add documentation

source/mom/npzd_src.F

@@ -865,6 +865,9 @@ C     graz_Z(i,j) is grazing of zooplankton j on zooplankton i
         ing_D = gmax*zprefD/thetaZ*bioz
         graz_D(1) = ing_D*biodiaz
         morp_D = nupt*biodiaz                  ! linear mortality
+!     this formulates N uptake by diazotrophs as the product of growth rate and an N-limitation term
+!     it is a mistake introduced by Schmittner et al. (2008)
+!     the term should read: biono3/(k1n + biono3)*avej_D*biodiaz
         no3upt_D = biono3/(k1n + biono3)*npp_D ! nitrate uptake
 #   if defined O_npzzd
         ing_D2 = gmax2*z2prefD/thetaZ2*bioz2