On extreme atmospheric and marine nitrogen fluxes and chlorophyll-a levels in the Kattegat Strait

[摘要] A retrospective analysis is carried out to investigate the importance of the vertical fluxes of nitrogento the marine sea surface layer in which high chlorophyll a levels may cause blooms of harmfulalgae and subsequent turn over and oxygen depletion at the bottom of the sea. Typically nitrogen isthe limiting factor for phytoplankton in the Kattegat Strait during summer periods (May to August)and the major nitrogen inputs come from the atmosphere and deep-water entrainment. The extremereoccurrence values of nitrogen from atmospheric wet and dry deposition and deep-water fluxentrainments are calculated by the periodic maximum method and the results are successfullycompared to a map of chlorophyll return periods based on in-situ observations. The one-year returnof extreme atmospheric wet deposition is around 60 mg N m^-2 day^-1 and30 mg N m^-2 day^-1 for deep-water entrainment. Atmospheric nitrogen dry deposition is insignificant in the context of algalblooms. At longer time-scales e.g. at 10-year return, the nitrogen deep-water entrainment is largerthan the extreme of atmospheric wet deposition. This indicates that the pool of nitrogen releasedfrom the sea bottom by deep-water entrainment forced by high winds greatly exceeds theatmospheric pool of nitrogen washed out by precipitation. At the frontal zone of the Kattegat Straitand Skagerrak, the nitrogen deep-water entrainment is very high and this explains the high 10-yearreturn chlorophyll level at 8 mg m^-3 in the Kattegat Strait. In the southern part, the extremechlorophyll level is only 4 mg m^-3 according to the statistics of a multi-year time-series of watersamples. The chlorophyll level varies greatly in time and space as documented by a series ofSeaWiFS satellite maps (OC4v4 algorithm) of chlorophyll ScanFish and buoy observations from anexperimental period in the Kattegat Strait. It is recommended to samplein-situ chlorophyll observation collocated in time to the satellite overpasses of e.g. SeaWiFS and ENVISAT MERIS toensure improved mapping of the chlorophyll levels in the Danish waters.

[学科分类] 大气科学