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Research on subtidal hydrothermal vent ecosystems at Milos, Hellenic Volcanic Arc (Aegean Sea), suggested that vent activity increased the species richness of sessile epibenthic assemblages. Based on 303 species found in 6 sites (3 close to vents, 3 farther away), the present paper uses correspondence analysis and species/samples curves to examine the species composition and richness of these assemblages. Differences due to vent proximity were more important than those due to bottom depth and distance from the shore. Diversity was confirmed to be higher near the vents, although none of the 266 species found at the vent sites can be considered as obligate vent-associated species. Seven different, although not mutually exclusive, hypotheses are discussed to explain the pattern of increased epibenthic species diversity at the vent sites, namely: (i) vents represent an intermediate disturbance, inducing mortality by the emission of toxic fluids; (ii) higher winter temperature allows for the occurrence of warm-water species, which add to the regional background; (iii) venting disrupts the homogeneity of the water bottom layer, increasing bottom roughness and hence habitat heterogeneity; (iv) deposition of minerals and enhanced bioconstruction by Ca enrichment increment habitat provision; (v) fluid emission induces advective mechanisms that favour recruitment; (vi) vents emit CO2, nutrients and trace elements that enhance primary productivity; and (vii) bacterial chemosynthesis add to photosynthesis to provide a diversity of food sources for the fauna. [ABSTRACT FROM AUTHOR]

Abstract: The performances of two commonly used non-destructive sampling procedures for rocky benthic assemblages (i.e. photography and visually assessed quadrats) were compared. A damaging human activity, date mussel (Lithophaga lithophaga) harvesting (DMH), was chosen. Directly impacted sites were compared with reference conditions (controls). Both visual quadrats and photography were equally able to detect differences between impacted situations and controls. However, visual quadrats showed high variability among replicates and estimated higher species richness for controls, while photography did so for impacts. Pooling photos in a ‘sum photo quadrat’ showed that differences between the two methods are due to sampling unit size rather than sampling procedure itself. As a small sampling unit is unavoidable with photography, visual quadrats should be preferred in investigating shallow rocky reefs for their larger size; however, longer working time underwater with quadrats does not allow for numerous replicates. Pooling many photos to reach sampling sizes comparable with those of quadrats may be a valid alternative to reconcile image resolution and areal coverage. [Copyright &y& Elsevier]

Abstract: Quantification and comparisons of the structure of subtidal marine communities are essential for answering a suite of basic and applied ecological questions. However, time constraint associated with sampling underwater by scuba diving is a major obstacle to achieve adequate sampling effort. We tested a technique for rapid-assessment of hard substrate epibenthic communities, based on counts of the frequency of different taxa within sampling units (quadrats). This technique was compared to a broadly used technique, based on visual estimation of percent cover of taxa, to determine whether a significant amount of information is lost when using the rapid assessment technique. We applied both techniques in three case studies from the Ligurian Sea, in the NW Mediterranean. Field measurements showed that the same communities can be surveyed twice as fast with the frequency-count compared to the cover-estimation. Structural variables of communities as species richness and diversity yield almost identical values with the two techniques while dominance showed underestimated by frequency-count when compared to the cover-estimation. Multivariate analyses indicated that the two techniques yield similar results, highlighting similar patterns of variation in all three case studies, particularly when data are transformed (square-root or fourth-root transformation). Our findings suggest that applying the frequency-count technique to the assessment of benthic communities is an effective strategy for increasing sampling effort when working underwater at depth. [ABSTRACT FROM AUTHOR]