Cristina Gambi

Deep-Sea Biodiversity in the Mediterranean Sea: The Known, the Unknown, and the Unknowable

Deep-sea ecosystems represent the largest biome of the global biosphere, but knowledge of their biodiversity is still scant. The Mediterranean basin has been proposed as a hot spot of terrestrial and coastal marine biodiversity but has been supposed to be impoverished of deep-sea species richness. We summarized all available information on benthic biodiversity (Prokaryotes, Foraminifera, Meiofauna, Macrofauna, and Megafauna) in different deep-sea ecosystems of the Mediterranean Sea (200 to more than 4,000 m depth), including open slopes, deep basins, canyons, cold seeps, seamounts, deep-water corals and deep-hypersaline anoxic basins and analyzed overall longitudinal and bathymetric patterns. We show that in contrast to what was expected from the sharp decrease in organic carbon fluxes and reduced faunal abundance, the deep-sea biodiversity of both the eastern and the western basins of the Mediterranean Sea is similarly high. All of the biodiversity components, except Bacteria and Archaea, displayed a decreasing pattern with increasing water depth, but to a different extent for each component. Unlike patterns observed for faunal abundance, highest negative values of the slopes of the biodiversity patterns were observed for Meiofauna, followed by Macrofauna and Megafauna. Comparison of the biodiversity associated with open slopes, deep basins, canyons, and deep-water corals showed that the deep basins were the least diverse. Rarefaction curves allowed us to estimate the expected number of species for each benthic component in different bathymetric ranges. A large fraction of exclusive species was associated with each specific habitat or ecosystem. Thus, each deep-sea ecosystem contributes significantly to overall biodiversity. From theoretical extrapolations we estimate that the overall deep-sea Mediterranean biodiversity (excluding prokaryotes) reaches approximately 2805 species of which about 66% is still undiscovered. Among the biotic components investigated (Prokaryotes excluded), most of the unknown species are within the phylum Nematoda, followed by Foraminifera, but an important fraction of macrofaunal and megafaunal species also remains unknown. Data reported here provide new insights into the patterns of biodiversity in the deep-sea Mediterranean and new clues for future investigations aimed at identifying the factors controlling and threatening deep-sea biodiversity.

Benthic microbial loop functioning in coastal lagoons: a comparative approach

Abstract Coastal lagoons are highly variable and dynamic systems that have been rarely investigated in terms of benthic microbial loop. We present here the results of a comparative study aimed at investigating factors and benthic processes potentially affecting microbial loop functioning in three lagoon systems (Goro, Lesina and Marsala lagoons). The three lagoons were characterised by different geo-morphological, trophic and ecological features. We determined organic matter quantity and biochemical composition, exo-enzymatic activities, bacterial biomass and bacterial carbon production together with heterotrophic nanoflagellate and meiofauna biomass. These variables allowed providing estimates of biomass ratios between different benthic compartments and gathering information on C transfer in different systems. The results of this study indicate that organic matter composition played a primary role on microbial loop ability of channelling C-biomass to higher trophic levels. In coastal lagoons, indeed, quantity and quality of sediment organic matter control rates of organic matter degradation, turnover rates (through breakdown of large macromolecules) and utilisation by benthic heterotrophic organisms (bacterial C production). Exo-enzymatic activities in all lagoons investigated were generally low, when compared to coastal marine systems, and lowest rates were observed in systems characterised by refractory organic pools. Our results indicate that the autotrophic contribution to the biopolymeric carbon pools, which influences the ratio of autotrophic to heterotrophic biomass, is a key factor regulating the functional efficiency of the benthic system. The structure of the benthic microbial loop varied according to the different characteristics of the lagoons: from a classical “detritus sink” system (i.e., the Marsala lagoon, where organic matter was highly refractory), to the “source” system (Lesina lagoon, which displayed an efficient C transfer to higher trophic levels, mediated by benthic bacteria).

Meiofauna of the Adriatic Sea: present knowledge and future perspectives

Owing to technical problems and difficult taxonomic identification, meiofauna have been generally less studied than macrofauna. However, the role of meiofauna in marine ecosystem functioning, and their effective and rapid response to anthropogenic alterations and climatic changes have recently been acknowledged, leading to increasing scientific and applied interest. At present, systematic and biogeographic knowledge of the meiofauna of the Adriatic Sea is extremely heterogeneous, because most of the data are limited to a few taxa and the sampled areas are scattered, being located mainly in the coastal areas of the northern basin. Analysis of the composition and distribution of meiobenthic groups in the Adriatic Sea highlights the presence of several endemisms. Meiofauna also include bioindicator taxa, which allow assessment of the quality of marine sediments; this is particularly useful in systems characterised by the synergistic effect of different forms of anthropogenic impact, such as the Adriatic basin. Current knowledge about the ecology of the meiofauna and use of this component in applied ecological studies, along with the availability of a standardised protocol for the analysis of meiofaunal assemblages, allows us to recommend formal acknowledgement of the need to integrate information derived from the analysis of macrofauna with information derived from the study of meiofauna. Future research based on the simultaneous use of both of these benthic components will allow faster and more accurate evaluation of the response of coastal marine ecosystems to anthropogenic disturbance.

