Modelling nitrogen and phosphorus limitation on phytoplankton growth in Narva Bay, South-Eastern Gulf of Finland

The relative roles of nitrogen and phosphorus in the limitation of phytoplankton growth in Narva Bay, south-eastern Gulf of Finland, were studied by combining the results of numerical modelling and nutrient enrichment experiments.Mo delled biomass-based intracellular nutrient concentrations (nutrient functions) were used to estimate the limiting nutrient in Narva Bay.Nutr ient functions – NF ∈ [0; 1] for nitrogen and PF ∈ [0; 1] for phosphorus – define the dependence of the phytoplankton growth rate on nutrients: NF = PF = 1 corresponds to nonlimitation of phytoplankton growth by nutrients, whereas NF = 0 or PF = 0 to zero growth.T he biotests indicated the response of phytoplankton growth to an increase in nutrient concentration in the surrounding water.Thr ee locations were selected for detailed analyses of temporal variations in the nutrient functions: the offshore station N12, station N8 at the mouth of the River Narva, and coastal station 38.T he biotests were performed at the same stations. NF and PF reached values of 0.9 prior to the spring bloom. With the onset of the spring bloom, NF decreased rapidly and remained below 0.1 in the open part of Narva Bay for the rest of that period.I n the coastal zone, NF was in excess of 0. 1, with a local maximum in the river mouth area. PF decreased to 0.3–0.4 in the open bay after the spring bloom.I n the coastal zone PF remained above 0.4, with a certain increase from the midsummer minimum towards the end of summer.The numerical modelling results clearly show that nitrogen limits phytoplankton growth in Narva Bay.Ph osphorus limitation may occur only for a limited period and over a limited area at the Narva River mouth and other coastal locations.I n general, the biotests backed up the modelling results, the main exception being in the open bay during summer.The model does not account for nitrogen fixation, however.S ince N-fixing cyanobacteria were prevalent in the offshore area, the addition of phosphorus led to enhanced phytoplankton growth at station N12.