Converting the unit of Chl-a from (mg/L) to (mmol C m/3)

[Yunhao]

Hi everyone,

I'm trying to set the initial/boundary conditions for Telemac3d-AED model and could anyone show me the correct way of converting the unit of Chl-a from (mg/L) to (mmol C m/3)?

Since the molar mass of Chl-a (C55H72MgN4O5) is 893.51g/mol, to my knowledge the conversion should be: 1 g/m3 = 1/893.51*55*1000 mmol C m/3, is that right?

I've searched through the forum and the AED manual but no solid answer was found, any suggestion is greatly appreciated!

Best,

Yunhao 

[matt.hipsey]

Hi Yunhao

The Chl-a : C ratio can change in a algal population over time (eg based on photosynthesis rate) but usually in our models we assume its constant as a first step.

In AED the parameter is in-built and called Xcc. The model simulates biomass in terms of C, but can covert this into Chl-a for output purposes, which is stored in the DIAG variable that is called PHY_TCHLA

The parameter for Xcc is read in here:

https://github.com/AquaticEcoDynamics/libaed2/blob/a86773c1d260a8ea731ecf0f5303a7951c62e668/examples/aed2_phyto_pars.nml#L65

Many water quality modelling papers will report such a parameter, and so you can find the right value that suits your purpose (eg depending on which species/group you are trying to model)

All the best

Matt

[Yunhao]

Hi Matt,

Thank you for the quick reply and the detailed explanation, it really helps a lot! 

Now I’m trying to set the initial/boundary conditions of several algae species (green, diatom & cyano), while in the field data the only information about phytoplankton is Chl-a concentration with qualitative determination about which species is dominant. In this case how do we usually set the initial concentration (pd%p_initial) of each algae species? It seems like I need to divide the Chl-a concentration into different algae species based on their proportion, which is unknown or needs to be assumed…

With my best regards,

Yunhao

在 2019年9月24日星期二 UTC+8上午6:52:41，matt.hipsey写道：

[Thomas Petzoldt]

Hi,

to add my two cents:

1) an empirical study across a trophic gradient was published by Kasprzak et al. (2008) in Limnologica, see https://doi.org/10.1016/j.limno.2008.07.002

It contains the common rule of thumb factor and a field study showing that in reality it is highly variable. See also Fig. 1 in Cloern et al (1995) Limnology and Oceanography how the Chl:C ratio varies.

2) mol mass calculations can easily be done with the R package marelac from Karline Soetaert et al. https://CRAN.R-project.org/package=marelac

Thomas P.

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