NETWORK ANALYSIS ON NITROGEN CYCLING IN A COASTAL LAGOON

Network analysis was applied to nitrogen cycling data from 6 ecosystem components (phytoplankton, Potamogeton pectinatus, Ruppia cirrhosa, d etritus plus heterotrophs, sediment, and dissolved inorganic nitrogen) in Tancada Lagoon (Ebro River delta, NE Spain) to ascertain (1) the r elative importance of internal recycling versus external exchanges, (2 ) the main biogeochemical processes and (3) the fate of nitrogen impor ted to the lagoon. We compared cycling from 2 periods of the year: May -October, dominated by freshwater inputs, and November-April, dominate d by seawater. Nitrogen recycling in the lagoon is equal to or more im portant than exchanges with other ecosystems, as indicated by the Finn Cycling Index (FCI). FCI values are the percentage of total flow in t he network which is associated with internal cycles and not with impor ts or exports. These internal cycles involve flows from a compartment that can be traced through 1 or more other compartments and back to th e original. FCI values were 62% during the freshwater period and 52% d uring the seawater period. Nitrogen input to the lagoons was mainly as detritus plus heterotroph nitrogen (0.81 and 0.51 mmol N m-2 d-1 duri ng the fresh- and seawater periods respectively). Most nitrogen export was as phytoplankton (0.2 mmol N m-2 d-1 during both periods). Denitr ification rates were comparable to other exports (0.17 and 0.09 mmol m -2 d-1 during the fresh- and seawater periods respectively). Sediment played a key role in the internal recycling of nitrogen. A total of 80 % of the phytoplankton nitrogen flow came indirectly from ammonium flu xes between sediment and water during the freshwater period, and 60% d uring the seawater period. Thus several differences were noted in the N cycles between these 2 periods involving both the amount of recyclin g and the relative importance of different processes to the fate of ni trogen.