blow up issue solved!

tracer.stat indicates undesirable increase in concentrations of some state variable(s) at the 7th time step:

         1  0.7887250195E+03
         2  0.7887190683E+03
         3  0.8475861522E+03
         4  0.9302648776E+03
         5  0.1044380477E+04
         6  0.1170169324E+04
         7  0.2228768128E+11
         8  0.2853781480E+11
         9  0.5271543990E+11
        10  0.7216373356E+11

After commenting and uncommenting source/sink terms in the equations, I discovered that this was due to fmort1 and fmort2 not property flushed into the water column:

        tra(ji,jj,1,jpgoc) = tra(ji,jj,1,jpgoc) &
                           ! source1: flushing
                           + (1.-f_p2)*flushdia*(1.-frld(ji,jj))*z_ia/e3t(ji,jj,1) &
                           ! source2: linear mortality
                           +0.
!                          + 0.7*fmort1 &
                           ! source3: quadratic mortality
!                          + fmort2


After commenting fmort1 and fmort2 terms above, tracer.stat looks more reasonable:

         1  0.7887289636E+03
         2  0.7887230246E+03
         3  0.7890159536E+03
         4  0.7902228379E+03
         5  0.7905440777E+03
         6  0.7908932570E+03
         7  0.7910913500E+03
         8  0.7913530813E+03
         9  0.7915290453E+03
        10  0.7917645388E+03