I did a field survey using CTD to measure temperature, salinity and chlorophyll-a concentration. I'm using MIDAS CTD+ (Valeport), for chlorophyll-a concentration I used a Chlorophyll Fluorometer sensor. After I looked at the data I did't find chlorophyll-a concentration data, I just saw the fluorometer (c), fluorometer (f), and fluorometer (r) data. Can anybody help define from the 3 data that represent the chlorophyll-a concentration in the water or how to calculate from fluorometer data (ug/l) into chlorophyll-a concentration data (mg/m3)?
Hi Eko,
Without having taken water samples for chl-a analysis, you need to use the fluorometer data with extreme caution - there is no unique fluorescence efficiency for phytoplankton (it changes between populations and over time within populations, depending on physiological and environmental state).
That said, Valeport clearly do a lab calibration using phytoplankton culture, which gives you an estimate of chl-a, so although you can't state from the fluorometer data alone what the chl-a concentration truly was, you can infer relative spatial and temporal changes (but beware diurnal variability from fluorescence quenching!).
From your text, it seems that you've found the chl-a estimate in one of those columns of data (c) or (f) I expect. The units ug/l : micrograms per litre, are identical to mg/m3 : milligrams per metre cubed (micrograms -> milligrams is a factor of 1000; 1 litre is 10 cm x 10 cm x 10 cm so the litres -> m3 conversion is also a factor of 1000). So you have your chl-a data from the fluorometer already: the only work you could do to improve the data quality would require in situ sampling (at the same time and over a range of depths/locations as your fluorometer profiles) for chl-a determination.
I hope that helps a little, Jill.
Hello, Eko,
In my opinion, the fluorometer provides the first estimate of chlorophyll-a with rapid measurement, but if a more accurate Chla concentration is required, generally, you should synchronously measure the water samples at discrete depths with the High-performance liquid chromatography (HPLC) method, which is regarded as the standard determination of Chla concentration.
Then you get a fluoresence profile (its shape could be trusted) and discrete HPLC Chla (its absolute value is trusted), and apply an interpolation method to extend the HPLC results to the complete profile based on the fluorescence shape.
If you don't have HPLC measurement, you could use the satellite-based Chla concentration (like MODIS GSM Chla product), to regress an empirical relationship between satellite Chla and surface in situ fluorescence. But be careful of the fluorescence quenching effect (an abnormal fluorescence decline at surface at high light condition).
I developed a robust method to correct the quenching effect based on the estimated mixed layer depth, I recommend you to have a look at it.
Any more questions are welcome.
Xiaogang
Agreed. If you really want an estimate of in situ chlorophyll, some direct measure is needed. If you don't have an HPLC, acetone extraction of water samples is another tried and true standard method (with all the same caveats the Jill mentioned above regarding quenching, natural variability, etc). Also keep in mind the raw fluorescence is on a percentage scale which may be different than the concentration scale (e.g., ug/L) on some instruments. I have found rapid in situ measurements of fluorescence are great for showing relative change, such as identifying the sub-surface chlorophyll max - but not necessarily for actual concentrations.
Is it possible to convert MODIS giving nFLH (normalized fluorescence line-height) (W cm-2 um-1 sr) product into chlorophyll concentration (mg m-3) ???? Please provide relevant information.
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