Horizontal and vertical profiles of light hydrocarbons in sea water related to biological, chemical and physical parameters

In situ-measurements of light (C²-C⁴) nonmethane hydrocarbons (NMHC) in sea water were performed during a ship cruise in spring 1991 in the Mediterranean, the Atlantic and the North Sea. The investigated regions were characterized with respect to biological (phytoplankton density, chlorophyll concentration), chemical (concentration of dissolved organic carbon, DOC) and physical parameters (irradiation intensity, wind velocities). These data were analysed for possible correlations with NMHC concentrations. An anticorrelation was found between NMHC concentrations and wind velocity. There were indications of a correlation between alkene concentration and the concentration of photochemically reactive DOC. For regions of low phytoplankton densities, significant positive correlations between biological activity (i.e.. chlorophyll concentration and cell density) and the concentrations of all NMHC except ethane were obtained. In contrast, in regions of an active growing phytoplankton bloom the concentrations of all NMHC except n-butane were inversely related to biological activity. In the vertical profiles, the alkanes often showed pronounced concentration maxima in depths of 1-10% relative light intensity, i.e., at the bottom of the euphotic zone. Alkene concentration maxima occurred in the upper layer of the euphotic zone. During periods of very low wind velocities, i.e., low mixing in the ocean surface layer and low emission rates, the concentrations of alkenes in surface water exhibited a distinct diurnal pattern with the concentrations increasing during day time and remaining constant at night.