red algae? hematite precipitation? you could easily decide by optical microscopy

You do not say where the lake is located. Algae are a possibility, but that should become clear with a simple optical microscope. Have you filtered the water to see what is trapped in the filter? If the answer is that the cloudiness is sediment- and there has been no intense rain- then the answers may well lie in a small underwater landslide, or the release of gas. The slight increase of temperature is interesting. Overturn of the bottom water is not likely to increase the temperature of the surface water... unless there has been a big increase in heat flow from the volcano... and that is likely to have other visible manifestations. If the crater has a history of the catastrophic release of CO2 then I imagine it might be smart to review the procedures outlined in your disaster planning documentation, but I believe the risk is small from your description. If it is clearly not algae and if temperature and acidity continue to increase, I would urge increased caution until you get some more lab analysis.

actually, if that lake is in the photo, that water looks more like muddy, than red...

I agree it looks muddy. The acidification could be from the release of CO2 or H2S. How deep is the lake? Deep lakes can trap a significant amount of dissolved gas and catastrophic releases have had deadly outcomes. Increase in heat and the turbation of the lake waters probably indicate an increase in volcanic activity. You might want to review the articles on Lake Nyos. You should probably monitor air quality while sampling the lake. Burning feeling in the eyes or lungs indicates the presence of H2S. Unusual shortness of breath or headaches can indicate a low oxygen atmosphere.

It is said before, use optical microscopy to decide is it alga or inorganic.

I agree completely wit the answers of Ferenc Kristaly

The colour that the picture shows is far from red but anyway it could be interesting to check the comparison between the chemistry of the water before the change and the actual gas contents (SO2 for example). In some cases the water of volcanic crater lakes changed from blu or green to red, in proximity (before or after) of phreatic eruptions. https://www.researchgate.net/publication/266477967_A_way_to_hydrothermal_paroxysm_Colli_Albani_Volcano_Italy

Article A way to hydrothermal paroxysm, Colli Albani Volcano, Italy

Dear colleagues,

there are several lakes in Africa where gas outbursts took place or were looming, e.g., Lake Kivu or Lake Nyos and elsewhere in the world, even in Germany (Lake Laach, which is one of the most recent feature in terms of volcanic-related depressions). Along the East African Rift several studies have been conducted- see Tiercelin et al. 1991 to 1992. The pH is far from being strongly acidic and H2S does not seem to be the case for the headache. Has ever been tested the lacustrine system for the presence of gas seepages with bubbles of methane or CO2? Simply inflame it with a lighter and you will get first-hand information. Red and brown colors may also  have been derived from Fe-bearing oxides and hydroxides washed into the lake. But I think before we are embarking on a journey of an endless speculation what the cause might be for a poodle of brown muddy water we need a bit more substantial information as to the chemistry and mineralogy. Have the fine-grained lacustrine sediments sampled only for a closer look under the microscope or a routine XRD or XRF? Unless this has been done I suggest doing some homework first. Sorry for this blunt response but it is to maintain a certain level in the discussion and not drift away.

Harald G. Dill

Hi Uriah, a change in colour such as the one you are observing could be result of 1. Runoff which seems to be  not the case, or release of gas through the water body. CO2 could be that gas. Do you have access to a portable gas fluxmeter such as the West Systems fluxmeter to measure flux of diffusive CO2 through soils?  There are also in the market gas sensing tubes (Draeger) that can be used to measure quickly in situ the amounts of dissolved gases in the lake water. These tubes are suitable for CO2, HCl, HF, H2S, and organics.

The slight change in pH, temperature, TDS, EC in the lake could be due to evaporational process (skin effects) rather than changes in the hydrothermal system that feeds the lake. Do you have time series of S/Cl or S/F ratios that can give insights on input of magmatic volatiles into the lake? These ratios will give good  information of changes in the input of volatiles into the lake. What´s the name and location of the lake? Have you published the geochemical monitoring data of this lake? If so, could you post some of the publications to check for the time series and see if we can give you some ideas of what could be happening in the lake you are monitoring. 

Regards, María Martínez

I forgot to mention that this change in colour could be the result of overturn of the lake because of a landslide underwater. How deep is the lake?  Do you observe any bubbling on its  surface?

Although the color is a little more greenish than I would expect, it is possible that iron bacteria bloomed to take advantage of the influx of ferrous iron from anoxic bottom waters.  The greening color may be the result of an algal bloom at the same time.

Thanks Maria Cruz. Apparently I need to carry out some on-the-spot assessment using gas sensors. This is in progress. We are on the stage of data gathering, the first of its kind, therefore the data is yet to be published. The Crater Lake is situated on the Jos Plateau in Nigeria, West Africa, close to the prominent Cameroon volcanic line. I will feed you back with my findings soonest.