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| Titel |
How significant are Phaeocystis colony blooms in the present-day Southern North Sea compared to a reconstructed pristine situation: a model study |
| VerfasserIn |
Christiane Lancelot, Paul Passy, Nathalie Gypens |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250099040
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| Publikation (Nr.) |
 EGU/EGU2014-14783.pdf |
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| Zusammenfassung |
| We performed simulations with the existing MIRO&CO 3D ecological model to assess the
present-day magnitude and geographical extent of undesirable Phaeocystis colony blooms in
the Southern Bight of the North Sea (SNS) receiving nutrient inputs by large European rivers.
The criterion of 4 106 Phaeocystis cells L-1 of Lancelot et al. (2009) is used to scale the
presence of undesirable bloom. These simulations are compared with a reconstructed pristine
SNS ecosystem making use of nutrient inputs calculated by the Seneque/Riverstrahler model
of the river system when all human activities on the watershed have been erased.
Interannual variability is considered by performing model runs for two contrasted
meteorological years: wet (2001) and dry (2005). Results show a large excess of
nitrogen (N) and phosphorus (P) delivery to the SNS of i.e. respectively 12 and 5
times the pristine situation. In contrast the total silicon (Si) delivery is decreased
with respect to natural conditions although not shown for every river. Qualitatively,
pristine river inputs are shown a large excess Si for pristine but excess N for both
2001 and 2005, when compared to the agreed N, P and Si requirement of coastal
diatoms.
Responding to the river nutrient inputs, either natural or of anthropogenic origin,
phytoplankton blooms are simulated in the vicinity of the river mouths, especially in the
eastern SNS receiving 78-98% of the direct river inputs and where nutrients cumulate along a
SW-NE gradient, allowing the bloom to form a wide ribbon parallel to the coast. A short
time-delay is simulated between the western and eastern SNS due to light limitation imposed
by the large sediment inputs discharged by the Thames. A spring diatom-Phaeocystis
succession is simulated for both present-day and pristine. Pristine diatoms dominate the bulk
of the phytoplankton community; Phaeocystis colonies develop in the whole domain,
especially in the Thames and Scheldt river plume though their biomass remains low and
never exceed the threshold of 4 106 cells L-1. In contrast under present-day conditions,
Phaeocystis colonies start growing when diatoms reach their maximum in April, co-occur
with and supplement them in May and June. Overall wet weather conditions are a little more
favorable to Phaeocystis than diatoms. The geographical coverage of undesirable
Phaeocystis blooms varies between dry/wet years but potentially reaches 72% of the
modeled SNS domain distributed in three spots: the eastern band (90%), the Thames
plume (6%) and a small offshore area (4%) possibly connected to the eastern band. |
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