Dear Colleagues

The ideal idea that decompostion of organic matter (especially leaves and stems) leads to pH decompostion seems not to work with some of my results. I refer to a small pond near a dune system, here in Camaçari (Bahia, Brazil), that was acidified in the late eitghties due to bad environmental management. At that time pH values reached 1.8. A rehabilitation program started around 1992 to seal the contaminated dune and pH rose for some years to around 3.0. In the first years of this decade, it was noticed that the sealing was not working properly as pH varied erratically during the year. An amendment of the sealing was conducted in 2005 with the use of proper sealing material. Suddenly, an aquatic plant dominated the littoral (Elleocharis bonariensis) and leaves from bushes and small trees from the nearby dune changed the system. Since then, pH values have been increasing steadily and fish mortality tests indicate an astonishing improvement in results (no mortality in 96h test) and the water also returned to its dark brown colour. We have not assessed the role of vegetation in pH increase, and then we ask more experienced colleagues, if is it possible that the nutrient removal from sediments by the aquatic plants and the leaves from the adjacent vegetation may induce a pH increase ? In theory, one would think that an increase in decompostion rates would lead to a pH decrease, but this seems to go on the other direction.

I wonder if anyone has any possible hint on what's could be happening.

Best wishes from Brazil

Eduardo M. da Silva

Perhaps the organic acids from decomposition are acting as pH buffers and actually increasing pH. I would not expect them to increase the pH higher than about 4.5 at the most.

If the aquatic plants are submersed with a dense population, and the water has some hardness, pH can increase within the mat of vegetation during the day as photosynthesis removes CO2 and HCO3 (if the species is a bicarb user). Typically during the night community respiration (CO2 release) brings the pH back down.

Although the plants may be responsible for the shift in pH, I would investigate the possibility that the sealing material was the cause. It seems like this would be something you could test with a quick lab experiment if you can get access to some of the sealent that hasn't been exposed to the acidic water.

Dear Colleagues

Thank you for your interesting comments. The pH has increased steadily from around 3.2, in 2005, when the second sealing happened to ca. 5.0 presently. During this time the Eleocharis plants thrived as ****. We are presently looking at the decomposition rate of the plants, and the first results indicate that these values are very low in comparison to other data from aquatic plants elsewhere. These results do not go along with the idea that pH has any contribution from the local macrophytes.

Regarding possible daily changes in pH values, we have thought in doing some night measurements, but personnal security has been a reason for not doing this, we cannot guarantee for the security of our students.

I will follow Sam recomendation and try to develop some experiment in the laboratory, testing the same kinds of water, and when I have the results I will post them.

Cheers

Eduardo