Let's assume no malfunction of our YSI sonde, as we are very sure of that. Recently we had spiking chlorophyll a measurements in water from Duluth-Superior Harbor (western Lake Superior), ranging from 16.4-17.9 µg/L, clearly in the "eutrophic" spectrum for this system. A microscopic analysis on slides indicated very few algae specimens (expected for this time of year), including a lack of tiny cyanobacteria (which we first thought it might be). Looking for ambient red signals using fluorescence microscopy also didn't show anything notable. One thought is that something (perhaps leaf litter) is breaking down and filling the water with active chlorophyll or its byproducts (like phaeophytin). I welcome any thoughts or ideas on what could be causing this.
CDOM might be causing the sensor to give a false-positive. Try diluting an organic-rich water sample with expected chl-a content with filtered clear water. The y-intercept (chl-a on ordinate, dilution factor (
Thanks, Michael. I think our chance to test this has passed, but this could be it. Do you know what component of CDOM would be fluorescing in the same range as chlorophyll? Non-pigment compounds?
Hi Euan
Sometimes you get lysed or broken cells which are growing in the upper waters and there are free chloroplasts floating around. I've seen this in samples at this time of year. Also Superior has very tiny old centrics which in some years accumulate at deeper levels( in the size range less than 3-4 um.).. and there are tiny chrysophytes in the range of < 3 um which I also found at deeper depths in Superior How are you preserving the phytoplankton from this layer? Superior has a good number of planktonic ciliates ( usually most abundant in spring and fall) which eat phytoplankton as well as some which have zoochlorella. I suspect with chla that high it is not leaf litter but a deep accumulation of algae or
Do you mean picocyanobacteria ("=tiny cyanobacteria") in your phytoplankton samples? They can be really difficult to see... Otherwise, I too would explore the CDOM option.
Dear Euan, Most probably the observed high levels can be due either to the pigmented bacteria or to some kind of yellow substances.
This happened to us in an equatorial lake in French Guiana, but it was more like 130 µg chla/ L determined with a Turner fluorometer. After HPLC analysis, most of it was bacteriochlorophyll.