The role of spatial scales and habitat heterogeneity in patterns and drivers of deep-sea benthic biodiversity and ecosystem functioning

Ciraolo, Alessia Caterina (2023) The role of spatial scales and habitat heterogeneity in patterns and drivers of deep-sea benthic biodiversity and ecosystem functioning. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Deep-sea environments support ecosystem functioning and services, however, the challenge in accessing deep-sea sediments limits intensive investigations of interactions between diversity, functioning, and spatial heterogeneity. The influence of spatial heterogeneity strongly depends on the scale of study, adding to the challenge. Nonetheless, understanding the potential role of spatial heterogeneity can inform efforts to protect and conserve deep-sea environments. In field experiments, I examined potential effects of different scales of spatial heterogeneity - habitat heterogeneity (large scale, 100s of km), a natural water column oxygen gradient (medium scale, tens of km), and organic matter addition (small scale, cm to m) - on deep-sea benthic ecosystem function and biodiversity. I found that large-scale habitat heterogeneity affects benthic macro- and microbial community assemblages, as well as functional and taxonomic infaunal diversity but with no obvious effect on benthic nutrient fluxes. Whereas quantity and quality of organic matter best explained variation in microbial community composition, interaction of several environmental factors acting over a large spatial scale complicated understanding of variation in infaunal communities. At a medium spatial scale (hypoxia), the quantity and quality of food shaped infaunal community variation where filter-feeder polychaetes dominated surface sediments. Bioturbation features decreased spatially with decreasing O2 levels and response to the O2 gradient varied with scale. The smaller video camera field of view suggested significantly lower megafaunal density and diversity during the period of reduced oxygen concentrations, but not in sediment trace diversity and density. In contrast, the larger field imaged with rotary sonar documented biological pit distribution, a significantly greater proportion of reworked seafloor area, and increases in pit size with increased oxygen levels. Furthermore, North Pacific infaunal composition could not explain variation in nutrient fluxes (mostly influxes) in relation to hypoxia. In contrast to microbial community composition that explained variation in benthic nutrient flux and organic matter degradation among three Atlantic continental slope habitats. At small scales, phytodetrital deposition did not affect oxygen consumption and short-term nutrient cycling. However, adding labeled phytodetritus demonstrated that polychaetes in particular influence rapid carbon uptake and C cycling, irrespective of bottom-water oxygen concentrations.

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