Benthic O-2 exchange across hard-bottom substrates quantified by eddy correlation in a sub-Arctic fjord

Oxygen exchange across the seabed is a key measure for quantifying benthic carbon turnover. However, technical limitations have restricted such measurements to sedimentary areas. Few assessments on hard-bottom substrates exist although such biotopes are frequent along many coastlines. In spring 2009, we used the non-invasive eddy correlation technique to quantify the in situ O-2 exchange rate across 3 typical substrates in West Greenland: (1) in soft sediments, the O-2 exchange in darkness averaged 2.0 mmol m(-2) h(-1). The rate exceeded the O-2 exchange quantified from microprofile measurements by 80%, but was not significantly different from parallel core incubations. (2) Measurements across consolidated sand and stones revealed a high activity with average O-2 exchange similar to that of sedimentary areas. Despite being net heterotrophic, the benthic primary production on average amounted to 27 mmol O-2 m(-2) d(-1), which per unit area is similar to the pelagic gross production of the central fjord. Carbon fixed by benthic diatoms and coralline red algae was efficiently recycled by microbes or grazed by the abundant epifauna. (3) A vertical cliff covered with sea cucumbers also revealed high O-2 consumption rates (1.2 mmol m(-2) h(-1)). The O-2 exchange derived from the eddy correlation approach exhibited pronounced short-term variability reflecting the interplay of several dynamic controls, most importantly short-term variations in local hydrodynamics. All together, measurements across hard-bottom substrates revealed active benthic communities that mediate high carbon turnover rates.