Abstract
Cold seep communities with distinctive chemoautotrophic fauna occur where hydrocarbon-rich fluids escape from the seabed. We describe community composition, population densities, spatial extent, and within-region variability of epifaunal communities at methane-rich cold seep sites on the Hikurangi Margin, New Zealand. Using data from towed camera transects, we match observations to information about the probable life-history characteristics of the principal fauna to develop a hypothetical succession sequence for the Hikurangi seep communities, from the onset of fluid flux to senescence. New Zealand seep communities exhibit taxa characteristic of seeps in other regions, including predominance of large siboglinid tubeworms, vesicomyid clams, and bathymodiolin mussels. Some aspects appear to be novel; however, particularly the association of dense populations of ampharetid polychaetes with high-sulphide, high-methane flux, soft-sediment microhabitats. The common occurrence of these ampharetids suggests they play a role in conditioning sulphide-rich sediments at the sediment-water interface, thus facilitating settlement of clam and tubeworm taxa which dominate space during later successional stages. The seep sites are subject to disturbance from bottom trawling at present and potentially from gas hydrate extraction in future. The likely life-history characteristics of the dominant megafauna suggest that while ampharetids, clams, and mussels exploit ephemeral resources through rapid growth and reproduction, lamellibrachid tubeworm populations may persist potentially for centuries. The potential consequences of gas hydrate extraction cannot be fully assessed until extraction methods and target localities are defined but any long-term modification of fluid flow to seep sites would have consequences for all chemoautotrophic fauna.
Highlights
Cold seeps are sites where fluids enriched with hydrocarbons, primarily methane, emerge from the seabed
Photographs and physical samples indicated two broad categories of carbonate formation: light-brown blocks embedded in soft sediments, usually associated with high densities of chemoautotrophic fauna, and referred to as ‘‘chemoherm’’ [14,29] or ‘‘fresh’’ [30]; and darker, more extensive, ‘‘weathered-looking’’ [30] rock associated with larger geomorphologic structures, sessile heterotrophic taxa and few, if any, chemoautotrophic fauna
Four mobile heterotrophic invertebrate taxa characteristic of surrounding slope habitats were commonly observed within and around seep sites: lithodid crabs were seen at the edges of sulphidic patches containing Calyptogena clams (Figure 3 D); pagurid crabs were common on carbonate and sediment substrata at Hihi, Builder’s Pencil and North Tower; predatory gastropod molluscs were common, at Piwakawaka, North Tower, Bear’s Paw, and Tui, and the regular echinoid Gracilechinus multidentatus occurred in high densities around the periphery of Tui (Figure 3 C)
Summary
Cold seeps are sites where fluids enriched with hydrocarbons, primarily methane, emerge from the seabed. They are known from both active and passive continental margins, typically occurring on the continental slope and much shallower [1,2,3]. Metazoan communities at seeps are characterised by high densities and relatively low diversity of invertebrate taxa which have evolved symbioses with methane- or sulphide-oxidising bacteria, enabling them to exploit the energy potential of the emerging fluids [4]. Cold seeps are known to occur on continental margins throughout the global ocean, including the Arctic and Antarctic [5,6] variations in the composition of seep communities in different parts of the world are not well resolved [7]. Concern has been raised about the threats that seep sites and their communities face from trawling and future mining for gas hydrates [8,9], and the need to provide relevant ecological information to inform management strategies for protection of potentially vulnerable seep communities [10]
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