Measuring during a full day

One of the possibilities explaining the high pH would be photosynthetic activity, removing the CO2 from the water. The samples are taking close to the surface, but the Weerwater is not a deep lake. At the same time the wind will stir the water and additional CO2 will be solved, so it’s hard to tell which effect is stronger. The only thing we know is that photosynthesis needs light and therefore it makes sense to measure the pH in different light conditions. To investigate the effect I decided to take samples every couple of hours, at just one point of the Weerwater, close to where I live.

The Maastrichtkwartier has a funny area with a jetty, together shaped – more or less – as a boat.

Because the location is close to where I live, I was able to walk down to this spot, do some sampling and return to home again. Earlier I noticed that the pH of the samples will vary with the (micro-) location. Even samples taken just one metre apart (some in a sunny place, others more in the shadow) will differ in their pH value. That’s why I took three different samples this time, only about a couple of metres apart. The samples were measured immediately after bringing them up and discarded afterwards, except for the third one. The third one was measured again 46-64 hours later, to check whether the pH changed a lot.

I have to admit that I sampled around midnight and early in the morning again, but I was not enough of a fanatic to get up in the middle of the night as well. At least I was not willing to do so for now, not having seen any effects yet.

Again the buffers were measured before and after the experiment and the accuracy is ok, with the precision being within a couple of hundreds.

Averages of three samples for every moment during the day, with an 99% Confidentiality Interval

Because I measured three samples, I was able to determine the standard deviation and calculate a 99% Confidence Interval (green lines). The whole Y-axis only covers 1 full pH point, so it is clear that the chance of the water actually being neutral is negligible. At the same time the differences between evening or midnight and sunny conditions during the day are also very small. It is interesting that for some moments the standard deviation was larger than for others. A very small part of it is the caused by the pH meter, but the location seems to be more influential.

In the end, the high pH found in several lakes can’t be the result of plants taking away CO2. There is a small effect, probably causing the pH to change with tens of a point, but certainly not bringing the pH up a whole point from neutrality (from 7 to 8 or more)!

In scientific terms our hypotheses would have been:

H0 photosynthesis does not change the pH from neutrality to a value around 8 (night values won’t be neutral, that is).

H1 photosynthesis is responsible for the high pH found in the lakes (and during the night the pH will drop to neutral or even acid values).

Not being able to reject the H0 is not proof that H1 is false, but in this case the variations are so small that H1 is nowhere in reach! In the past I plead for a “confirmation interval” in addition to the “rejection interval” (outside of the 95% or 99% confidentiality interval), to avoid wrong conclusions and here things are rather clear. There might be a very small photosynthetic effect, but it’s not enough to explain a pH of 8 or even higher. We could even say that the pH did not change significantly during the day and we have to look for another explanation.

46-64 hours later (depending on the age of the sample) I measured all third samples again and put them in a graph. The original values of the third samples (not the averages of the three shown in the graph above, because only sample 3 was kept!) are shown as a grey reference-line. Unlike previous series, the pH drop was rather small, but the temperature was also 10 oC lower. If biological activity is causing the drop, then it makes sense that the change was much smaller now, because of the lower temperature.

The same scale was used in both graphs, so it’s easier to compare the fluctuations during the day and the shift after two or three days.

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