| Abstract |
- Carbonate communities:The activity of anaerobic methane oxidizing microbes facilitates
precipitation of vast quantities of authigenic carbonate at methane seeps. Here we demonstrate
the significant role of carbonate rocks in promoting diversity by providing unique habitat
and food resources for macrofaunal assemblages at seeps on the Costa Rica margin
(400–1850 m). The attendant fauna is surprisingly similar to that in rocky intertidal shores,
with numerous grazing gastropods (limpets and snails) as dominant taxa. However, the
community feeds upon seep-associated microbes. Macrofaunal density, composition, and
diversity on carbonates vary as a function of seepage activity, biogenic habitat and location.
The macrofaunal community of carbonates at non-seeping (inactive) sites is strongly
related to the hydrography (depth, temperature, O2) of overlying water, whereas the fauna
at sites of active seepage is not. Densities are highest on active rocks from tubeworm
bushes and mussel beds, particularly at the Mound 12 location (1000 m). Species diversity
is higher on rocks exposed to active seepage, with multiple species of gastropods and polychaetes
dominant, while crustaceans, cnidarians, and ophiuroids were better represented
on rocks at inactive sites. Macro-infauna (larger than 0.3 mm) from tube cores taken in
nearby seep sediments at comparable depths exhibited densities similar to those on carbonate
rocks, but had lower diversity and different taxonomic composition. Seep sediments
had higher densities of ampharetid, dorvilleid, hesionid, cirratulid and lacydoniid polychaetes,
whereas carbonates had more gastropods, as well as syllid, chrysopetalid and
polynoid polychaetes. Stable isotope signatures and metrics: The stable isotope signatures
of carbonates were heterogeneous, as were the food sources and nutrition used by
the animals. Carbonate δ13Cinorg values (mean = -26.98‰) ranged from -53.3‰to +10.0‰,
and were significantly heavier than carbonate δ13Corg (mean = -33.83‰), which ranged from -74.4‰to -20.6‰. Invertebrates on carbonates had average δ13C (per rock) = -31.0‰
(range -18.5‰to -46.5‰) and δ15N = 5.7‰(range -4.5‰to +13.4‰). Average δ13C values
did not differ between active and inactive sites; carbonate fauna from both settings depend
on chemosynthesis-based nutrition. Community metrics reflecting trophic diversity (SEAc,
total Hull Area, ranges of δ13C and δ15N) and species packing (mean distance to centroid,
nearest neighbor distance) also did not vary as a function of seepage activity or site. However,
distinct isotopic signatures were observed among related, co-occurring species of
gastropods and polychaetes, reflecting intense microbial resource partitioning. Overall, the
substrate and nutritional heterogeneity introduced by authigenic seep carbonates act to promote
diverse, uniquely adapted assemblages, even after seepage ceases. The macrofauna
in these ecosystems remain largely overlooked in most surveys, but are major contributors
to biodiversity of chemosynthetic ecosystems and the deep sea in general.
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