This question arises in aquaculture, aquaponics, aquarium chemistry, & swimming pool management.

The short answer: No, we can't manage alkalinity with aeration.

Table of Contents

• Plain Language Summary

• The key take-away

• Quick background

• Why? The charge balance!

• Other processes that change CO₂

• Other effects linked to CO₂ (Water Quality Whack-a-Mole)

• So, How do you control alkalinity?

• WQ Playground: CO₂-[Alk]

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Plain Language Summary

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The Key Take-away

For practical purposes, our main take-away is this:

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Quick Background

Aeration increases dissolved oxygen (O₂) and lowers dissolved carbon dioxide (CO₂).

Our focus is on CO₂ because it reacts with water to produce the two major components of Total Alkalinity (TA), bicarbonate ion (HCO₃^-) & carbonate ion (CO₃⁼):

Note: Carbonic acid, H₂CO₃, is in parentheses to indicate that there never is much of it in solution (much less than 1%): It's relatively slow to form, but once formed, it rapidly dissociates to HCO₃^-.

CO₂ also impacts the concentrations of the other ions that make up alkalinity, most notably borate (B(OH)₄^-).

Those reactions are reversible, so any change in dissolved CO₂ (such as from aeration) changes the concentrations of those ions.

That has led some to conclude that, because aeration lowers CO₂, it also must lower alkalinity.

Though that might sound reasonable...it’s wrong.

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Why? The Charge Balance!

Changes in dissolved CO₂ affect the concentrations of all TA ions, but differently and in a way that maintains the charge balance.

For example, as aeration lowers the concentration of CO₂, the bicarbonate ion (HCO₃^-) concentration also decreases, but the carbonate ion (CO₃⁼) concentration increases.

As a result, the single-negative charges lost with the decrease of HCO₃^- ions are partly replaced by the double-negative charges gained from the increase of CO₃⁼ ions.

Carbonate has bicarbonate's back.

As described in the following optional section, the other TA ions pick up the slack so that there is no change in charge balance, which means that there is no change in alkalinity.

So, stated more concisely...

1. Alkalinity is defined by the charge balance among its ions

2. Changes in dissolved CO₂ do not alter that charge balance

3. That means that changes in dissolved CO₂ do not alter alkalinity

4. Aeration changes dissolved CO₂

That means that aeration doesn't change alkalinity.

That's the correct text-book conclusion when aeration is considered in isolation of all other aquatic processes.

We could end here, but aquaculture systems host a web of many interacting biological, chemical, & physical processes.

We'll gain some useful insight into aquaculture water quality by briefly looking into two related topics:

• other processes that change CO₂

• other effects linked to CO₂ changes

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Other Processes That Change CO₂

Aeration isn't the only aquaculture process that changes CO₂:

micro- & macroalgal uptake of carbon dioxide remove dissolved CO₂

respiration by animals, plants, & microorganisms adds dissolved CO₂

carbonating culture water (diffusing CO₂ gas) adds dissolved CO₂

In each case, when CO₂ is removed (as in aeration) or added, and isolated from other processes, alkalinity is unchanged.

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Other Effects linked to CO₂ changes

We've seen that changes in dissolved CO₂ themselves do not impact alkalinity.

But aeration and the other processes cited above do more than change CO₂; they have other effects which do impact alkalinity.

An algae example

Algal uptake of CO₂ is accompanied by uptake of inorganic nitrogen compounds needed to build plant biomass.

The two more important forms of those nitrogen compounds change alkalinity in opposite ways:

• nitrate (NO₃^-) uptake increases alkalinity

• ammonia (NH₃) uptake decreases alkalinity

CO₂ and pH

pH is sometimes called a "master variable". When it changes, so do many core aquaculture processes.

CO₂ and pH are inversely related:

For example:

• aeration & algal uptake...

  • ⇩ lower dissolved CO₂
  • ⇧ raise pH (only to a certain point when aerating)

• carbonation & respiration...

  • ⇧ raise dissolved CO₂
  • ⇩ lower pH (to a certain point when carbonating)

Some pH-dependent processes increase alkalinity...

• algal uptake of nitrate
• dissolution of calcium carbonate (CaCO₃)

...and others decrease alkalinity...

• nitrification — bacterial production of nitrate from ammonia
• calcification — formation of shrimp carapaces, fish bones...

There are many such pH-dependent processes in culture water, each pulling in a different direction and at a different rate.

What's the net effect of CO₂-induced pH changes on alkalinity?

On a small scale — I'm thinking of a low-biomass, non-reef home aquarium — the system might reach a steady-state in which aeration plays its part in the overall alkalinity balance.

But in commercial-scale production, in which biomass is high and changes greatly over a harvest cycle, controlling alkalinity with aeration alone is impractical at best.

Let's re-state the main take-away with a bit of nuance:

Water Quality Whack-a-Mole

The issue we're facing here looms over most every discussion of aquaculture water quality: The system is highly interconnected, so we have to manage the whole system, not just one part of it.

Otherwise, when we make an adjustment to solve one water-quality problem, others may pop up unexpectedly, and we'll find ourselves playing a game of Water Quality Whack-a-Mole.

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So, How do you control alkalinity?

We'll touch on this briefly and expand on it in a separate post.

Let's use some reasoning based on what we learned above:

• managing alkalinity means that we have to change alkalinity
• you know that CO₂ doesn't change alkalinity, and you know why: because it doesn't change the charge balance among the TA ions
• so, managing alkalinity boils down to finding ways to change the charge balance among the TA ions

For now, we'll just note that we adjust alkalinity by adding chemical reagents that change the charge balance.

A few examples of common adjustment reagents:

• increase alkalinity

  • sodium bicarbonate (NaHCO₃)
  • sodium carbonate (Na₂CO₃)
  • calcium hydroxide (CaCO₃)

• decrease alkalinity

  • muriatic acid (aka hydrochloric acid of a certain strength)
  • sulfuric acid (H₂SO₄)

How much of each reagent is needed?

Sorting out the consequences of a reagent addition is not a trivial problem, especially as each of those reagents changes pH, too.

Additionally, it depends on the system's water temperature, salinity, current pH, and current alkalinity.

That's a task for the visual approach to water-quality management used in the Water Quality Map.

It's also a topic for a future post.

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[Bonus] The connection with the sea

Finally, from a broader perspective that pops up regularly in the news, the global rise in atmospheric CO₂ is increasing the dissolved CO₂ content of the upper mixed layer of the sea.

The effect of added dissolved CO₂ on seawater chemistry is the same as described here for aquaculture water chemistry:
pH decreases and alkalinity remains unchanged.

That's an important result in discussions of the sea's role in the global carbon cycle.

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