Dear Anila,

It is not surprising that you get a negative coefficient of correlation. The coefficient of correlation between two variables is equal to the ratio of their covariance and nonzero product of their standard deviations. The correlation coefficient is between -1 and 1.

With my best regards

Prof. Bachir ACHOUR

It is my guess that you would like a biological explanation. Due to bicarbonate uptake by algae in the sunlight both the pH and the DO will normally increase. So, could you be more specfic about the aquatic system in which you observed this? I could think of a situation (in the light) where the bottom oxygen demand of the mud prevails over the oxygen production rate by the photosynthetic organisms.

Yes, I need a biological explanation. The temperature of the waterbody was between 25-28. pH between 6.5-7.2 mostly and DO ranged between 2.5- 26 mg/l .

Was it a standing (stagnant) water body or (fast) running water? What about the algal biomass? And, was there a thick organic-rich sediment layer under the water? The pH is rather low for a system with high photosynthetic rates. Fermentation process in the mud?

Thanks for the answer. It's a standing water body. It's  organic rich  and regarding algal biomass, I haven't calculated yet.  & don't know about the fermentation process..

Dear Anila,

It is hard to judge from this distance what may be the case, but I think that the bottom (mud) plays a role in this inverse correlation pH vs. DO. What you may try is to incubate some hundred litres of this (pond) water (without sediment) in a container. Place it near the pond under the same conditions (light, temperature) and monitor the pH along with the DO for an extended period. See what happens then.

(KInd regards, Jan) 

Thanks a lot for the suggestion. Will try this.

Regards

ANILA AJAYAN

Higher dissolved O2 concentration (DO) may well be accompanied by higher dissolved CO2 concentration, both to a possible stimulation effect at the metabolic activity, or because higher (aerobic) fermenter's backpressure (*) tends to increase both the concentrations. Higher dissolved CO2 concentration may be accompanied by a tendency for lowering the pH, owing to a right-hand displacement of the equilibrium with the bicarbonate ion: 2H2O + CO2 ↔ H3O+ + HCO3-. So, the pH may indeed decrease as DO increases. 

(*) About the backpressure effect you may want to check answers to another question at this forum:  https://www.researchgate.net/post/Can_any_one_tell_me_what_will_happen_if_we_increase_pressure_1-2_bar_inside_fermenter_back_pressure_during_yeast_fermentation