UC San Diego

Methane seeps in the deep sea are chemosynthetic ecosystems that host highly productive communities, are spatially heterogeneous and temporally dynamic, and enhance regional diversity on all continental margins. The goal of this dissertation was to examine the relative influences of chemical, substrate, and spatial heterogeneity on macrofaunal community structure and dynamics within three Pacific Ocean seep ecosystems. The discovery of the Del Mar methane seep, only tens of meters across but with many microhabitats and associated species, highlights the ecosystem services seeps offer humans. These include exporting primary production to support food webs, sequestering inorganic carbon, providing habitat for fishery species, and enhancing biodiversity through microhabitat heterogeneity. The effects of substrate and seepage heterogeneity on community structure and dynamics were investigated at two methane seeps : Mound 12, Costa Rica and Hydrate Ridge, Oregon. Deployment of substrates representative of chemosynthetic ecosystems [authigenic carbonate (seeps), wood (sunken wood falls), bones (whale falls), and biogenic material (seep ecosystem engineers)] at sites with varying influence of active fluid seepage led to several key findings regarding macrofaunal colonization patterns. After about one year, active seepage best explained the structure of colonizing macrofaunal communities, which had species identities and densities resembling native carbonates and were dominated by seep-endemic gastropods like Provanna spp., Pyropelta spp., and Neolepetopsidae. At inactive sites, colonization trends were more variable with reduced macrofaunal densities but higher diversity compared to active sites. There was a mixture of regional margin species, some seep endemics, and (at Hydrate Ridge) substrate specialists that bored into wood (Xylophaga washingtona) and bone (Osedax spp.). As bare substrates were colonized or when carbonates were transplanted between active and inactive sites, the trophic structure as measured by stable isotopes ([delta¹³C, [delta¹⁵N) better resembled in situ carbonate communities at active seeps. These results suggest that niche-based dynamics regulate diversity patterns on hard substrates at active methane seeps, while competition-colonization tradeoffs and successional dynamics become increasingly important for nearby inactive carbonate communities. A metacommunity framework will be useful for those interested in maintaining the integrity of chemosynthetic ecosystems as the human footprint expands in the deep sea