Attack of the Algae Part 2

Yesterday I started an algae control protocol for my garden pond and decided to monitor the progress with spectroscopy. I’m simply measuring the absorbance and fluorescence (405 nm excitation) of the pond water using a Vernier Spectravis spectrophotometer. Here are the results, compared with yesterday’s spectra.

Fluorescence spectra (ex. 405 nm) of garden pond water following treatment of the pond with algaecide.
Absorbance spectra of garden pond water following treatment of the pond with algaecide.

The data on 06/01 were collected about 3 hours after the algaecide was added to the pond and today’s data were collected 24 hours after the pond was treated. I’m hesitant to draw any conclusions from these results until more data can be collected, but the differences over 24 hours leads me to start speculating.

If we assume that the 405 nm peak in the fluorescence spectra can be treated as an internal standard (a big if), then the fluorescence peak due to chlorophyll at 690 nm is going in the wrong direction. The peak has grown over 24 hours, suggesting that the algae concentration as increased. Yesterday was a cool but sunny day, so the weather may have allowed for algae growth to out-compete the algaecide.

The general shape of the two absorbance spectra is very similar, with the one obtained today having a slightly lower background. Yesterday, I speculated that the high background was due to scattering since the pond water is very cloudy. If that analysis is correct, then the decrease in background could indicate that the pond water is becoming less cloudy. Again, these thoughts shouldn’t be considered as results interpretations; rather, they are better classified as thoughts I’m having about the data. We’ll see tomorrow

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