Seasonal variations of Benthic Carbon and Nutrient fluxes in the southern North Sea

Abstract

In shallow water regions, such as the coastal areas of the North Sea, nutrient and carbon cycles are driven by a close coupling of benthic-pelagic processes. Due to shallow water depths, a substantial amount of organic matter which is produced via primary production in surface waters is transferred to the seafloor. Most of the organic matter is degraded within surface sediments and nutrients such as NH4 or PO4 are transported back into the water column, whereas a small amount of organic carbon is buried within the sediment. Consequently, benthic carbon and nutrient fluxes have a direct impact on biological and geological processes such as the availability of nutrients in the water column, nutrient budgets or the storage of carbon within marine sediments. Even though coastal carbon and nutrient cycles are intensively investigated, their seasonal and diurnal variability is poorly understood. The aim of this study was to quantify benthic carbon and nutrient fluxes in the southern North Sea during different seasons. The results are used for carbon and nutrient mass budgets. Furthermore processes which affect carbon and nutrient fluxes over seasonal and diurnal scales were identified. Chapter 2 and 3 present a seasonal study on benthic carbon and nutrient fluxes in the southern North Sea. During five cruises on RV Heincke, carried out in June 2012, August 2012, March 2013, November 2013 and March 2014, carbon and nutrient cycles were studied with the benthic lander NuSObs (Nutrient and Suspension Observatory) and shipboard sampling techniques. Chapter 4 presents laboratory experiments in which the diurnal variability of benthic carbon and nitrate fluxes were studied in light:dark cycles. The first manuscript (chapter 2) presents benthic oxygen and nitrogen fluxes derived from in situ incubations and pore water data. Both oxygen and nitrogen fluxes followed the seasonal cycle with highest fluxes in summer and autumn and lowest fluxes in winter. Detailed investigations of the benthic macrofauna and tracer flux studies showed the importance of faunal induced transport of solutes along the sediment water interface for the benthic oxygen consumption in summer and autumn. Over spatial scales the suspension feeder Ensis directus had a considerable impact on benthic oxygen consumption. Estimated recycling efficiencies of organic bound carbon revealed that most of the carbon (76-93 %) and nitrogen (87-97 %) reaching the seafloor is remineralized within surface sediments. The second manuscript (chapter 3) discusses benthic silicic acid (Si(OH)4) fluxes which were determined with different sampling techniques including in situ incubations with the benthic lander NuSObs, ex situ incubations and calculated fluxes based on pore water profiles. A comparison of the different sampling techniques shows that in shallow water coastal areas in situ techniques are required for a precise quantification of benthic fluxes. Strong seasonal variations in silicic acid effluxes were measured by in situ and ex situ incubations with highest fluxes in summer and autumn and lowest fluxes in winter. Estimated annual rain rates of biogenic silica (bSi) reaching the seafloor of the southern North Sea are within a range of 1.7 to 2.2 mol bSi m-2 a-1. The third manuscript considers the results of laboratory experiments on diurnal cycling of oxygen and nitrate in a coastal sediment. An oxygen optode (PyroscienceTM) and an optical nitrate sensor (SatlanticTM) were applied to closed microcosm experiments in order to monitor oxygen and nitrate continuously in incubation experiments. A diatom dominated sediment was incubated over 12 hour light:dark shifts. During daylight oxygen was most likely produced by benthic primary producers and during night time consumed by heterotrophs and the oxidation of reduced solutes. The consumption of oxygen was regulated by the presence or absence of benthic macrofauna. Monitoring nitrate continuously with the nitrate sensor revealed that at the onset of light, while sediments and bottom waters were anoxic, a nitrate reducing or assimilating process takes place within surface sediments.