Domain of the Pond, Part 2: A Chemical Mystery

Explanation:

Hover over the above photo to peer
into the pond's underlying chemistry.

Last week we explored the complexities of the food web of forest ponds found in the Klamath Mountains of northwestern California. This week, we are exploring the chemistry of the waters themselves ... and have discovered a bit of a mystery.

The discovery is that the waters seem to be pH neutral. This means that they have a pH of about 7, which is neither acidic nor alkaline.

Why is this a mystery? Because ponds of this type are filled with humic and tannic acids from falling leaves and decaying vegetation, and typically the waters are acidic (meaning the pH level is well under 7). But it's pH neutral.

How can this be?

I have potentially unraveled the reason for this ... and there are clues below the soil, clues from the submerged plants, and a big clue from an algae growing at the bottom of the pond.

If you moused over the main image above, you saw my "x-ray" cross-section of the pond, with a complex array of chemical reactions. (Here's a larger image.) Let's tease that apart, and solve the mystery, bit by bit.

Rainfall percolates through the soil into groundwater, shown here as
the red wavy lines. Moving from left to right, the water percolates
through subsurface limestone that characterizes much of the
Klamath Mountains -- surprisingly, because the limestone was
formed when these ancient mountains were at sea level
during the Tertiary Period.

So what happens is that the pH-neutral groundwater (pH ~7)
moves through the calcium carbonate of the limestone,
resulting in calcium cations (Ca⁺²), bicarbonate (HCO₃^-),
hydroxide (OH^-), and carbon dioxide (CO₂),
the result of which is water with an alkaline pH > 7,
which enters the pond through ground seepage.

Next, as that water enters the pond system, the calcium cations (Ca⁺²) get taken up as macronutrients by the submergent vegetation of the pond (such as pondweeds). The vegetation also absorbs the carbon dioxide, as plants do, and produces oxygen (O₂).

At the same time, the calcium, bicarbonate, and hydroxide radicals are taken up by a most peculiar algae found in the pond, called chara. Chara is unusual because it is rooted in the pond sediment like a vascular plant, but it is not a vascular plant. Chara also is unique in that it is covered with a light green gritty material called marl, which is essentially lime -- calcium carbonate, CaCO₃. Marl, along with good old H₂O, is produced by the algae. And chara typically grows best in ponds with abundant calcium. Hmmm....

So what's next? Some of the chemicals entering the pond enter into other reactions, including entering into a balanced equation of bicarbonate plus hydrogen ions (H⁺) producing (and being produced from) carbonic acid (H₂CO₃) which is acidic (pH < 7). The bicarbonate is alkaline with a pH > 7.

Meanwhile, on the other side of the ecosystem, trees are dropping their leaves ... the soil is eroding sediment into the pond ... and organic matter (OM) settled on the pond bottom is decomposing.
All this action introduces humic and tannic acids which have a pH less than neutral, that is, <7.

And finally, here is the answer to why the pond is pH neutral.
The alkalinity from the bicarbonate reaction balances
the acidity from the carbonic acid and the humic and
tannic acids, resulting in a pH of about 7 ... neutrality!

And I'll bet you thought that a pond was just a simple pool of water, eh?

Reference:
Marcot, B. G. 1978. Flora and fauna of existing and potential slump pond sites in Six Rivers National Forest with recommendations for management. M.S. Thesis, Humboldt State University, Arcata CA.
Marcot, B. G. 1990. Limnology, vegetation, and classification of Coast Range slump-formed ponds. Northwest Science 64(1):55-63.

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