Availability of phosphorus sources for blooms of phaeocystis pouchetii (haptophyceae) in the north sea: Impact of the river rhine

The distribution and uptake of various phosphorus fractions such as soluble reactive phosphorus (SRP), enzymatically hydrolysable phosphorus (EHP) and soluble unhydrolysable phosphorus (SUP) were studied in coastal waters of the North Sea with special emphasis on those areas inhabited by dense blooms of Phaeocystis pouchetii (Hariot) Lagerheim. A comparison was made between the uptake rates of SRP and the capacity of the phytoplankton to utilize the EHP fraction as a phosphorus source. In areas not directly influenced by the discharge of the river Rhine the phytoplankton depleted the SRP down to the detection level of 0.02 μmol·dm⁻³, while the concentration of EHP was always higher. Inorganic phosphate (SRP) was rapidly assimilated by the microplankton (algae and bacteria) as measured by the uptake of ³²P-orthophosphate. Concurrently, the alkaline phosphatase activity (APA) was high and appeared to be associated with the phytoplankton mainly. The various measurements indicated that the plankton was P-limited. The calculated rate of enzymatic hydrolysis of EHP was highly variable, but could provide the cells with orthophosphate at a rate comparable with that eeasured for the direct uptake of inorganic phosphate.The diversity and availability of phosphate sources in the area near the river Rhine varied strongly. The abundant supply of phosphorus by the river to the coastal waters can cause further growth of the phytoplankton, unrestricted by nutrient limitation. Here it seemed that EHP and APA played only an insignificant role. The dominance of Phaeocystis in these coastal waters is discussed in view of its capacities to exploit transient phosphate supply.