The atmospheric impact on fluxes of nitrogen, POPs and energy in the German Bight

SummaryThe external forcing of the German Bight system is largely due to the atmosphere. Energy fluxes that drive mixing processes and biological productivity, as well as atmospheric nutrient inputs outside the Elbe estuary, are important factors for biomass production. This study is based on the KUSTOS experiments focusing on air-sea exchange with intensive observations of a) radiative fluxes at the surface of the drifting water body; b) atmospheric surface layer parameters determining the mixing conditions in the planetary boundary layer; c) the speciation of atmospheric nitrogen compounds; d) and changes in aerosol and gas composition during transport over sea. These episodic data were complemented by a) synoptic data analysis of water and air temperature, wind, pressure and water vapour pressure over the sea; b) corresponding oceanic data on heat advection and mixed layer depth from an oceanic model driven by observations in the atmosphere; c) computations of the highly variable heat and radiative fluxes with the mesoscale atmospheric model METRAS; d) long-term atmospheric deposition measurements of nutrients in the German Bight; e) investigations of the atmospheric processes responsible for the formation of coarse particulate nitrate by means of a new aerosol submodel in the METRAS transport model. We present detailed seasonal or annual budgets for fluxes of heat, momentum, nitrate, ammonium, persistent organic pollutants. The atmospheric fluxes of heat and chemical matter are compared with load and fluxes in the water column in order to identify when and where the atmospheric impact is relevant and detectable. Spatial and temporal variability is discussed for the fluxes of heat, momentum and nitrogen. From the budgets we identify categories of potential atmospheric impact. Apart from the category “no atmospheric impact≓ valid e.g. for Cr, As, Ni and phosphate, we identify 4 others: 1) “atmosphere driven≓: short term, local dominant impact for Heat and momentum; 2) “episodic atmospheric impact≓: long term, local and dominant impact with large fluxes involved for radiation, PCB, Pb; 3) “persistent atmospheric pollutant≓: long term dominant but regionally indifferent impact for α- and γHCH; 4) “steadily perturbing the marine ecosystem≓: long term, widespread impact superimposed on the dynamic system driven by marine biology for nitrate, ammonium.