Abstract
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.
Highlights
Trophic ecology of detrital-based food webs is still poorly understood
After including gut contents and sediments as two food sources in the training set, the Linear Discriminant Analysis (LDA) grouped all deposit feeders (DF) collected during low food supply with gut contents, whereas during high food supply E. rostrata was grouped with sediments and O. mutabilis showed a mixture of both sources
We used compound-specific stable isotope ratios of amino acids (CSIA-amino acids (AAs)) to show that megafaunal deposit feeders (DF) were secondary consumers of detritus on the abyssal plain at Station M. This was shown by their trophic position, which was two trophic levels higher than the surrounding sediments
Summary
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. In other species the gut anatomy allows for efficient processing of ingested food by gut microbes[23] In this sense, the guts of some deep-sea holothurians have been found to hold high abundance of prokaryotes relative to the surrounding sediments[23,24,25]. The guts of some deep-sea holothurians have been found to hold high abundance of prokaryotes relative to the surrounding sediments[23,24,25] These microbial communities seem to increase the efficiency of deposit feeders to exploit the limiting resources, their functional role and whether they are selected from the environment or have a symbiotic origin remain unknown. Gut microbial communities might be a crucial yet overlooked component of abyssal food webs
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