Biodiversity and food web patterns in the deep sea: why food quality matters

Abstract

Despite multiple drivers of ocean change, few studies have addressed the role of food quality in structuring deep-sea benthic communities and food webs, and therefore its role in ecosystem processes (functioning). Based on a review of published data, I showed that food quality (i.e., nitrogen, pigment, lipid, carbohydrate, protein content) alters marine trophic guild composition, and thus food web structure, differently. In sampling heterogeneous submarine canyon and chemosynthetic ecosystems as model environments for food web and biodiversity studies, I found that patchy food quality distribution in Barkley submarine canyon influenced macroinfaunal community structure more strongly at smaller spatial scales (10’s of m), whereas major stressors (i.e., oxygen/depth) acted over larger scales (100’s of m). Increased food patchiness at a hydrate outcrop site (Barkley Hydrates), compared to sites located 20 and 600 m away, was related to increased macroinfaunal trophic diversity. Food quality explained substantial variation (~ 33 %) in macroinfaunal community structure, but H₂S toxicity likely explained much of the remaining variation. Increasing spatial resolution of analysis (i.e., ≤ 10 m) indicated a 9-15 m influence radius around the most methane-active site (i.e., more depleted δ13C, indicative of chemosynthesis). Macroinfaunal total abundance at Barkley Hydrates tracked temporal changes in chemosynthetic organic matter, however, some taxa (i.e., Ampharetidae) apparently matched recruitment to phytodetrital pulses, much like the background community. An in situ enrichment experiment demonstrated modest differences in infaunal community species and functional trait composition in food patches enriched with either Chaetoceros calcitrans (a diatom) or Nannochloropsis oculata (a lipid rich eustygmatal alga). Megaepifaunal visits to the enrichment patches increased in N. oculata over the first two weeks of the experiment, perhaps responding to increased abundance of potential infaunal prey. This research demonstrates a structuring role for food quality both in benthic communities and in food webs, with different effects on organisms of different sizes (e.g., macrofauna and megafauna).