Oxygen dynamics of marine sediments on different spatial scales

Abstract

The oxidation of organic material in marine sediments leads to an oxygen uptake and thereby a flux of O2 across the sediment / water interface. This flux, being relatively easy to quantify, is an important parameter in order to assess benthic mineralization rates. Too date, knowledge about global fluxes of particulate organic carbon to the sediments is for the most part derived from benthic oxygen uptake studies. Furthermore, oxygen dynamics in photic sediments gives information about magnitude and distribution of benthic primary production. However, the number of in situ studies is still limited and in some areas (e.g., the oligotrophic Subtropical Gyres) not even sufficient to allow for reliable estimates of benthic mineralization rates. The relevance of benthic photosynthesis in shallow subtidal zones is also still largely unexplored. Furthermore, recent work indicates strong spatial variability of sediment oxygen dynamics on various spatial and temporal scales. Knowledge about scales and magnitudes of this variability is essential for site comparison and up-scaling and, again, calls for additional studies of benthic oxygen fluxes and their dynamics. During this study, benthic oxygen distributions and fluxes were investigated in contrasting environments on very different scales - both in the laboratory and in situ. The aim was to improve our understanding of driving factors and distribution of benthic mineralization processes. The studied spatial scales ranged from ~0.1mm (light driven heterogeneities in production- and respiration rates in coastal sandy sediment) to several 1000km (transects in the South Pacific). For microscale studies in 2D, planar optode technology was further developed. Application of this advanced technology enhanced spatial resolution as well as accuracy, and facilitated the concurrent determination of the light field within the sediments. It was found that diffraction and light scattering in sandy sediments resulted in strong heterogeneities in the distribution of scalar irradiance. Local rates of respiration and photosynthesis were clearly correlated to the irradiance distribution, indicating a tight coupling between autotrophic and heterotrophic communities on a sub millimeter scale. To study benthic mineralization rates and -primary production in subtidal, photic sediments in the Kattegat, an in situ multi-parameter approach was chosen. Using a benthic crawler ("C-MOVE") as a platform, measurements with planar optodes, profiling microelectrodes, and incubation chambers, all attached to the crawler were conducted simultaneously. Complemented with eddy correlation measurements of benthic oxygen fluxes, this approach allowed to cover different aspects of benthic oxygen dynamics on largely different spatial scales. By combining the different measurements it was possible to identify some fundamental characteristics of the chosen area. Considerable benthic primary production was found with a high contribution of macroalgae as opposed to microphytobenthos. However, even at high light intensities, the sediments still proved to be net heterotrophic. Changes in light regime and mechanical sediment perturbations resulted in long periods of changing oxygen distributions, indicating that non-steady-state situations are prevalent at that site. Oxygen distribution and fluxes displayed a large spatial variability and dynamics that could, to a large extent, be attributed to faunal activity. In contrast to this highly dynamic and productive coastal sediment variability and fluxes in the South Pacific Gyre proved to be much smaller. In situ and ex situ microelectrode oxygen profiling allowed to constrain benthic mineralization rates in this most oligotrophic oceanic region to 0.4 to 1.5 gC m-2 yr-1, around 2% of typical values found in the Kattegat. Mathematical modeling of the microprofiles revealed that almost all bioavailable organic matter was remineralized within the first few millimeters of the sediment. However, oxygen was not used up in the upper sediment layer, diffused further downwards, and was still present at a depth of eight meter below the seafloor as measured with fiber optical sensors on piston cores. These measurements represent the deepest oxygen penetration ever reported. Along transects between the rim and the center of the gyre, little difference in the general pattern of deep oxygen penetration was found and mathematical modeling of the steady state diffusion-reaction equation suggested completely oxic sediments and a flux of oxygen to the underlying basalt, up to 70m below the sediment surface.