Deep-sea communities rely on organic matter (OM) produced almost exclusively in surface waters. Pulses of fast sinking material are their main food source, but here we show that when the quantity of that material is low, small particles can sustain deep zooplankton communities. Small particles are often overlooked as a source of OM to deep food webs, yet they are found in higher concentrations than larger, faster sinking particles, and their relative contribution to the total pool is expected to increase in the future. Our results highlight the importance of small particles as a food source sustaining deep food webs, which may become more widespread given the predicted changes in overlying surface waters.

No abstract

Many species of micronekton perform diel vertical migrations (DVMs), which ultimately contributes to carbon export to the deep sea. However, not all micronekton species perform DVM, and the nonmigrators, which are often understudied, have different energetic requirements that might be reflected in their trophic ecology. We analyze bulk tissue and whole animal stable nitrogen isotopic compositions (δ15N values) of micronekton species collected seasonally between 0 and 1250 m depth to explore differences in the trophic ecology of vertically migrating and nonmigrating micronekton in the central North Pacific. Nonmigrating species exhibit depth‐related increases in δ15N values mirroring their main prey, zooplankton. Higher variance in δ15N values of bathypelagic species points to the increasing reliance of deeper dwelling micronekton on microbially reworked, very small suspended particles. Migrators have higher δ15N values than nonmigrators inhabiting the epipelagic zone, suggesting the consumption of material during the day at depth, not only at night when they migrate closer to the surface. Migrating species also appear to eat larger prey and exhibit a higher range of variation in δ15N values seasonally than nonmigrators, likely because of their higher energy needs. The dependence on material at depth enriched in 15N relative to surface particles is higher in migratory fish that ascend only to the lower epipelagic zone. Our results confirm that stark differences in the food habits and dietary sources of micronekton species are driven by vertical migrations.

. Nitrogen stable isotope ratios (δ 15 N) and body size were used to describe the size-based trophic structure of a deep-sea ecosystem, the Avilés submarine Canyon (Cantabrian Sea, Southern Bay of Biscay). We analyzed δ 15 N of specimens collected on a seasonal basis (March 2012, October 2012, and May 2013, from a variety of zones (benthic, pelagic), taxa (from zooplankton through invertebrates and fi shes to giant squids and cetaceans), or depths (from surface to 4700 m) that spanned nine orders of magnitude in body mass. Our data reveal a strong linear dependence of trophic level on body size when data were considered either individually, aggregated into taxonomical categories, or binned into size classes. The three approaches render similar results that were not signifi cantly different and yielded predator : prey body mass ratios ( PPMR ) of 1156:1, 3792:1 and 2718:1, respectively. Thus, our data represent unequivocal evidence of interspecifi c, size-based trophic structure of a whole ecosystem based on taxonomic/functional categories. We studied the variability in δ 15 N not explained by body mass ( W ) using linear mixed modeling and found that the δ 15 N vs. log 10 W relationship holds for both pelagic and benthic systems, with benthic organisms isotopically enriched relative to pelagic organisms of the same size. However there is a marked seasonal variation potentially related to the recycling state of the system.

Trophic ecology of detrital-based food webs is still poorly understood. Abyssal plains depend entirely on detritus and are among the most understudied ecosystems, with deposit feeders dominating megafaunal communities. We used compound-specific stable isotope ratios of amino acids (CSIA-AA) to estimate the trophic position of three abundant species of deposit feeders collected from the abyssal plain of the Northeast Pacific (Station M; ~ 4000 m depth), and compared it to the trophic position of their gut contents and the surrounding sediments. Our results suggest that detritus forms the base of the food web and gut contents of deposit feeders have a trophic position consistent with primary consumers and are largely composed of a living biomass of heterotrophic prokaryotes. Subsequently, deposit feeders are a trophic level above their gut contents making them secondary consumers of detritus on the abyssal plain. Based on δ13C values of essential amino acids, we found that gut contents of deposit feeders are distinct from the surrounding surface detritus and form a unique food source, which was assimilated by the deposit feeders primarily in periods of low food supply. Overall, our results show that the guts of deposit feeders constitute hotspots of organic matter on the abyssal plain that occupy one trophic level above detritus, increasing the food-chain length in this detritus-based ecosystem.