to provide a significant contribution to the vertical mixing in the deep ocean, that is a core component of the global ocean overturning cycle.
The modelling of hydrodynamics around the seamounts is based on the theoretical concept that if a flow in perfect geostrophic balance encounters an isolated topographic obstacle (e.g. a seamount) an anticyclonic circulation following the isobaths is generated and any fluid inside it is trapped in a stagnant area above the obstacle. This flow pattern is commonly known as Taylor column. In the real ocean a more complex picture evolves mainly depending on the strength of the impinging flow, the geometry of the obstacle and the oceanic background conditions. In general, a steady current encountering and crossing a seamount generates upwelling on the upstream side and downwelling on the downstream side of the obstacle. Enhanced water mixing has been observed in several seamount locations (Lueck and Mudge 1997, Toole et al. 1997, Ledwell et al. 2000). This has significant implications for the passive material transport and retention in the vicinity of the seamount (Roberts et al. 1974, von Stackelberg et al. 1979, Turnewitsch et al. 2004). Asymmetrical distribution of sedimentary deposits is often observed, due to the acceleration of the impinging flow that probably cause the erosion or lack of sediment deposition on one flank of the seamount. Heinz et al. (2004) observed lower levels of superficial sedimentary organic carbon at the summit of the Great Meteor Seamount than along the slopes and the adjacent abyssal plain and suggested that this may be due to the accumulation of material from the top during lateral and vertical transport and to resuspension processes. To isolated water from the surrounding ocean may have a pronounced influence on the retention of planktonic or other organic material in the vicinity of a seamount. Goldner and Champman (1997) have concluded that the retention of particles by the seamount circulation was dependant on the speed of the ramp up of the forcing motion and on the fact that particles close to the region of maximum residual current would be retained longer.
It has been suggested that the hydrodynamic conditions around seamounts may enhance productivity, especially in the case of the so called “shallow seamounts”, whose summit region enters the photic one (Dower et al. 1992, Comeau et al. 1995, Genin 2004). This could be due to the upwelling that occurs over the seamount which may bring deeper and richer waters above the seamount. Here the nutrients may be used if other conditions, such as light and water column stability, are suitable (Perissinotto et al. 1990, Dower et al. 1992, Sime-Ngando et al. 1992). Significantly higher and long-lasting chlorophyll levels have been observed in the proximity of seamounts compared to the surrounding ocean (Dower et al. 1992, Mourino et al. 2001, Nycander and Lacasce 2004). Less commonly, seamounts may exhibit reduction of primary productivity indicators such as chlorophyll-a (Venrick 1991) compared to the neighbouring open ocean. These contrasting results may be due to the large extent of temporal and spatial variability of the physical conditions (e.g. thermocline) both at seasonal and shorter-term scales (Bode and Fernandez 1992).
The scenario of enhanced biological activity and organic matter sedimentation in the water column is consistent with the observed presence of high biomass benthic communities over the seamount (Rogers 1994). Moreover, complex hydrodynamics generated by local constraints may also remove the organic matter (and organisms) produced at the seamount site, leading them away towards other ecosystems (open sea and coastal areas, Parker and Tunnicliffe 1994). Also by an ecological point of view the seamounts are more complex than what expected, and may deeply influence the ecological pathways in large areas.
The seamounts are considered hotspots of marine biodiversity characterised by an abundant and diversified ichthyofauna and very rich benthic filter-feeding communities mainly composed by sponges, hydrozoans, scleractinian corals, antipatharian corals and gorgonians (Koslow et al., 2000). This richness is due to different factors such as the high productivity, the coexistance of different biocoenoses with different bathymetric requirements in a relatively limited space, the role of seamounts as stepping stones in the organism distribution (Thurman &amp; Burton, 2001), and the high rate of endemisms.
The vertical distribution encouraged by seamounts not only enhances the existence of different habitats very close to each other but also a different bathymetric distribution of juveniles and adults of the same species (bathymetric ontogenic migration), which tend to prefer different depths. The ability of these vertical migrations of organisms to transfer nutrients from shallow to deep waters, or vice versa, together with the snowfall of animal and plant detritus that concentrates over them, helps seamounts to engender rich communities and high levels of biomass (Koslow, 1997). The peaks of most of the seamounts in the Mediterranean area are several hundred metres beneath the surface of the sea, which prevents any photosynthetic community from settling on them. However, in the case of the Vercelli seamount, the shallowness of its peaks (50 m depth) together with the transparency of the waters in the area, means that shallow water communities, like the coralligenous assemblage, characterised by bioermes produced by coralliner algae can develop. This fact includes the Vercelli ridge between the seamounts studied up to the present day with the greatest biodiversity. Finally, regarding the dispersion of benthic species, seamounts have been regarded as “stepping stones” or areas of species expansion (Thurman &amp; Burton, 2001).
The importance of seamounts in giving life to endemisms is determined by the history of the tectonic plates and by their function as zones for recolonising and extending species (Butler et al., 2001; Lutjeharms &amp; Heydorn, 1981; Mullineaux &amp; Mills, 1997). Some seamounts or groups of seamounts can be isolated from the surrounding water because of the currents around them, enhancing species isolation. Unfortunately no data about the percentage of endemisms are until now available for the Mediterranean Sea.
Seamounts are therefore recognized to be important nursery habitats and/or refuges for many species of fish and invertebrates. Destruction of these important habitats may therefore enhance degradation of shallow-water reefs, while understanding these unusual communities might help in the recovery efforts of costal ecosystems.
The fish communities of the seamounts are composed by species belonging to many orders and families sharing some eco-morphological and physiological characteristics, allowing them to live in a habitat with strong currents and a high flux of organic matter (Koslow et al., 2001). The seamount fish are strong swimmers, characterised by a high metabolism, long life cycles and slow growth rates (Leman &amp; Beamish, 1984).
The main hard bottom assemblages that we expect to find on the slopes of the Vercelli seamount are the following:
1) Coralligenous assemblage
2) The so called “Twilight Zone” (TZ), the hard bottoms comprised between 60 and 100-120 m depth. In this area live peculiar assemblages characterised by sponges and branched corals, like several species of gorgonians, the parasitic zoanthid Gerardia savaglia and the black coral Antipathella subpinnata recently described as an ubiquitarian species distributed in the TZ all around the Mediterranean coasts
3) The deep-water coral reefs dominated by two colonial scleractinians, Lophelia pertusa and Madrepora oculata, and by the deep sea black coral Leiopathes glaberrima. These deep-water biocoenoses increase the complex three-dimensional structure of the bottom providing several ecological niches for a great number of the species (Rogers, 1999; Tursi et al. 2004). The white coral reefs may function as nurseries for many deep-water species and centres of spreading for the associated fauna, having positive “spill-over” effects on the deep-water demersal resources (Fossa et al., 2002). <<<