Colonization, diversity, and trophic patterns of macrofauna on hard substrates reflect environmental heterogeneity in deep-sea Pacific methane seep communities

Background/Question/Methods

Methane seeps could represent appropriate ecosystems in which to apply metacommunity theory in order to consider the way different sources of heterogeneity influence species diversity and community assembly. Methane seep environments commonly occur on continental margins at depths of several hundred to 2000m. Dynamic and heterogeneous physical and geochemical processes control patterns of fluid flow associated with seeps. Methane seeps harbor chemosynthetic ecosystems in which diverse microbial consortia use reduced compounds (methane and sulfide) to fix organic carbon, and simultaneously precipitate carbonate. Therefore, methane seeps are patchy and hierarchical in nature, existing as a mosaic of habitat patches whether viewed at the scale of decimeters or kilometers. We investigated how the spatial, chemical, and structural heterogeneity inherent to methane seeps influenced colonization dynamics, diversity, and trophic patterns in the community. Substrate colonization experiments were performed at two sites: Hydrate Ridge off Oregon, and Mound 12 off Costa Rica. Different substrates (carbonate, wood, shells, bone) were deployed in replicate at sites of active fluid seepage and inactive areas. About one year later, experiments were collected and all invertebrates were identified and counted. We asked what types of heterogeneity were associated with the density, abundance, and diversity of colonizing macrofauna. Additionally, we used natural stable isotopes (δ¹³C and δ¹⁵N) and isotopic tracer studies to examine whether food web patterns vary with respect to heterogeneity. 

Results/Conclusions

At both Oregon and Costa Rica seeps, chemical heterogeneity appeared to be the factor most responsible for creating patterns in colonizing assemblages and food webs. Fluid flow (ANOSIM, R=0.44, P<0.002) was more important than substrate type (R=0.24, P<0.005) in defining communities. Macrofaunal densities and biomass were highest on experimental substrates placed in active seepage. Similar taxa characterized substrates in active sites separated by thousands of kilometers, including ampharetid and dorvilleid polychaetes, and provannid and pyropeltid gastropods. These results suggest that the abiotic conditions associated with different patches may be critical in sorting species according to niches. However, one-year deployments were insufficient for complete recovery, as diversity lagged that of natural substrates. At the Oregon seeps, δ¹³C values were similar for taxa occupying different substrates, but they were consistently lighter at active seeps (-29.8‰) than at inactive sites (-21.0‰), suggesting a trophic reliance on microbes distributed across chemical gradients. Stable isotopic ratios suggest a dependence on multiple chemoautotrophic pathways in both active and inactive settings, and tracer experiments identified specific metabolic pathways that contribute carbon and nitrogen to macrofauna.