Network analysis was applied to nitrogen cycling data from 6 ecosystem components (phytoplankton, Potarnogeton pectinatus, Ruppia cirrhosa, detritus plus heterotrophs, sediment, and dissolved inorganic nitrogen) in Tancada Lagoon (Ebro fiver delta, NE Spain) to ascertain (1) the relative importance of internal recycling versus external exchanges, (2) the main biogeochemical processes and (3) the fate of nitrogen imported to the lagoon. We compared cycling from 2 periods of the year: May-October, dominated by freshwater inputs, and November-April, dominated by seawater. Nitrogen recycling in the lagoon is equal to or more important than exchanges with other ecosystems, as Indicated by the Finn Cycling Index (FCI) FCI values are the percentage of total flow in the network which is associated with internal cycles and not with imports or exports. These internal cycles involve flows from a compartment that can be traced through 1 or more other compartments and back to the original. FCI values were 62% during the freshwater period and 52% during the seawater period. Nitrogen input to the lagoons was mainly as detritus plus heterotroph nitrogen (0.81 and 0.51 mm01 N m-' d -' during the fresh-and seawater periods respectively) Most nitrogen export was as phytoplankton (0.2 mm01 N m-' d-l during both periods). Denitrliication rates were comparable to other exports (0.17 and 0.09 mmol m-' d -' during the fresh-and seawater penods 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 fluxes between sediment and water during the freshwater period, and 60% during the seawater period. Thus several differences were noted in the N cycles between these 2 periods involving both the amount of recycling and the relative importance of different processes to the fate of nitrogen.
Nitrogen and phosphorus released into the water of a main drainage channel in the Ebro Delta was measured during a period of rice cultivation. An increase in total nitrogen and phosphorus in its different forms in accordance with the increase in cultivated area drained was observed. A significant correlation between the release of nutrients into the water and the nutrient load, for both nitrogen and phosphorus, if we consider the release of nutrients per unit of length, of the channel and increased total phosphorus content of the water that flows through it during the period of rice cultivation. Physical and chemical changes in the water as a result of ricefield metabolism may explain the differences observed in nitrogen and phosphorus cycles between Ebro and Rh6ne Deltas, two mediterranean deltaic systems where a large area with similar drainage system is used for rice cultivation.
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