Seasonal sedimentation of autochthonous material from the euphotic zone of a coastal system

The seasonal sedimentation of particulate matter across the pycnocline was recorded in the Kattegat during the period of stratification (March–October 1989). The proportion of viable phytoplankton cells of the total carbon flux (63 g C m−2) was low (18%). Faecal pellets contributed on average less than 5%. The major component of the sinking matter consisted of detrital material (amorphous particles with few recognizable components) which was estimated mainly to originate from the activity of microzooplankton. The average C/N ratio of the particulate biogenic matter trapped at the pycnocline was 8•6, but only 6•4 for the suspended particulate organic matter in the mixed layer indicating that nitrogen is more effectively recycled than carbon in the euphotic zone. This mechanism allows for an additional new organic carbon production of 33% compared to that which can be estimated from the external loading of nitrogen. Over the entire period the average daily vertical flux of carbon corresponded to 14% of the phytoplankton biomass indicating a high efficiency in the transport of organic material from the euphotic zone to the bottom.