Fish-farm impact on metazoan meiofauna in the Mediterranean Sea: Analysis of regional vs. habitat effects.

The worldwide exponential growth of off-shore mariculture is raising severe concerns about the impacts of this industry on marine habitats and their biodiversity. We investigated the metazoan meiofaunal response to fish-farm impact in four regions of the Mediterranean Sea. Meiofaunal assemblages were investigated in two habitats (seagrass meadows of Posidonia oceanica and non-vegetated soft bottoms) comparing sites receiving faeces and uneaten food pellets from fish farms to control sites. We report here that, consistently across different regions, the meiofaunal abundance typically responded positively to fish-farm effluents. Biodeposition caused also significant changes in assemblage structure and the reduction in the richness of higher meiofaunal taxa, but the multivariate analysis of variance revealed that the effects were region- and habitat-specific. In non-vegetated systems, three of the four regions investigated displayed significant effects of the fish farms on richness of meiofaunal taxa. In vegetated habitats, meiofauna did not respond to biodeposition (except in one region), suggesting that seagrass meadows can mask the effects of fish-farm effluents on benthic biodiversity. We conclude that different indicators of fish-farm impact are needed in vegetated and non-vegetated benthic systems.

Trophic conditions and meiofaunal assemblages in the Bari Canyon and the adjacent open slope (Adriatic Sea)

Due to their topographic features, submarine canyons are generally sites of intense shelf-slope exchanges of water and material, fuelling the deep basins with large quantities of sediment exiting from the continental shelf. In order to provide new insights about the role of submarine canyons in controlling the relationships between food availability and benthic biodiversity patterns along the continental slope, we investigated the quantity and the biochemical composition of sediment organic matter and the abundance of meiofaunal assemblages in the sediments along two different branches of a canyon and in an adjacent open slope located in the Bari margin (Adriatic Sea). Our results highlight that even twin branches within the same canyon may exhibit very large differences in the quantity, depth-related patterns and biochemical composition of sediment organic matter as well as of meiofaunal abundance. We also report here that the trophic relationships in the canyon sediments are tightly connected with the hydrodynamic conditions and that the steeper and the more flushed the canyon the more hostile environment for the benthos.

Trophic state, ecosystem efficiency and biodiversity of transitional aquatic ecosystems: analysis of environmental quality based on different benthic indicators

Estuaries and coastal lagoons are characterized by a strong spatial and temporal variability of physicochemical characteristics and productivity patterns. In these environments, the magnitude and direction of the ecological responses to inorganic nutrient increase (i.e. eutrophication) are difficult to predict. In the framework of the project, New Indicators of Trophic state and environmental quality of marine coastal ecosystems and transitional environments (NITIDA), we analysed benthic indicators of trophic state, ecosystem efficiency, and environmental quality in four different transitional environments. The trophic state of the sediments was assessed in terms of quantity and bioavailability of sediment organic C pools; ecosystem efficiency was determined in terms of the prokaryote efficiency in exploiting enzymatycally degraded organic C; environmental quality was determined in terms of meiofaunal diversity. Here, we provide a synopsis of the results obtained and a meta-analysis of the scores assessments obtained using the different ecological indicators of environmental quality and demonstrate that trophic state, ecosystem efficiency, and biodiversity in transitional ecosystems are closely linked. We conclude that the assessment of the environmental quality of transitional ecosystems should be based upon a battery of trophic state indicators and ‘sensors’ of ecosystem functioning, efficiency, and quality.

