Primary Production and Nitrogen Uptake in a Shallow Coastal Sea (Gulf of Trieste, Adriatic Sea)

Primary production and uptake rates of nitrate and ammonium were measured together with environmental parameters (temperature, salinity, light and nutrients) in a shallow coastal area of the Adriatic Sea (Gulf of Trieste) over a 17-month period (October 1999 - February 2001). The field activity was addressed to the description of primary production processes in a coastal area characterised by both strong freshwater discharges and high seasonal variations of physical conditions of the water column. During the investigated period the spreading of freshwater was indicated by low surface salinity (from 19 to 35 PSU), which coincided with increased nitrate concentrations in the upper layers (from 3.3 up to 51 ?M) during late autumn-winter period and early spring. Elevated ammonium concentrations were measured mainly in the deeper part of the water column related to the in situ recycling processes, throughout summer and early-autumn months (2.12 to3.51 ?M). On average, nitrate was the main form of inorganic nitrogen (46-60%). Carbon and nitrogen uptake rates were measured for the first time simultaneously, using C-14 and N-15 labelled techniques. Daily rates of primary production, calculated on the basis of irradiance daily measurements, varied from 0.22 gC/m2/d in October 1999 to 19.07 gC/m2/d in September 2000, with an overall average value of 3.93±4.01 gC/m2/d. The late autumn-winter periods of both years were characterised by low production rates (below 5 gC/m2/d). From March 2000 on, primary production gradually increased to reach extremely high values (above 10 gC/m2/d) in May and again in the period from July to September. This period of high primary production coincided with the occurrence of peculiar gelatinous macroaggregates over the whole northern Adriatic area. The inorganic nitrogen uptake trend was similar to that of carbon: the highest rates of QNO3 (up to 579 nM/dm3/d) and QNH4 (up to 2383 nM/dm3/d) were also measured from June to October 2000. The lowest values ranging from 0.5 to 30 nM/dm3/d and from 20 to 199 nM/dm3/d in case QNO3 and QNH4, respectively, were measured in winter. On the whole, ammonium uptake prevailed over nitrate uptake during most of the time as indicated by calculated f-ratios (from 0.05 to 0.50), in spite of significant supply of the riverine-originated nitrate to the sea. Only during early spring increased primary production was supported mainly by nitrate assimilation (f-ratio: 0.53-0.73). This observation indicates, as already observed in other coastal and estuarine areas, the predominance of regenerated production with respect to new production, which underlines the importance of the recycling in situ processes in sustaining the ecosystem of the Gulf of Trieste.