Macrofaunal assemblages from mud volcanoes in the Gulf of Cadiz: abundance, biodiversity and diversity partitioning across spatial scales

Abstract

Abstract. The Gulf of Cadiz is an extensive seepage area in the South Iberian Margin (NE Atlantic) encompassing over 40 mud volcanoes (MVs) at depths ranging from 200 to 4000 m. The area has a long geologic history and a central biogeographic location with a complex circulation assuring oceanographic connectivity with the Mediterranean Sea, Equatorial and Northern Atlantic regions. The geodynamics of the region promotes a notorious diversity in the seep regime despite the relatively low fluxes of hydrocarbon-rich gases. We analyse quantitative samples taken during the cruises TTR14, TTR15 and MSM01-03 in seven mud volcanoes grouped into Shallow MVs (Mercator: 350 m, Kidd: 500 m, Meknès: 700 m) and Deep MVs (Captain Arutyunov: 1300 m, Carlos Ribeiro: 2200 m, Bonjardim: 3000 m, Porto: 3900 m) and two additional reference sites (ca. 550 m). Macrofauna (retained by a 500 μm sieve) was identified to species level whenever possible. The samples yielded modest abundances (70–1567 ind. per 0.25 m2) but a number of species among the highest ever reported for cold seeps. Among the 366 recorded species, 22 were symbiont-hosting bivalves (Thyasiridae, Vesicomyidae, Solemyidae) and tubeworms (Siboglinidae). The multivariate analyses indicated significant differences between Shallow and Deep MVs: The environmental conditions at the Shallow MVs makes them highly permeable to the penetration of background fauna leading to high diversity of the attendant assemblages (H': 2.92–3.94; ES(100): 28.3–45.0; J': 0.685–0.881). The Deep MVs showed, in general, contrasting features but were more heterogeneous (H': 1.41–3.06; ES(100): 10.5–30.5; J': 0.340–0.852) and often dominated by one or more siboglinid species. The rarefaction curves confirmed the differences in biodiversity of Deep and Shallow MVs as well as the approximation of the latter to the reference sites. The Bray–Curtis dissimilarity demonstrated the high β-diversity of the assemblages, especially in pairwise comparisons involving samples from the deeper MVs. Diversity partitioning assessed for species richness, Hurlbert's expected number of species and Shannon–Wiener index confirmed the high β-diversity across different spatial scales (within MVs, between MVs, between Deep and Shallow MVs). We suggest that historical and contemporary factors with differential synergies at different depths contribute to the high α-, β- and γ-diversity of the mud volcano faunal assemblages in the Gulf of Cadiz.