| Abstract [eng] |
The increased availability of reactive phosphorus may favour cyanobacterial blooms, which is a phenomenon well known around the globe. Cyanobacteria are capable to proliferate intensively and form huge biomass in the water column, later settling to sediments in a form of rapidly degrading organic matter. Related microbial decomposition processes may lead to the formation of hypoxia/anoxia at the bottom water layers and, ultimately, to the release of dissolved inorganic phosphorus from sediments. Regenerated phosphorus supports nitrogen fixing cyanobacteria bloom forming a self–sustaining cycle. In this thesis, P cycling was investigated in the Curonian Lagoon by analysing external and internal phosphorus loads and the relationship between phosphorus release and cyanobacteria blooms. At two sites, representing dominant sediment types in the lagoon, we characterized P pools and mobility, via combined pore water analysis, calculation of diffusive exchange and flux measurement in core incubations. Annual balance was also calculated, to analyse the role the whole lagoon as net sink or source of phosphorus. Muddy sediments had higher P content as compared to sandy sediments, and most of phosphorus pool was reactive. Muddy sites had consequently higher pore water DIP concentrations, maintaining higher diffusive and net fluxes across sediment-water interface. However, measured fluxes suggested that both sediment types were mostly P sinks, except for a large regeneration of DIP (nearly 30 µmol m-2h-1) recorded at muddy sediments during the cyanobacteria bloom. On average, 69 % of the total inflowing DIP from the Nemunas River is assimilated in the Curonian lagoon, however, during the intensive river discharge events (as in June 2013), about 97 % of inflowing DIP could be directly transported to the Baltic Sea. 76 % of outflowing phosphorus from the Curonian lagoon to the Baltic Sea is in particulate form. |
