Carbon mineralization of shelf and coastal sediments: A holistic approach using state of the art lander technology and the eddy-correlation technique.

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A significant fraction of the organic material produced or imported to our coastal seas is degraded within the sea-bed, a process mediated by infaunal invertebrates. The efficiency of this process has important implications for regional and global nutrient and element recycling. Benthic carbon mineralization represents the biogeochemical and biological conditions of marine ecosystems and can be readily measured. Ultimately, the fraction of material retained in the sediment record versus being degraded, regulates the CO2 and O2 concentration of our oceans (and atmosphere). The best and most widely used proxy for quantifying the benthic carbon mineralization is the benthic O2 exchange rate. Most commonly such rates are measured by chamber incubations or derived form high-resolution O2 microprofiles but, to obtain reliable data, measurements have to be performed directly at the seabed by so-called lander systems. Recently a new approach - the eddy-correlation technique - has been introduced to aquatic biology. The approach allows benthic O2 exchange rates to be quantified for several m2 of seabed and, unlike previous methodologies, is in principle non-invasive and does not affect potential regulating controls (i.e. hydrodynamics, light, fauna activity). In essence, the benthic O2 exchange is derived from simultaneous, rapid recordings of the unobstructed vertical current velocity and O2 concentrations in a small volume of water above the seabed. The technique is adapted from terrestrial research where it now represents a routine measurement for resolving gas exchange between ground and atmosphere (or aquatic areas and atmosphere). This novel approach overcomes the primary limitations of traditional approaches by integrating the signal from a large heterogeneous seabed and by not obstructing factors regulating the benthic exchange rates. Benthic chambers do, however, offer detailed insight on the benthic dynamic that not can be provided by the eddy-correlation approach. The three different approaches are thus truly complementary and together they provide powerful means of measuring ecosystem processes at ecosystem relevant scale. We propose concerted deployments of three benthic lander types to obtain benthic O2 exchange rates by microsensors, chambers and eddy-correlation. The two first lander types will be made available to the project while the latter will be realized within the project. Complemented with online camera surveys for characterizing the seabed and the macro- and mega fauna communities the measurements will provide detailed insight in the spatial variability of diagenetic activity from spatial scales of a few mm (transecting microprofile arrays), dm (chamber incubations), m (by eddy-correlation) to km by measuring along transects around topographic seabed structures. Temporal variations in benthic O2 exchange rates as induced by variations in regulating controls (hydrography, tide, turbidity, exchange of water masses, light, fauna activity) will be evaluated, in situ, by 1-5 days deployments of the developed eddy-lander and a transecting microprofiler. The eddy-lander will also be deployed in a number of hard-bottom environments (reefs, rocks) that generally not are assessable for chamber and microprofiling equipment to, for the first time, fully include these common, but rarely studied environments in estimates for coastal carbon turn over.