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

Abstract

Abstract.A retrospective analysis is carried out to investigate the importance of the vertical fluxes of nitrogen to the marine sea surface layer in which high chlorophyll a levels may cause blooms of harmful algae and subsequent turn over and oxygen depletion at the bottom of the sea. Typically nitrogen is the limiting factor for phytoplankton in the Kattegat Strait during summer periods (May to August) and the major nitrogen inputs come from the atmosphere and deepwater entrainment. The extreme reoccurrence values of nitrogen from atmospheric wet and dry deposition and deep-water flux entrainments are calculated by the periodic maximum method and the results are successfully compared to a map of chlorophyll return periods based on in-situ observations. The one-year return of extreme atmospheric wet deposition is around 60 mg N m −2 day −1 and 30 mg N m −2 day −1 for deep-water entrainment. Atmospheric nitrogen dry deposition is insignificant in the context of algal blooms. At longer time-scales e.g. at 10-year return, the nitrogen deep-water entrainment is larger than the extreme of atmospheric wet deposition. This indicates that the pool of nitrogen released from the sea bottom by deep-water entrainment forced by high winds greatly exceeds the atmospheric pool of nitrogen washed out by precipitation. At the frontal zone of the Kattegat Strait and Skagerrak, the nitrogen deep-water entrainment is very high and this explains the high 10-year return chlorophyll level at 8 mg m −3 in the Kattegat Strait. In the southern part, the extreme chlorophyll level is only 4 mg m −3 according to the statistics of a multi-year time-series of water samples. The chlorophyll level varies greatly in time and space as documented by a series of SeaWiFS satellite maps (OC4v4 algorithm) of chlorophyll ScanFish and buoy observations from an experimental period in the Kattegat Strait. It is recommended to sample in-situ chlorophyll observationCorrespondence to: C. B. Hasager (charlotte.hasager@risoe.dk) collocated in time to the satellite overpasses of e.g. SeaWiFS and ENVISAT MERIS to ensure improved mapping of the chlorophyll levels in the Danish waters.

Impact of biomass burning on ocean water quality in Southeast Asia through atmospheric deposition: field observations

“…Atmospheric inputs of nutrients (nitrogen (N), phosphorus (P) species and iron) have been hypothesized to play an important role in the chemical and biological dynamics of open oceans and aquatic ecosystems although delivered as a diffuse flux in contrast to localized river inputs (Menzel and Spaeth, 1962;Duce, 1986;Duce et al, 1991;Prospero et al, 1996;Spokes et al, 2000;Whitall et al, 2003). The role of atmospheric inputs as an annual source of nutrients to the euphotic zone has recently been evaluated (Duce, 1986;Duce et al, 1991;Hasager, 2003;Duce et al, 2008). Increasing quantities of atmospheric fixed N of anthropogenic origin, entering the open ocean, could account for up to about a third of the ocean's external (non-recycled) N supply and up to 3% of the annual new marine biological production (Duce et al, 2008).…”

High Spatial and Temporal Variability of Dry Deposition in a Coastal Region

“…The reason for such conclusions is that the observed distribution of the dry deposited ammonia is supported by too coarse (in both time and space) a network of measurements. In the same study Hasager et al [5] concluded, on the basis of the results from a 30 km grid spacing model, that the nitrogen input to the marine surface by dry deposition is insignificant for algal blooms in the area of the Kattegat strait. However, the present study indicates that local deposition events of much larger amplitude can occur under certain situations, for example when the sea-breeze significantly modifies the large-scale flow.…”

“…In spite of this, even large scale transport and deposition models have been regarded as successful in reproducing the observed state (e.g. [5]). The reason for such conclusions is that the observed distribution of the dry deposited ammonia is supported by too coarse (in both time and space) a network of measurements.…”

Impact of biomass burning on surface water quality in Southeast Asia through atmospheric deposition: field observations

“…Atmospheric inputs of nutrients (nitrogen (N), phosphorus (P) species and iron) have been hypothesized to play an important role in the chemical and biological dynamics of open oceans and aquatic ecosystems although delivered as a diffuse flux in contrast to localized river inputs (Menzel and Spaeth, 1962;Duce, 1986;Duce et al, 1991;Prospero et al, 1996;Spokes et al, 2000;Whitall et al, 2003). The role of atmospheric inputs as an annual source of nutrients to the euphotic zone has recently been evaluated (Duce, 1986;Duce et al, 1991;Hasager, 2003;Duce et al, 2008). Increasing quantities of atmospheric fixed N of anthropogenic origin, entering the open ocean, could account for up to about a third of the ocean's external (non-recycled) N supply and up to 3% of the annual new marine biological production (Duce et al, 2008).…”