Abstract
Hadal trenches are depocenters for organic material, and host intensified benthic microbial activity. The enhanced deposition is presumed to be reflected in elevated meiofaunal standing-stock, but available studies are ambiguous. Here, we investigate the distribution of meiofauna along the Atacama Trench axis and adjacent abyssal and bathyal settings in order to relate the meiofauna densities to proxies for food availability. Meiofauna densities peaked at the sediment surface and attenuated steeply with increasing sediment depth. The distribution mirrored the vertical profile of the microbial-driven oxygen consumption rate demonstrating a close linkage between microbial activity and meiofauna density. Meiofaunal standing-stock along the trench axis varied by a factor of two, but were markedly higher than values from the abyssal site at the oceanic plate. Overall, meiofaunal densities poorly correlated with common proxies for food availability such as total organic carbon and phytopigments, but strongly correlated with the microbial benthic O2 consumption rate. We argue that microbial biomass likely represents an important meiofaunal food source for hadal meiofauna. Observations from three trench systems underlying surface water of highly different productivity confirmed elevated meiofaunal densities at the trench axis as compared to abyssal sites on oceanic plates. Food availability appear to drive elevated abundance and variations in meiofauna densities in hadal sediments.
Highlights
Hadal trenches are depocenters for organic material, and host intensified benthic microbial activity
This study showed high meiofauna densities at the sediment surface that attenuated steeply with sediment depth, mirroring the vertical profile of microbial-driven volume specific O2 consumption
The meiofauna standing stock at the Atacama Trench axis was elevated compared to the adjacent abyssal plain site on the oceanic plate and comparable to values encountered at the nearby abyssal and bathyal sites on the continental plate
Summary
Hadal trenches are depocenters for organic material, and host intensified benthic microbial activity. The depth attenuation for meiofauna and microbial biomass is less pronounced[4] and the relative importance of microbes and meiofauna for the benthic food web structure and the turnover of deposited organic material increases towards the abyssal p lain[8]. The higher deposition of organic material at the trench floor would be expected to be mirrored in elevated density and biomass of m eiofauna[16,17,32], but the few available assessments of the meiofauna standing stock in hadal trench settings do not provide conclusive evidence for this pattern[14,21,32]. The density and biomass of meiofauna varies by more than one order of magnitude among different trenches[15,16,33,34], and the variability does not always correlate to the common proxies for benthic food availability such as total organic carbon (TOC) or phytodetrital material[18,32,35,36]. Living microbes could represent an important food source for m eiofauna[37–39], and cell counts or benthic O2 consumption, reflecting microbial activity[8], might serve as alternative proxies for food availability in deep-sea settings
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