Changes between Version 4 and Version 5 of EwEugNutrientCyclingAndNutrientLimitationInEcosim
 Timestamp:
 20101122 01:34:58 (10 years ago)
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EwEugNutrientCyclingAndNutrientLimitationInEcosim
v4 v5 8 8 Primary production rates for producer pools ''j'' are linked to free nutrient concentration during each simulation through assumed MichaelisMenten 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. 9 9 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.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. 11 11 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 overresponse 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.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 overresponse 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. 13 13 14 14 Chattering can be reduced by using the RungeKutta 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.