Abstract
The seasonally stratified continental shelf seas are highly productive, economically important environments which are under considerable pressure from human activity. Global dissolved oxygen concentrations have shown rapid reductions in response to anthropogenic forcing since at least the middle of the twentieth century. Oxygen consumption is at the same time linked to the cycling of atmospheric carbon, with oxygen being a proxy for carbon remineralisation and the release of CO2. In the seasonally stratified seas the bottom mixed layer (BML) is partially isolated from the atmosphere and is thus controlled by interplay between oxygen consumption processes, vertical and horizontal advection. Oxygen consumption rates can be both spatially and temporally dynamic, but these dynamics are often missed with incubation based techniques. Here we adopt a Bayesian approach to determining total BML oxygen consumption rates from a high resolution oxygen time-series. This incorporates both our knowledge and our uncertainty of the various processes which control the oxygen inventory. Total BML rates integrate both processes in the water column and at the sediment interface. These observations span the stratified period of the Celtic Sea and across both sandy and muddy sediment types. We show how horizontal advection, tidal forcing and vertical mixing together control the bottom mixed layer oxygen concentrations at various times over the stratified period. Our muddy-sand site shows cyclic spring-neap mediated changes in oxygen consumption driven by the frequent resuspension or ventilation of the seabed. We see evidence for prolonged periods of increased vertical mixing which provide the ventilation necessary to support the high rates of consumption observed.
Highlights
Shelf seas and carbon Continental shelf seas play a disproportionately important part in the global cycling of carbon
Cefas benthic landers were placed at four sites during the Shelf Sea Biogeochemistry (SSB) program (Thompson et al 2017, Tables 1, 2, Fig. 1), two of which are included in this study: East of Haig fras (EHF), a benthic lander situated east of Haig Fras (Hull et al 2017a), a 45 km long submarine rocky outcrop and Nymph Bank Time-series (NB), a lander on the Nymph Bank, West of Celtic Deep (Hull et al 2017b)
In 2014 the central Celtic Sea began to stratify in late March, as confirmed by a thermistor array at the Celtic Deep (CD) site
Summary
Shelf seas and carbon Continental shelf seas play a disproportionately important part in the global cycling of carbon. They are estimated to provide 10–30% of the global oceanic primary production and 80% of organic carbon burial, while accounting for only 7% of the ocean surface area (Bauer et al 2013). Shelf seas can vary over short temporal and spatial scales, long term and spatially broad averages do not well represent their dynamics. These knowledge gaps hinder our ability to design numerical models that can correctly reproduce the carbon dynamics of these regions, which limits their predictive potential (Aldridge et al 2017; Diesing et al 2017; Solan et al 2020)
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