What Your Water Test Kit Isn't Telling You
A typical water test kit covers the basics: temperature, pH, salinity, dissolved oxygen, maybe ammonia. That's enough to keep lobster alive under normal conditions. But it's not enough to understand why a system that looks fine on paper is still delivering mediocre survival rates, or why lobster arrive stressed despite what the shipper says is perfect transit water.
Several water quality parameters rarely make it onto an operator's daily checklist — not because they're unimportant, but because the test equipment costs more and the concepts are less intuitive. This post covers the four parameters most likely to be
Alkalinity: The Buffer Your pH Depends On
pH is the parameter everyone monitors. Alkalinity is the one that determines whether your pH holds steady or swings overnight. Most operators know one and ignore the other.
Alkalinity measures the water's capacity to resist acidification — specifically, the concentration of bicarbonate and carbonate ions that act as a chemical buffer. In practical terms: high alkalinity means pH is stable even when CO2 production changes. Low alkalinity means pH can drop significantly within hours when biofilter activity, lobster respiration, or organic loading increases.
For marine lobster systems, you want total alkalinity in the range of 100–200 mg/L as CaCO3. Natural seawater typically runs around 120–130 mg/L. Recirculating systems tend to deplete alkalinity over time as nitrification — the process that converts ammonia to nitrate — consumes bicarbonate in the process. A system that isn't getting enough new water to replenish alkalinity will see slow pH drift downward over days or weeks.
One facility running a near-closed recirculation loop couldn't understand why their pH kept reading 7.4 despite regular sodium bicarbonate additions. The root cause was a carbonate depletion cycle: they were adding bicarb, the system was consuming it through nitrification, and no one was measuring alkalinity directly. Once they started testing alkalinity weekly and adjusting doses accordingly, pH stabilized and stayed within 0.1 units of target for the rest of the season.
Dissolved CO2: The Invisible Stressor
Dissolved oxygen gets measured constantly. Dissolved carbon dioxide almost never does — even though CO2 toxicity can impair lobster at concentrations that are easy to reach in recirculating systems.
CO2 is a byproduct of respiration. Lobster produce it, bacteria in your biofilter produce it, and in a closed or semi-closed system it accumulates unless you actively remove it. The mechanism of harm is straightforward: CO2 in the blood lowers blood pH (a condition called hypercapnia), which interferes with oxygen binding at the gill and disrupts internal pH regulation. At CO2 levels above roughly 20–25 mg/L, most marine species show measurable stress responses. Above 30–40 mg/L, survival is compromised.
In a well-designed recirculating system with good aeration and surface agitation, CO2 typically stays below 10 mg/L. But systems with poor degassing — dense stocking, minimal water surface exposure, overstocked sumps — can run chronically elevated CO2 even while oxygen looks normal. This is a common scenario in facilities that added capacity without upgrading their degassing.
Off-gassing CO2 is simpler than adding oxygen. Increasing surface turbulence, adding a packed tower or cascade aerator, or improving turnover through the sump are all effective. The key is measuring CO2 first so you know whether you have a problem. CO2 test kits are inexpensive, and online calculators can estimate dissolved CO2 from pH and alkalinity measurements if you're monitoring both.
Total Gas Pressure: When the Water Itself Becomes Dangerous
Total Gas Pressure (TGP) is a measure of all dissolved gases combined — nitrogen, oxygen, CO2, and others — expressed relative to atmospheric pressure. When TGP exceeds 100% saturation, water is supersaturated with gas. Pump that water into a tank and the excess gas can come out of solution in lobster tissues, causing gas bubble disease.
Gas bubble disease is exactly what it sounds like. Gas emboli form in the hemolymph and block circulation, leading to lethargy, lesions, exophthalmia (bulging eyes), and death. It's frequently misdiagnosed as infection or handling injury because the presentation isn't always obvious unless you're looking for it specifically.
The most common source of supersaturation in lobster facilities is poorly designed pump systems — specifically, entraining air on the suction side of a pump and then forcing it into solution under pressure. A small leak in a suction line, a vortex at the pump intake, or a leaking mechanical seal can all introduce air. Cascade systems fed by gravity can also supersaturate if the water drops into a confined space with no way for excess gas to escape.
Measuring TGP requires a saturometer or dissolved gas probe — a bit more investment than a basic test kit, but worth it if you're seeing unexplained mortality with symptoms that don't fit ammonia or oxygen profiles. TGP above 103% is considered potentially harmful for sensitive species. Above 110%, mortality risk is high.
How These Parameters Interact
The reason these parameters get overlooked is that they don't announce themselves — and they tend to compound each other quietly.
High CO2 depresses pH. Low alkalinity means that pH drop isn't buffered. Low pH changes the ratio of toxic to non-toxic ammonia (in this case, reducing toxicity at lower pH — but only until you hit the point where the pH itself is stressful). Gas supersaturation can be worsened by temperature changes, since colder water holds more dissolved gas.
A system running at the edge on all four parameters simultaneously — elevated CO2, low alkalinity, moderate TAN, and periodic TGP spikes from a leaky pump seal — will produce confusing mortality patterns that no single fix resolves. The only way to diagnose it is to measure all of it at once.
A Practical Testing Protocol
You don't need to run a full panel every day, but here's a reasonable schedule for a serious holding operation:
-
Daily: temperature, dissolved oxygen, pH, salinity, TAN
-
Weekly: alkalinity, dissolved CO2 (or estimate from pH + alkalinity)
-
Monthly or after any system change: TGP, nitrite, nitrate
-
After large receipts: TAN, TGP, dissolved oxygen within 12 hours of arrival
The GasXchanger from APS is designed specifically to handle dissolved gas management — both adding oxygen and stripping CO2 in a single pass — which addresses two of the four parameters above. But the prerequisite for any solution is knowing what you're dealing with in the first place.
If you're not sure where to start or which parameters are most relevant to your specific setup, reach out to APS at aquaproduction.ca. We can walk through your system design and help identify where your current testing gaps are likely to be causing losses.
