Vertical distribution of dissolved and particulate fluorescence in the Black Sea

Abstract

Continuous profiles of fluorescence in three channels [chlorophyll, dissolved organic matter (DOM) and flavin] were taken simultaneously in the Black Sea using a pump profiling system. Other parameters measured on each pump cast included temperature, salinity, beam attenuation coefficient ( c), nutrients, oxygen and hydrogen sulfide. Chlorophyll fluorescence showed two maxima, a distinct primary maximum at the bottom of the euphotic zone, and a weak secondary maximum at the depth of the sulfide interface. The secondary maximum was associated with a particle maximum and a maximum in microbial activity (electron transport system). Fluorescence in the DOM and flavin channels increased steadily with depth from the surface to 366 m. Flavin fluorescence showed small peaks near the depth where nitrate concentrations approached zero, well above the depth of the secondary chlorophyll maximum. We attribute the secondary chlorophyll maximum solely to the presence of photosynthetic bacteria of the genus Chlorobium.