Seasonal coupling between riverborne nutrients, net productivity and hypoxia

Abstract

Seasonal dynamics of net productivity in the northern Adriatic Sea and in the northern Gulf of Mexico is coherent with the dynamics of freshwater discharge from their major rivers, the Po River and the Mississippi River, respectively. Oxygen deficit in the bottom waters also shows significant correlation with the river flow, implying a time-lag of 2 months in the northern Gulf of Mexico, and a time-lag of 4 months in the northern Adriatic Sea. Fluxes of organic matter resulting from ‘new’ primary production are potentially high, and may be sufficient to induce hypoxia in a highly stratified water column. A large fraction of nutrients in the Po River and the Mississippi River likely originates from synthetic fertilizers and detergents. The above findings, therefore, not only demonstrate close coupling between riverborne nutrients, net productivity and hypoxia, but also show that the anthropogenic nutrient loads can easily over-come the homeostatic potential of a coastal marine ecosystem.