Oceanologia (Jun 2007)
Modelling nitrogen and phosphorus limitation on phytoplankton growth in Narva Bay, south-eastern Gulf of Finland
Abstract
The relative roles of nitrogen and phosphorus in the limitationof phytoplankton growth in Narva Bay, south-eastern Gulf of Finland,were studied by combining the results of numerical modellingand nutrient enrichment experiments. Modelled biomass-based intracellularnutrient concentrations (nutrient functions) were used to estimatethe limiting nutrient in Narva Bay. Nutrient functions - NF∈ [0; 1] for nitrogen and PF ∈ [0; 1] for phosphorus - definethe dependence of the phytoplankton growth rate on nutrients:NF = PF = 1 corresponds to non-limitation of phytoplankton growthby nutrients, whereas NF = 0 or PF = 0 to zero growth. Thebiotests indicated the response of phytoplankton growth to anincrease in nutrient concentration in the surrounding water.Three locations were selected for detailed analyses of temporalvariations in the nutrient functions: the offshore station N12,station N8 at the mouth of the River Narva, and coastal station38. The 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 below0.1 in the open part of Narva Bay for the rest of that period.In the coastal zone, NF was in excess of 0.1, with a localmaximum in the river mouth area. PF decreased to 0.3-0.4 inthe open bay after the spring bloom. In the coastal zone PFremained above 0.4, with a certain increase from the midsummerminimum towards the end of summer. The numerical modelling resultsclearly show that nitrogen limits phytoplankton growth in NarvaBay. Phosphorus limitation may occur only for a limited periodand over a limited area at the Narva River mouth and other coastallocations. In general, the biotests backed up the modelling results,the main exception being in the open bay during summer. The modeldoes not account for nitrogen fixation, however. Since N-fixingcyanobacteria were prevalent in the offshore area, the additionof phosphorus led to enhanced phytoplankton growth at station N12.
