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Macrofauna biodiversity and ecosystem functioning in the deep-sea. Mediterranean sediments: analysis at different spatial scales

April 10, 2013

The overall aim of the study was to investigate the relationship(s) between macrofauna diversity and ecosystem functioning (BEF) in the deep Mediterranean sea at different spatial scales: small scales (100s of meters to kilometres), a larger spatial scale (100s of kilometres) and at a macroscale (1000s of kilometres) and in relation to major chemical-physical and trophic variables. In order to accomplish this objective, I tested the following hypotheses: 1. the deep-sea macrofauna diversity and ecosystem functioning are significantly related; 2. shape and strength of the BEF relationships vary across spatial scales of observation and environmental settings. The study has been conducted with a dual approach, first understanding the macrofaunal biodiversity patterns changing spatial scales of investigation and comprising different environmental features and than testing the presence and the nature of a relationship between diversity and ecosystem functioning. On a macro-scale (1000s kilometres), it has been detected a major longitudinal decrease in macrofaunal standing stock and species richness moving eastward along the Mediterranean Sea. The influence of depth on macrobenthic assemblages variability was weaker than the longitude. Clear dissimilarities emerged in species composition not only between basins (Westerns vs Eastern) and among stations at different depth, but also among sites within the same basin and among stations at the same depth but at different longitude. Beside longitude and water depth, also the availability of food and, secondarily, the sedimentary features had some influence on macrofauna standing stock and biodiversity patterns in the deep Mediterranean Sea. At a smaller (from 100s of meters up to kilometres) spatial scale along the Sardinia slope, the macrobenthic stock displayed major changes across water depths, rather than between stations at the same depth. The macrofauna species composition (i.e. turnover diversity) largely varied along the one-kilometre spatial scale and with increasing water depth, even if below 1500m depth communities were more homogeneous both across and along depths. At a larger (100s kilometres) spatial scale, the comparison of two different continental slopes (Balearic and Sardinian slopes) revealed that macrofaunal standing stock, alpha and turnover diversity varied significantly between the two slopes. Overall, higher values of macrofaunal abundance, biomass and alpha diversity characterized the Sardinia slope compared to the Balearic slope. Along both slope systems the turnover diversity in macrobenthic species composition significantly differed between stations at different depth up to 1800 m. Below that depth, that is among stations at 1800m and 2400m water depth, the macrofaunal species composition appeared to be homogenous. Previous studies on a quasi-global scale observed the presence of a positively exponential biodiversity and ecosystem functioning relationship linking nematodes biodiversity and various deep-sea ecosystem functioning and efficiency proxies, which would mean a prevalence of mutualistic interactions in deep-sea communities. The presence of a positive and significant relationship between the deep-sea macrofaunal biodiversity inhabiting the deep Mediterranean Sea and different measures of ecosystem functioning was observed here, at the largest spatial scale of investigation. The biodiversity was positively linked also to ecosystem efficiency and functional diversity, underlying that high diversified macrofaunal communities may carry on efficiently different functions of the deep-sea ecosystem and sustain the presence of a higher number of functional traits, that in turn can enhance higher levels of ecosystem efficiency. On a macroscale it’s possible to think to the presence of mutualistic interactions also for the deep-sea macro-fauna. When the BEF relationship was assessed at a smaller spatial scale (Sardinia slope), almost all the relationships detected during a first analysis, appeared not significant if the effect of environmental characteristics was statistically discarded. The co-variation of environmental factors at a local scale may overwhelm the specific effect of diversity and thus drive or make idiosyncratic the relationship between diversity and ecosystem functioning. Considering a spatial scale of 100s of kilometers and comprising both slope systems, positive and causal BEF relationships emerged. When the two investigated slope areas were considered separately and the effect of environmental features was discarded, opposite results have been reported. Along the Balearic slope, the relationships remained significant, leading to think to a causal nature of the relations between the macrofauna biodiversity and ecosystem functioning. Along the Sardinia slope any BEF relationship subsisted, being overwhelmed by the environmental factors considered. That is that the larger the spatial scale considered, the higher the frequency of positive and highly significant BEF relationships. I can conclude that the shape, but in particular the strength, of the BEF relationships varied across different spatial scales of observation and environmental settings. When different spatial scales and environments (i.e. slope systems) were considered, different macrobenthic communities with disparate functional traits were involved each time. It may be possible that the different functional traits included and/or excluded at different spatial scales might have a certain influence in driving the relationships between biodiversity and functions of an ecosystem. Results emerged from the present study indicate that overall positive species interactions characterized the deep macrobenthic component inhabiting the Mediterranean basin. Nevertheless, these interactions and the enhancement of functions of the ecosystem may be highly sensitive to the spatial scale and system considered.

Baldrighi, E, Università Politecnica delle Marche,  2010/2011. Doctoral Thesis.

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