Intra- and inter-spatial variability of meiofauna in hadal trenches is linked to microbial activity and food availability.

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.

Effect of patch size and in-patch location on the infaunal macroinvertebrate assemblages of Zostera marina seagrass beds.

Much recent work on patch-occupancy dynamics has been concentrated in terrestrial ecosystems, with few examples evident from soft-sediment marine habitats. Seagrass landscapes have recently been recognised to be potentially ideal marine models for the study of such ecological concepts. Infaunal macroinvertebrate assemblages of two patch sizes of the seagrass Zostera marina were compared: small (<15 m diameter) and large (>30 m diameter), using an unreplicated random block design. Further comparison was made between infaunal assemblage composition at the edge and centre of each patch. Univariate statistical analysis of data indicated significantly greater total numbers of taxa in samples from large patches than in small. Multivariate analyses indicated significant differences in assemblage composition due to both patch size and in-patch location, and revealed that differences were due to small changes in the relative abundances of many taxa. Possible mechanisms underlying the observed variations of assemblage composition with patch size and in-patch location are discussed. Although the present results support some of the theories relating to the control of infaunal assemblage composition, explanations are not applicable across all taxonomic groups. At the scale of the present study, seagrass patch size and edge-effects appear to be less significant than 'regional' factors, which relate to relatively small variation in environmental parameters, for the structuring of infaunal macroinvertebrate assemblages.