question on chlorophyll

[seashellingolds]

Hi All

Maybe this is too basic question but I simply cannot get it.
In the Fennel's model, we calculate the new phytoplankton using P(new)=P(old)+cff*(Nnew)
However, the calculation of chlorophyll uses N(old) instead of N(new). Is there any reason about that?
Thanks a lot

[kfennel]

Phytoplankton is calculated using the implicit scheme as described in the comments right before the start of the ITER LOOP. This is done to avoid negative concentrations, in the case of phytoplankton growth, of the nutrients that are being consumed in the process. Chlorophyll synthesis is different in that it is not connected to the other variables in the sense of mass conservation. In other words, the sum of all the N, P, Z and D variables is conserved, but chlorophyll isn't. There is no danger of creating any negative concentrations while chl is being synthesized, hence no implicit treatment there.