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Marine Ecology Progress Series

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MEPS 289:63-78 (2005)  -  doi:10.3354/meps289063

Modelling diatom and Phaeocystis blooms and nutrient cycles in the Southern Bight of the North Sea: the MIRO model

C. Lancelot1,*, Y. Spitz2, N. Gypens1, K. Ruddick3, S. Becquevort1, V. Rousseau1, G. Lacroix3, G. Billen4

1Université Libre de Bruxelles, Ecologie des Systèmes Aquatiques, CP-221, Boulevard du Triomphe, 1050 Brussels, Belgium
2Oregon State University, College of Oceanic and Atmospheric Sciences, 104 COAS Admin. Bldg., Corvallis, Oregon 97331, USA
3Royal Belgian Institute of Natural Sciences, Management Unit of the North Sea Mathematical Models, Gulledelle 100, 1200 Brussels, Belgium
4Université Pierre & Marie Curie (Paris VI), UMR-CNRS 7619 Sisyphe, 4 Place Jussieu, 75005 Paris, France

ABSTRACT: The link between anthropogenic nutrient loads and the magnitude and extent of diatom and Phaeocystis colony blooms in the Southern Bight of the North Sea was explored with the complex ecosystem model MIRO. The model was adapted for resolving the changing nutrient loads, the complex biology of the bloom species and the tight coupling between the benthic and pelagic compartments that characterise this shallow coastal shelf sea ecosystem. State variables included the main inorganic nutrients (nitrate [NO3], ammonium [NH4], phosphate [PO4] and dissolved silica [DSi]), 3 groups of phytoplankton with different trophic fates (diatoms, nanophytoflagellates and Phaeocystis colonies), 2 zooplankton groups (copepods and microzooplankton), bacteria, and 5 classes of detrital organic matter with different biodegradability. The capability of the MIRO model to properly simulate the observed SW–NE gradient in nutrient enrichment and the seasonal cycle of inorganic and organic C and nutrients, phytoplankton, bacteria and zooplankton in the eastern English Channel and Southern Bight of the North Sea is demonstrated by running the model for the period from 1989 to 1999. The MIRO code was implemented in a simplified multi-box representation of the hydrodynamic regime. These model runs give the first general view of the seasonal dynamics of Phaeocystis colony blooms and nutrient cycling within the domain. C, N and P budget calculations show that (1) the coastal ecosystem has a low nutrient retention and elimination capacity, (2) trophic efficiency of the planktonic system is low, and (3) both are modulated by meteorological forcing.

KEY WORDS: North Sea · Eutrophication · Phaeocystis · Carbon budgets · Nutrients · Ecological modelling

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