Biodiversity response to experimental induced hypoxic-anoxic conditions in seagrass sediments

The effects of induced hypoxic-anoxic conditions on the metazoan meiofaunal assemblages and nematode diversity were investigated with an in situ experiment in a Posidonia oceanica meadow. The experiment, of the duration of five months, was performed in three experimental sets of plots. Two of them were enriched with organic matter to induce anoxic conditions (1 set with sucrose and 1 set with sugar plus nutrients, i.e. nitrogen and phosphorus) whereas the last set of plots was kept undisturbed and used as Control. Metazoan meiofauna displayed a fast response to the induced anoxic conditions with an immediate reduction of the richness of taxa (only nematodes and copepods tolerated the hypoxic-anoxic conditions). Nematodes were the most tolerant organisms as their species richness did not change in hypoxic-anoxic conditions, but their species composition and trophic structure displayed significant changes. Some genera (Desmoscolex and Bolbolaimus) were replaced by other (Chromadorella, Sabatiera and Polysigma) more tolerant to the extreme conditions. No significant differences were observed in the Control plots, whereas in treated plots, selective deposit feeders and predators decreased significantly, being replaced by non-selective deposit feeders and epistrate feeders. These results indicate that, events causing a reduction in oxygen availability, can have an impact on the nematode beta-diversity and functional diversity with potential important implications on the benthic food web and functioning of the seagrass systems.

Short-Term Impact of Clam Harvesting on Sediment Chemistry, Benthic Microbes and Meiofauna in the Goro Lagoon (Italy)

The effects of clam harvesting on sediment organic matter, bacteria, heterotrophic nanoflagellates, ciliates and meiofauna were investigated through a short-term experiment carried out in the Goro lagoon during summer 2000. Sediment samples were collected in two areas: an impacted and an undisturbed (both 25 m2), before and after (5, 24 and 48 h) clam harvesting. Immediately after sediment reworking, total organic matter content decreased for about 20%. Different organic compounds displayed different redeposition patterns resulting in the alteration of the biochemical composition of sediment organic matter. Bacterial abundance decreased significantly after clam harvesting, but microbial enzymatic activities and bacterial C production were relatively unaffected. Heterotrophic nanoflagellates displayed a positive response to sediment disturbance, whereas ciliate and meiofaunal abundance did not display any significant response. Results of this short-term field experiment suggest that clam harvesting determines a positive impact on nutritional availability of sediment organic matter, thus facilitating organic carbon transfer to higher trophic levels of the benthic microbial loop.

Relationships between meiofaunal biodiversity and prokaryotic heterotrophic production in different tropical habitats and oceanic regions.

Tropical marine ecosystems are among the most diverse of the world oceans, so that assessing the linkages between biodiversity and ecosystem functions (BEF) is a crucial step to predict consequences of biodiversity loss. Most BEF studies in marine ecosystems have been carried out on macrobenthic diversity, whereas the influence of the meiofauna on ecosystem functioning has received much less attention. We compared meiofaunal and nematode biodiversity and prokaryotic heterotrophic production across seagrass, mangrove and reef sediments in the Caribbean, Celebes and Red Seas. For all variables we report the presence of differences among habitats within the same region, and among regions within the same habitat. In all regions, the richness of meiofaunal taxa in reef and seagrass sediments is higher than in mangrove sediments. The sediments of the Celebes Sea show the highest meiofaunal biodiversity. The composition of meiofaunal assemblages varies significantly among habitats in the same region. The nematode beta diversity among habitats within the same region is higher than the beta diversity among regions. Although one site per habitat was considered in each region, these results suggest that the composition of meiofaunal assemblages varies primarily among biogeographic regions, whereas the composition of nematode assemblages varies more considerably among habitats. Meiofauna and nematode biodiversity and prokaryotic heterotrophic production, even after the removal of covariate effects linked with longitude and the quantity and nutritional quality of organic matter, are positively and linearly linked both across regions and within each habitat type. Our results confirm that meiofauna and nematode biodiversity may influence benthic prokaryotic activity, which, in turn, implies that diversity loss could have negative impacts on ecosystem functioning in these systems.

Ecosystem effects of dense water formation on deep Mediterranean Sea ecosystems: an overview

Natural episodic events, such as gravity flows, submarine landslides, and benthic storms can determine severe modifications in the structure and functioning of deep-sea ecosystems. Here, we report and compare the ecosystem effects produced by dense water formation events that occurred in the Gulf of Lions (NW Mediterranean) and the Aegean Sea (NE Mediterranean). In both regions, the rapid sinking of cold dense waters, driven by regional meteorological forcings, results in important immediate modifications that can be summarised in: (i) increased organic matter content in the deep basin; (ii) diminished benthic abundance; and (iii) changes of benthic biodiversity. At longer time scale the analysis reveals, however, different resilience times in the two regions. The Gulf of Lions is characterized by a very fast (months) recovery whereas the Aegean Sea shows much longer (45 years) resilience time. New long-term studies are further needed to identify the potential effects that changes in the duration, intensity and frequency of episodic events could have on the structure, biodiversity and functioning of the deep Mediterranean Sea under environmental and climate change scenarios.