Short-term variability of primary production and inorganic nitrogen uptake related to the environmental conditions in a shallow coastal area (Gulf of Trieste, N Adriatic Sea)

Primary production (PP) and nitrate (QNO₃) and ammonium (QNH₄) uptakes were measured together with other environmental parameters from October 1999 to February 2001 in the Gulf of Trieste (N Adriatic Sea). Their trends showed a high variability because of the combined effects of meteorological conditions, water circulation and river discharges. PP ranged from 0.2 to 15.9 μmol C dm^–3 d^–1, whereas QNO₃ varied from 0.8 to 442 nmol N dm^–3 d^–1, showing a trend similar to that of carbon basically ascribable to the autotrophic activity. QNH₄ ranged from 20 to 1308 nmol N dm^–3 d^–1 and it reached the highest values during the declining phases of phytoplankton blooms, indicating that bacterial community can also be involved in its uptake. Regenerated PP generally prevailed over the new production (depth-integrated f-ratios from 0.05 to 0.50). C/N uptake ratios by planktonic community (annual average of 16 ± 11) showed the repetitive carbon overconsumption (23–33) during periods of high production, and lower values (2–13) during the post-bloom phases and in the months of scarce autotrophic activity. Residence time of freshwater in the area (1 d in November and January, up to 23 d in July) indicated the fast export of low salinity waters in winter and their longer permanence in summer. These values were closer to the ammonium turnover times (1–34 d) than to the nitrate ones (2–831 d). Riverine nitrate load (3–67 t N d^–1) generally exceed the biological demand of this nutrient (uptakes from 0.2 to 8 t N d^–1), whereas the ammonium load (0.1–3.3 t N d^–1) was almost always insufficient (uptakes from 2.6 to 33 t N d^–1). These results evidenced the major role of physical transport and recycling processes to regulate, respectively, nitrate and ammonium availability in this shallow ecosystem.