Metabolism of inorganic nitrogen and phosphorus dissolved in a water column of a tropical coastal lagoon in Caribbean Columbia

Cienaga Grande de Santa Marta (Colombia), is a shallow coastal lagoon with evident signs of eutrophication, and with one of the world's highest primary production rates. To determine the influence of inorganic nutrients in driving and controlling such high production, the relationship of phytoplankton primary production to the amount of dissolved inorganic nitrogen and phosphorous (DIN, DIP), and their liberation - consumption rates, was studied. Two sites were compared, contrasted by the influence of fresh-water and marine inputs, during four tri-monthly samplings from October 1996 to September 1997. Pelagic gross primary production was variable (28-693 mu gCl(-1)h(-1)). Ambient concentrations of DIN and DIP were higher than those measured about ten years earlier, showing an advance in the eutrophication process. Contrary to previous years, DIN (mainly ammonium) and DIP were positively correlated to primary production, indicating that the system is currently more controlled by dissolved nutrients. On average, supply of NID tended to satisfy the N demanded for gross primary production, while DIP was found in much higher concentrations. This allows the maintenance of such high primary productions, regardless of the low N/P ratios (frequently<1). The occasional deficiency of N and P, in relation to the demand, is indicative of the use of reserves and of recycling processes in the pelagic milieu. DIP and DIN net metabolism was quite variable and was apparently not related to primary production or to other biotic and abiotic variables, evidencing the complexity of the processes involved.