DON.f

!> @file
!> @brief This subroutine calculates change in dissolved organic nitrogen concentration.
!> @details This subroutine calculates change in dissolved organic nitrogen concentration as developed for the 2012 Master Plan models.
!> Refer to Appendix D-1 'Eco-hydrology Model Technical Report' from the 2012 Master Plan report for detailed
!> descriptions and development of these water quality routines.

!> @author  Eric White - The Water Institute of the Gulf


!> @param[in]     TempW(N,2)      current water temperature in compartment (deg Celsius)
!> @param[in]     chem(N,ichem,1) array with previous timestep water quality constituent concentrations (mg/L)
!> @param[in]     muph            phytoplankton photosynthesis rate (1/day)
!> @param[in]		kdet20			reaction rate @ 20 deg-C for detritus dissolution
!> @param[in]		thetadet		reaction rate coefficient for detritus dissolution

!> @param[out]     DChemSum       change in concentration of water quality constituent during timestep

!> @param 		det				detritus concentration at previous timestep
!> @param 		don				DON concentration at previous timestep
!> @param 		kdet			temperature dependent reaction rate for detritus dissolution
!> @param 		kdon20			reaction rate @ 20 deg-C for DON hydrolysis
!> @param 		thetadon		reaction rate coefficient for DON hydrolysis
!> @param 		kdon			temperature dependent reaction rate for DON hydrolysis
!> @param 		kresp20	    	reaction rate @ 20 deg-C for phytoplankton respiration
!> @param 		thetaresp		reaction rate coefficient for phytoplankton respiration
!> @param 		kresp			temperature dependent reaction rate for phytoplankton respiration
!> @param         rnd             nitrogen-to-chlorphyll A stoichiometric mass ratio


      
!      Subroutine DON(DChemSum,ichem,mex,j,k)
! kday now global parameter - no longer needed to be passed into subroutine   
      Subroutine dDON(DChemSum,ichem,j)

	use params
      
      implicit none
      integer :: ichem,j
      real :: det,don
      real :: kdet
      real :: kdon
      real :: rnd
      real :: dChemSUM
      
!>> DON routine (eq. 20 of 2012 Master Plan Appendix D-1)

!>> previous time step WQ concentrations
	det = chem(j,9,1)
	don = chem(j,10,1)
      
!>> temperature-dependent detritus dissolution hydrolysis rate coefficient 
	kdet = kdet20*thetadet**(Tempw(j,2)-20.)
      
!>> temperature-dependent DON hydrolysis rate coefficient 
	kdon20 = 0.015
	thetadon = 1.047
	kdon = kdon20*thetadon**(Tempw(j,2)-20.)
      
!>> nitrogen-detritus stoichometric mass ratio
      rnd = 0.072
      
!>> change in dissolved organic nitrogen concentration
      dChemSUM = rnd*kdet*det-kdon*don
 
      return
      end
      
      !JAM Oct 2010 Chem #10

!	FNoC=1./5.681
!	fnodin=Chem(j,1,1)/(Chem(j,3,1)+10E-08)
!      DChemSUM=DChemSUM+decay(ichem,9)*					! Hydrolysis
!     &	Chem(j,9,1)*FNoC+decay(ichem,2)*Chem(j,ichem,1)

!	return
!	end
	
c-------------------------------
c        1 = NO3 + NO2
c        2 = NH4 
c        3 = DIN (1+2)
c        4 = Organic N
c        5 = TP
c        6 = TOC
c        7 = DO
c	   8 = Live Algae
c	   9 = Dead Algae = Detritus
c        10= DON
c        11= DOP
c        12= DIP   !Partition SRP = ParP*TP
c        13= ChLa  !Partition Chla= ParCla*LivA
c        14= POP !-EDW used to say 14=TKN !-EDW
c-------------------------------
	
c***********************End Subroutine for DeadA --> DON****************************************