Processes driven by the Small Sized Organisms at the Water-Sediment Interface

Abstract

The small sized organisms including prokaryotes (bacteria and archaea), protozoa and metazoan meiofauna (< 250 µm) are the driving forces for biogeochemical fluxes in surficial deepsea sediments under oxic conditions. The relative proportion of small sized organisms increases along trophic gradients from eutrophy to oligotrophy or from the continental margin towards the mid oceanic deep-sea. They can consume up to 10% of freshly sedimented organic matter per day. The small sized fauna consumes and respires the largest part of organic matter, while macrofauna is instrumental in incorporating fresh detritus into the sediment, structuring the environment and thus facilitating microbial processes. Small organisms, in particular prokaryotes, can adapt to amount and quality of organic matter input. Under nutrient starvation probably a large proportion of the prokaryotic community is dormant and is reactivated during sedimentation events. On time scales of 7–10 days (metabolism) to 2–3 weeks (biomass increase) they can react to pulses of deposition of organic material. However, the history of food supply influences the speed of adaptation and effectiveness of growth. At stations close to continental margins estimates of organic matter input from sediment traps largely disagree with measurements of benthic respiration, carbon turnover or estimates obtained from geochemical modelling. This discrepancy is much smaller at mid-oceanic stations. Lateral inputs from productive shelf seas into the deep-sea are suspected to cause this discrepancy.