Nutrients, standing crop and primary production in western coastal waters of the adriatic sea

Nutrients and standing crop over a 2-year period and primary production rates over a 1-year period were measured at three stations located 1-5, 6 and 15 nautical miles (NM) from the coast, 150 km south of the Po river mouth. Both nutrients and standing crop (the latter measured as chlorophyll a concentration and phytoplankton cell density) showed a concentration gradient from west to east with marked changes of average values from 6 to 15 NM off the coast. Nutrients and standing crop appeared to be positively correlated and both showed an inverse dependence on salinity which was mainly influenced by freshwater inputs from the major Italian river, the Po. Primary production rates showed values of about 260 and 210 g C m- e year- l at the 1-5 and 6 NM stations respectively, indicating the presence of a narrow coastal belt with productivity levels significantly higher than those previously reported for other Adriatic areas affected by the Po. The offshore station at 15 NM gave an average annual production of about 120 g C m- 2 year- l which lies in the range reported for lagoons and coastal waters in the Po delta area. The photosynthetic efficiency was similar at the 1 '5 and 6 NM stations (respectively 4-04-2-3 and 3.54-2.7) but significantly lower at the 15NM station (1-9 4- 1 '7). This difference reflects either different nutrient levels in inshore and offshore stations or different structures in phytoplankton community: diatoms and phytoflagellates prevailed at 1 "5 and 15 NM, respectively. Photosynthetic efficiency increased with increasing concentration of nutrients. The behaviour was found to be linear as a function of total dissolved phosphorus, and hyperbolic as a function of total dissolved nitrogen. This different dependence suggests a surplus of nitrogen with respect to phosphorus for photosynthetic processes in the study area, which is consistent with the assumption of phosphorus as limiting element often made for the northern Adriatic.