Seasonal and spatial variations of benthic oxygen and nitrogen fluxes in the Helgoland Mud Area (southern North Sea).

Benthic oxygen and nitrogen fluxes were quantified within the years 2012 to 2014 at different time series sites in the southern North Sea with the benthic lander NuSObs (Nutrient and Suspension Observatory). In situ incubations of sediments, in situ bromide tracer studies, sampling of macrofauna and pore water investigations revealed considerable seasonal and spatial variations of oxygen and nitrogen fluxes. Seasonal and spatial variations of oxygen fluxes were observed between two different time series sites, covering different sediment types and/or different benthic macrofaunal communities. On a sediment type with a high content of fine grained particles (<63 µm) oxygen fluxes of −15.5 to −25.1 mmol m −2 d −1 (June 2012), −2.0 to −8.2 mmol m −2 d −1 (March 2013), −16.8 to −21.5 mmol m −2 d −1 (November 2013) and −6.1 mmol m −2 d −1 (March 2014) were measured. At the same site a highly diverse community of small species of benthic macrofauna was observed. On a sediment type with a low content of fine grained particles (<63 µm) high oxygen fluxes (−33.2 mmol m −2 d −1 August 2012; −47.2 to −55.1 mmol m −2 d −1 November 2013; −16.6 mmol m −2 d −1 March 2014) were observed. On this sediment type a less diverse benthic macrofaunal community, which was dominated by the large bodied suspension feeder Ensis directus, was observed. Average annual rain rates of organic carbon and organic nitrogen to the seafloor of 7.44 mol C m −2 y −1 and 1.34 mol N m −2 y −1 were estimated. On average 79% of the organic bound carbon and 95% of the organic bound nitrogen reaching the seafloor are recycled at the sediment–water interface. [ABSTRACT FROM AUTHOR]