A STOICHIOMETRIC MODEL OF NUTRIENT CYCLING IN THE NORTHERN ADRIATIC SEA AND ITS RELATION TO REGENERATION PROCESSES

From a linear regression analysis of an extended set of concentration data collected between 1972 and 1981 (n=20 000) a stoichiometric model of nutrient regeneration was developed for the shallow northern Adriatic, the most eutrophic region in the Mediterranean Sea. The model (AOU:N:Si:P=-610:15:47:1) differs significantly from a widely accepted 'oceanic' model (AOU:N:Si:P=-276:16:15:1), mainly because of denitrification in the sediment, different regeneration rates of phosphorus and nitrogen (regenerated mainly in the water column) vs. biogenic silicon (regenerated mainly in the sediments), and a phosphorus deficiency in decomposing organic matter from primary and secondary production. In water masses, mainly in surface layers, in which freshwater nutrient input and phytoplankton assimilation are dominant, the nutrient ratios (N/P=62, Si/N=1.1, and Si/P=65) are significantly different from ratios in subeuphotic regeneration layers (N/P=15, Si/N=3.3, and Si/P=48). These differences were ascribed to nutrient ratios in river waters which are 'anomalous', and to faster utilization of phosphorus by northern Adriatic phytoplankton compared with nitrogen and particularly silicon. © 1990.