In principle you can quantitatively and cheaply measure any colored substances (dissolved or suspended) with a spectrophotometer. Just experiment and find the best wavelength(s), and you should be able to measure the degree of pinkness as absorption or transmission.

Your associate is probably aware that the pink coloration is caused by high concentrations of beta-carotene (maybe also other carotenoids?) in algae, bacteria and zooplankton. Those pigments can in principle be quantified in a spectrophotometer. Usually, one would filter a known volume of lake water onto a GF/F filter, extract it (e.g., in 95% Ethanol, or Methanol, or Acetone... look up for an established method in the literature!) and scan the extract in the spectrophotometer. In this way, you won't have the particles (algae cells, maybe inorganic particles) disturbing your light path. The only problem with this method is that the chlorophyll which evidently is present wherever you have algae growing will be in the extract too. One of the chlorophyll's absorbance peaks is close to the carotenoid one and you can't separate the signals in the spectrophotometer.

Nevertheless, if carotenoid concentration is much higher than the chlorophyll, you should still get an arguably good approximation. The method of choice would however be to separate the pigments in an HPLC system, if you have access to one that has already been setup for pigment analysis. (Pigment analysis is more common among marine biologists, so it could be worth a try to ask around ;)

Actually, some people have provided protocols and formulae to quantify carotenoids spectrophotometrically even in the presence of chlorophylls, e.g., Wellburn and Lichtenthaler, http://link.springer.com/chapter/10.1007/978-94-017-6368-4_3 (click on "look inside"!) but there is more literature to be found. Good luck!

If you really have a pink color, it's likely a carotenoid or several, but not necessarily beta-carotene, which would be more yellow. I would guess lycopene or even spirilloxanthin (bacterial carotenoids) might be present. Others above are correct about general methodology - solvent extraction and spectrophotometry. If chlorophyll interference is suspected, a saponification step (pre-treatment with methanolic KOH) can remove the chlorophylls and lipids, leaving the carotenoids for assay. An absorption spectrum would be wise to run first, as assuming the 'standard' carotenoid wavelength of 430 nm is likely wrong here. Lycopene absorbs maximally around 490 nm. Alternatively, you might consider some kind of color wheel approach using digital imagery - need to look into how photographers and other image processing types handle that.

Hi everyone, thanks for all your responses. I've been travelling so haven't kept up to date with ResearchGate, hence my late acknowledgement. but I will pass these responses on to my associate. Quantitative analysis of the pigments is an obvious (and I would think essential) way to go, in addition to looking for other factors that influence to what extent these carotenoids make a lake look pink ( such as depth). The lake in question is called Pink Lake and is a tourist attraction but has lost a lot of its pinkness in recent years.

As Tobias said, HPLC would be the best way to go, but it isn't easy or cheap. I used a spectrophotometer to characterize the pigments in purple-sulfur bacteria from the monimolimnion of the Great Salt Lake (see photo: http://www.aslo.org/photopost/showphoto.php/photo/23/title/great-salt-lake-purple-sulfur-bacteria/cat/502). The pigments were extracted in acetone, although ethanol or methanol might work as well. Take a look at the following publications for the specifics:

Wurtsbaugh, W.A. and T.S. Berry. 1990. Cascading effects of decreased salinity on the plankton, chemistry and physics of the Great Salt Lake (Utah). Can. J. Fish. Aquat. Sci. 47:100-109.

Reflectance spectroscopy is ideal, as you are not disturbing the natural structure of the bacterial/algal ecosystem. Extraction by organis solvents shifts the absorption maxima of the pigments. See:

Wiggli, M., A. Smallcombe and R. Bachofen: Reflectance spectroscopy and laser confocal microscopy as tools in an ecophysiological study of microbial mats in an alpine bog pond. J. Microbiol. Methods 34, 173-182, 1999

Horath, Th., Th. Neu and R. Bachofen: Characterization of endolithic cyanobacteria in dolomite rocks in central Switzerland by reflection spectroscopy, scanning electron microscopy and laser scanning microscopy. Microbial Ecology 51, 353-364, 2006

Thanks Reinhard, I'll look in to that.

Adrian.