Increased diversity of sessile epibenthos at subtidal hydrothermal vents: seven hypotheses based on observations at Milos Island, Aegean Sea

Abstract

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 CO 2 , 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.

Environmental and Benthic Community Patterns of the Shallow Hydrothermal Area of Secca Delle Fumose (Baia, Naples, Italy)

“…At this geyser site we observed the absence of obligate vent-associated species, which were previously reported for deep-sea vent systems (Tarasov et al, 2005 and reference therein;Schander et al, 2010;Stevens et al, 2015). Our data are consistent with results from other coastal shallow water hydrothermal vents (Dando, 2010;Bianchi et al, 2011). In particular, the fauna around the geyser at the Secca delle Fumose was composed by the most representative species inhabiting the 'background' area (e.g., the polychaetes E. vittata and A. bilineata, the bivalves H. arctica and Venus verrucosa, the ophiuroid Amphipholis sp.…”

Hydrothermal influence on nearshore sediments of Kos Island, Aegean Sea

“…Here, the island of Milos represents a long-established research hotspot on complex interactions between hydrothermal fluids and the sedimentary environment as well as macro-and microfaunal communities and key climatic drivers (e.g. Dando et al 1995Dando et al , 2000Bianchi et al 2011;Giovannelli et al 2013;Yücel et al 2013;Megalovasilis 2014). Much less is known in this domain for several neighbouring islands, including Kos where the present study was conducted.…”

Persistent natural acidification drives major distribution shifts in marine benthic ecosystems

“…Most of the CO 2 vent systems studied so far were placed in very shallow waters (3 -5 m depth), and therefore are poorly representative of the broad range of habitats that occur on the continental shelf. Very few studies investigating the biological effects of these systems at larger water depths have been performed in hydrothermal vents in the Aegean Sea [12] and in the northern Gulf of California [13], at 10-35 m and at 70 -200 m, respectively. Dominant habitats at mesophotic depths (the deepest part of the photic zone where light penetration is low, typically from 30-40 m to over 150 m) are usually characterized by a large dominance of calcifying organisms, such as endangered coralligenous outcrops and rhodolith beds in the Mediterranean Sea, which display a notable carbonate production (170-660 g CaCO 3 m -2 yr -1 ) [15,16].…”

“…However, a small number of CO 2 vent systems have been investigated to date [9][10][11][12][13]. These systems share significant decreases in the diversity, biomass and trophic complexity of benthic marine assemblages, major declines in the number of many calcifying organisms, and increased abundance of erect macroalgae, seagrasses or soft-corals [9][10][11][12][13][14]. Most of the CO 2 vent systems studied so far were placed in very shallow waters (3 -5 m depth), and therefore are poorly representative of the broad range of habitats that occur on the continental shelf.…”