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N:Si ratios and the ‘balance of organisms’: PROWQM simulations of the northern North Sea
Tett, P.; Lee, J.-Y. (2005). N:Si ratios and the ‘balance of organisms’: PROWQM simulations of the northern North Sea, in: Mills, D.K. et al. (Ed.) Contrasting approaches to understanding eutrophication effects on phytoplankton. Journal of Sea Research, 54(1): pp. 70-91
In: Mills, D.K.; Gowen, R.J. (Ed.) (2005). Contrasting approaches to understanding eutrophication effects on phytoplankton. Journal of Sea Research, 54(1). Elsevier: Amsterdam. 1-124 pp., more
In: Journal of Sea Research. Elsevier/Netherlands Institute for Sea Research: Amsterdam; Den Burg. ISSN 1385-1101, more
Peer reviewed article  

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Keywords
    Diatoms; Eutrophication; Models; Nitrogen cycle; Ratios; Silicates; Simulation; ANE, North Sea [Marine Regions]; Marine

Authors  Top 
  • Tett, P.
  • Lee, J.-Y.

Abstract
    This paper, first presented at the EutWork meeting, includes short reviews of (i) the meaning of balance of organisms in the context of eutrophication and (ii) the simulation of the balance with numerical models. The ratio ψ of flagellate chlorophyll to total chlorophyll is used as a measure of the balance. The PROVESS Water Quality Model (PROWQM) was used to investigate the control of ψ by simulating changes during 1998 in water-column layering and the abundance of diatom-or flagellate-dominated microplankton communities at 59°N, 01°E, in the northern North Sea. This was the site of a study by the PROVESS project in autumn 1998 and near that of the FLEX investigation in spring 1976. Although not considered to be at risk of eutrophication, these waters have a high natural N:Si ratio and provide a good subject for numerical experiments concerning the control of the balance of organisms. PROWQM is a 1-D physical-biological model; its 2 microplankton compartments parameterise the microbial loop and take account of the metabolism of pelagic bacteria and protozoa as well as the photosynthesis, respiration and nutrient uptake of micro-algae. The effect of an initialisation with the long-term mean observed winter concentrations of nutrients in the northern North Sea (10 μM nitrate and 5 μM silicate) was compared with an initialisation with the same amount of nitrate but only 3 μM silicate. Simulations were run with the standard value of the surface drag coefficient and a doubled value. The latter resulted in a more accurate representation of the surface mixed layer, but did not show a midwater maximum of flagellate chlorophyll during summer. The simulation with doubled drag and initial N:Si of 10:5 was in good agreement with the long-term observed seasonal cycle of chlorophyll. Simulations with initial N:Si of 10:3 had substantially more flagellates during summer, as a result of diatom limitation by silica exhaustion. The model results support suggestions that the winter N:Si ratio must be perturbed to greater than 3:1 before the balance of organisms is much changed, if the balance is quantified by peak biomass or annual production. However, annual mean ψ was more influenced by physical conditions than by nutrient initialisation, showing that how data are analysed can have an influence on conclusions about whether the balance is disturbed. The simulated removal of nutrient up to the time of maximum chlorophyll was used to estimate values for diatoms and flagellates of the yield, q, of chlorophyll from nitrate or silicate. The silicate value (0.77 g chl (mol Si)- 1 in the best simulation) is novel. The yields from nitrate were 0.23-0.75 g chl (mol N)- 1, the range including both groups and all simulations. These were less than yields reported for inshore waters. Finally, the need for detailed study of the distributions of pelagic microflora and microfauna in late summer in the northern North Sea is pointed out.

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