After rereading the NEMURO code in nemuro.h, I believe that there is a mistake in the term
Bio(i,k,iopal)=Bio(i,k,iopal)+(GraPL2ZL+GraPL2ZP)*RSiN(ng)
Indeed, this would mean that all the grazing in terms of silica would be going to the opal pool, which is not correct as only a fraction of it should go to that pool.
According to Eq. 11 from Kishi et al. (2007), only the egestion is going to the opal pool, which is correct.
d(Opal)/dt=Mortality (PL)+egestion (ZL)+egestion(ZP)−decomposition(Opal to Si(OH)4)
I therefore believe that the line of code in nemuro.h should be
Bio(i,k,iopal)=Bio(i,k,iopal)+(EgeZL+EgeZP)*RSiN(ng)
This would also be consistent with the formation of detrital nitrogen (PON in the code and article).