CONTRIBUTION OF DISSOLVED ORGANIC NITROGEN FROM RIVERS TO ESTUARINE EUTROPHICATION

The bioavailibility of dissolved organic nitrogen (DON) in river water entering estuaries was examined for the Delaware and Hudson Rivers, U SA. Water collected from above the salinity intrusion zone of each riv er was filtered, brought to a salinity of 15 ppt, and inoculated with estuarine bacteria. Bacterial production rates (8 to 26 x 10(5) cells ml(-1) d(-1)) during the initial 2 d in these experiments were within the range measured in these and other estuaries, indicating that river ine dissolved organic matter can contribute to production of estuarine bacteria. Average DON concentrations decreased by 40 to 72% within th e 10 to 15 d time course of the experiments; the decreases in DON were accounted for by increases in microbial biomass plus remineralization to inorganic nitrogen. The time scale over which DON was utilized sug gests that in estuaries with residence times on the order of weeks to months, such as Delaware Bay, river inputs of the biologically availab le portion of DON are first utilized within the estuary. In contrast, in estuaries with residences times of less than a week, such as New Yo rk Bay, a portion of the biologically available DON may be utilized fi rst within the estuary, with the remainder exported and utilized in co ntinental shelf waters. The large proportion of the DON that was biolo gically available in these experiments, coupled with the knowledge tha t inputs of organic nitrogen can account for 20 to 90% of the total ni trogen loading to estuaries, suggests that organic nitrogen inputs may contribute more to estuarine and shelf eutrophication than was previo usly suspected. These experiments demonstrate that dissolved inorganic nitrogen (DIN) inputs underestimate, and total nitrogen inputs likely overestimate, the inputs of biologically available N to estuaries. In order to develop a 'biologically available nitrogen budget' for an ec osystem, DIN inputs, plus that portion of the organic N that is biolog ically available must be quantified.