Primary productivity regime and nutrient removal in the Danube estuary

The primary productivity regime, as well as the distribution of dissolved inorganic nutrients and particulate organic matter in the Danube estuary, were investigated during several cruises at different discharge regimes of the Danube River. The shallowness of the upper surface layer due to insignificant tidal mixing and strong stratification of the Danube estuary, as well as the high nutrient concentrations, are favourable for elevated primary production. The incident light levels at the bottom of the upper surface layer of the water column (0.5-3.0 m) were generally higher than 20% of the surface irradiance. Elevated chlorophyll (Chi) a concentrations with maxima at mid salinities were found during each survey. Within the upper mixed layer estimated primary production of 0.2-4.4 g m(-2) day(-1) is very high compared with estuaries of other major world rivers. Mixing diagrams of dissolved inorganic nutrients reveal removal of significant quantities of nutrients during estuarine mixing. These observations were consistent with the distribution of particular organic matter, which was negatively correlated to the nutrient distribution during each survey. C:Chl a ratios, as well as the elevated estimated production, indicate that biological transformation processes govern the nutrient distribution in this estuary. (C) 1997 Academic Press Limited.