Changes between Version 4 and Version 5 of EwEugNutrientCyclingAndNutrientLimitationInEcosim


Ignore:
Timestamp:
2010-11-22 01:34:58 (9 years ago)
Author:
shermanl
Comment:

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  • EwEugNutrientCyclingAndNutrientLimitationInEcosim

    v4 v5  
    88Primary production rates for producer pools ''j'' are linked to free nutrient concentration during each simulation through assumed Michaelis-Menten uptake relationships of the form ''P/B'',,''j'',,''=P/B'',,''max,j'',,'' N'',,''f'',,''/(K'',,''j'',,''+N'',,''f'',,'')'', where the parameters ''P/B'',,''max,j'',, and ''K'',,''j'',, are calculated as part of the Ecosim initialization using input estimates by the user of the ratios ''P/B,,max,j,, / P/B'',,''Ecopath,j'',, (Ecosim [wiki:EwEugGroupInfo Group Info] form).  The Michaelis constant ''K'',,''j'',, is set so that ''P/B'',,''j '',,''='P/B'',,''Ecopath,j'',,'when ''N,,f,,'',,,,is at the initial concentration determined by ''N'',,''T'',,'' - ∑'',,''I'',,'' ŋ'',,''i'',,'' B'',,''i'',,'when all ''B'',,''i'',, are at Ecopath base values). The user can increase sensitivity to changes in nutrient concentration (make ''P/B'',,''j'',, more variable with changes in ''N'',,''T'',, and'' N'',,''f'',,) by increasing the input ''P/B'',,''max,j'',,'' / P/B'',,''Ecopath,j'',, ratio. 
    99 
    10 The default free nutrient proportion pf is set at unity, which causes ''N'',,''f'',, to be virtually constant over time (and hence ''P/B'',,''j'',,'s to be virtually independent of nutrient concentration changes).  Thus to “turn on” nutrient limitation effects, you must set a lower value for pf, (e.g., 0.3) on the Ecosim parameters form. 
     10The default free nutrient proportion pf is set at unity, which causes ''N'',,''f'',, to be virtually constant over time (and hence ''P/B'',,''j'',,'s to be virtually independent of nutrient concentration changes).  Thus to "turn on" nutrient limitation effects, you must set a lower value for pf, (e.g., 0.3) on the Ecosim parameters form. 
    1111 
    12 Users should be aware that this simple approach to accounting for nutrient limitation can interact with the numerical method used to simulate very fast phytoplankton dynamics over time, to cause numerical instability or “chattering” in the values of phytoplankton biomass.  This happens mainly in cases where ''p'',,''f'',, is low, so that ''N'',,''f'',,is initially small.  Then any biomass decline in the system (e.g. due to decline in zooplankton biomass) results in a relatively large increase in ''N'',,''f'',,, which can cause an over-response in the calculated phytoplankton biomass(es) ''B'',,''j'',,, which then drives'' N'',,''f'',, to near zero, which in turn causes too large a decrease in calculated ''B'',,''j'',, for the next monthly Ecosim time step. 
     12Users should be aware that this simple approach to accounting for nutrient limitation can interact with the numerical method used to simulate very fast phytoplankton dynamics over time, to cause numerical instability or "chattering" in the values of phytoplankton biomass.  This happens mainly in cases where ''p'',,''f'',, is low, so that ''N'',,''f'',,is initially small.  Then any biomass decline in the system (e.g. due to decline in zooplankton biomass) results in a relatively large increase in ''N'',,''f'',,, which can cause an over-response in the calculated phytoplankton biomass(es) ''B'',,''j'',,, which then drives'' N'',,''f'',, to near zero, which in turn causes too large a decrease in calculated ''B'',,''j'',, for the next monthly Ecosim time step. 
    1313 
    1414Chattering can be reduced by using the Runge-Kutta integration option and/or higher pf settings.  Improved numerical integration procedures should allow us to avoid the problem entirely in future Ecosim versions, but at present the computational cost of avoiding the problem by 'brute force' (shorter simulation time steps) would be prohibitively expensive of computer time.