Benthic oxygen flux observations as a measure of ecosystem state and impacts: strengths, limitations, and available technologies

Measurements of benthic biogeochemical fluxes address key functions of benthic systems, e.g., remineralization, nutrient regeneration, production of biomass, or bioturbation. They can provide an integrative measure of processes across all size classes of benthic life; and they allow for quantitative comparisons to processes in other compartments of the marine environment, e.g. surface water primary productivity, vertical particle fluxes, organic matter supply to the benthos, and seepage and oxidation of reduced compounds from subsurface sources. As mining operations potentially alter key ecosystem functions from surface to seafloor, benthic flux observations seem highly suited for baseline studies and subsequent assessments of mining-related impacts. The presentation addresses the suitability of this approach with a focus on oxygen to assess community respiration rates. As terminal electron acceptor, oxygen is directly connected to carbon cycling, integrates over aerobic as well as anaerobic processes, and may be measured with sensors at high accuracy. Benthic community respiration rates could serve as a baseline parameter against which both impact and recovery could be assessed. The suitability for impact assessments depends on the robustness of the approach, the sensitivity of community respiration to environmental changes, as well as practical considerations including the potential of the method to be applied autonomously over large areas. Oxygen flux measurements are able to resolve large-scale patterns in biological and biogeochemical conditions, e.g., with respect to benthic biomass, organic matter availability, and seepage of reduced compounds. On the other hand, studies with benthic chambers, microsensors, planar optodes, and eddy correlation systems reveal strong variability in oxygen fluxes on virtually any spatial and temporal scale addressed. In-situ scaling studies and theoretical considerations suggest that substantial replication is needed in order to obtain representative quantifications of fluxes. As part of the FRAM Ocean Observing System, time-series of annually repeated in-situ oxygen flux measurements are obtained at the Long-Term Ecological Research (LTER) site HAUSGARTEN in eastern Fram Strait. These allow the quantification of natural levels of temporal variability in the deep sea against which impacts would be assessed. At HAUSGARTEN and other sites, improved flux observation methodologies and strategies are currently implemented for better areal and temporal coverage. This will serve to better understand baseline variation and, hence, increase our ability to identify man-made impacts