Understanding causes and consequences of increases in biodiversity. A case study based on nematodes from stable and dynamic continental margins and contrasting productivity regimes

Abstract

This thesis aimed to unravel processes driving meiofaunal community patterns in the deep sea. Benthic-pelagic coupling and various physical parameters, such as surface primary productivity, organic matter flux, sediment composition, chlorophyll and other pigments, fatty acids, and carbon and nitrogen, were measured as well as connectivity between studied areas. The study areas comprised four stations located at the abyssal plain of the Southern Ocean (SO) and ten stations situated at the lower continental shelf and mid-slope of the Western Iberian Margin (WIM). Benthicpelagic coupling was investigated by testing how surface primary productivity and export fluxes, as well as other environmental factors, relate to differences in meiofaunal diversity, density, and standing stocks, but also meiofauna distribution over local and regional scales. Moreover, spatial turnover and connectivity between bathymetrically different stations were analysed. In order to test our hypotheses, surface and benthic environmental parameters were calculated. At the SO, surface primary productivity values and particulate organic carbon fluxes were estimated. For both study areas, benthic environmental variables analysed included chlorophyll a and its derivatives, sediment composition, and total carbon and nitrogen. In addition, in the SO, sediment and nematode fatty acid concentrations were measured. Meiofauna and nematode community aspects included standing stock (biomass only for the SO), density and diversity for both areas, as well as nematode respiration for the SO. The potential role of connectivity was studied at the WIM through the use of 18S rDNA. Results showed that net surface primary productivity at the SO was positively associated with the diversity, abundance, and total fatty acid content of meiofauna and nematodes. This shows that primary production represents the fundamental energy source for the meiobenthos. The export of organic matter to the benthos is depth dependent though. This could be shown by a decrease with depth in organic matter arriving at the sea bottom exhibited at the WIM. These differences accounted for disparity of density and diversity of nematodes associated with a decrease in patchiness with increasing depth. Besides organic matter content, other factors, such as hydrodynamics, inferred through variation in sediment composition, shaped alpha and beta diversity in both studied environments by increasing habitat heterogeneity. Hydrodynamics were also identified as potential promoter of dispersal of selected nematodes. The lack of genetic differentiation between bathymetrically and geographically different areas indicated connectivity between the study areas.