Macrobenthos community structure and trophic relationships within active and inactive Pacific hydrothermal sediments

Abstract: Hydrothermal fluids passing through sediments create a habitat hypothesized to influence the community structure of infaunal macrobenthos. Here we characterize the density, biomass, species composition, diversity, distributions, lifestyle, and nutritional sources of macroinfauna in hydrothermal sediments in NE and SW Pacific settings, and draw comparisons in search of faunal attributes characteristic of this habitat. There is increasing likelihood that seafloor massive sulfide deposits, associated with active and inactive hydrothermal venting, will be mined commercially. This creates a growing imperative for a more thorough understanding of the structure, dynamics, and resilience of the associated sediment faunas, and has stimulated the research presented here. Macrobenthic assemblages were studied at Manus Basin (1430–1634m, Papua New Guinea [PNG]) as a function of location (South Su vs. Solwara 1), and hydrothermal activity (active vs. inactive), and at Middle Valley (2406–2411m, near Juan de Fuca Ridge) as a function of habitat (active clam bed, microbial mat, hot mud, inactive background sediment). The studies conducted in PNG formed part of the environmental impact assessment work for the Solwara 1 Project of Nautilus Minerals Niugini Limited. We hypothesized that hydrothermally active sites should support (a) higher densities and biomass, (b) greater dominance and lower diversity, (c) a higher fraction of deposit feeders, and (d) greater isotopic evidence for chemosynthetic food sources than inactive sites. Manus Basin macrofauna generally had low density (<1000ind.m−2) and low biomass (0.1–1.07gm−2), except for the South Su active site, which had higher density (3494ind.m−2) and biomass (11.94gm−2), greater dominance (R1D=76%), lower diversity and more spatial (between-core) homogeneity than the Solwara 1 and South Su inactive sites. Dominant taxa at Manus Basin were Spionidae (Prionospio sp.) in active sediments, and tanaids and deposit-feeding nuculanoid bivalves in active and inactive sediments. At Middle Valley, hot mud sediments supported few animals (1011indm−2) and low biomass (1.34gm−2), while active clam bed sediments supported a high-density (19,984indm−2), high-biomass (4.46gm−2), low-diversity assemblage comprised of largely orbiniid and syllid polychaetes. Microbial mat sediments had the most diverse assemblage (mainly orbiniid, syllid, dorvilleid, and ampharetid polychaetes) with intermediate densities (8191indm−2) and high biomass (4.23gm−2). Fauna at both Manus Basin active sites had heavy δ13C signatures (−17‰ to −13‰) indicative of chemosynthetic, TCA-cycle microbes at the base of the food chain. In contrast, photosynthesis and sulfide oxidation appear to fuel most of the fauna at Manus Basin inactive sites (δ13C=−29‰ to −20‰) and Middle Valley active clam beds and microbial mats (δ13C=−36‰ to −20‰). The two hydrothermal regions, located at opposite ends of the Pacific Ocean, supported different habitats, sharing few taxa at the generic or family level, but both exhibited elevated infaunal density and high dominance at selected sites. Subsurface-deposit feeding and bacterivory were prevalent feeding modes. Both the Manus Basin and Middle Valley assemblages exhibit significant within-region heterogeneity, apparently conferred by variations in hydrothermal activity and associated biogenic habitats. [Copyright &y& Elsevier]