The retrospective analysis investigates links between atmospheric nitrogen deposition and algal bloom development in the Kattegat Sea from April to September 1989-1999. The analysis is based on atmospheric deposition model results from the ACDEP model,hydrodynamic deep-water flux results, phytoplankton abundance observations from Danish and Swedish marine monitoring stations and optical satellite data. Approximately 70 % of the atmospheric deposition consists of wet depostion of highly episodicnature. The atmospheric deposition of nitrogen is of the same order of magnitude as the flux from the bottom waters. Yet the cumulative atmospheric deposition is always larger than the marine deep-water flux. The mixing of nutrient-rich water from belowthe pycnocline into the euphotic zone is also a process of highly episodic character and provides sufficient nitrogen to the euphotic zone to sustain larger algae blooms. The two nitrogen loading processes are correlated - mainly because both are to someextent related to the wind velocity - and the nitrogen input from both processes enables the build-up of algae blooms. Furthermore, the nitrogen supplied on a single day cannot sustain a bloom with an increase above 0.5 µg/l chlorophyll a, but severalconsecutive days of high nitrogen inputs create the potential for blooms. The physical and chemical conditions before and during a bloom revealed that blooms occurred under higher salinity and wind conditions on 2-6 days prior to the observed bloom.Blooms were dominated by diatoms and dinoflagellates species. Four algal blooms events were identified in the years 1990-1992. Chlorophyll a maps from SeaWiFS satellite are compare well to in-situ observations from the ScanFish mounted at a moving ship.