Accurate monitoring and prediction of water chemistry is a key factor in sustainable and profitable aquaculture. Among the most important parameters governing water quality in closed facilities, Total Inorganic Carbon (TIC) stands out as a critical, and until now overlooked, parameter. TIC describes the total inorganic carbon load in the water and therefore provides a more complete picture of the system’s CO₂ risk than individual measurements of pH or dissolved CO₂ alone.

For decision-makers, this is important because insufficient control of TIC can affect fish welfare, operational stability, the sizing of water treatment systems, and compliance with discharge requirements. This article explains what TIC is, how TIC is related to total alkalinity (TA) and pH dynamics, and why TIC should be included in both operational management and carbon accounting.

The article is based on Searas’ work in FHF project 910827, with Baring and Mowi as industry partners, and describes the development of a model-based TIC estimator as part of the digital decision-support system AquaWARE. The purpose is to highlight how a better understanding and monitoring of TIC can contribute to more robust decision-making in land-based aquaculture.

Understanding total inorganic carbon

Total inorganic carbon refers to the total concentration of all dissolved inorganic carbon compounds in an aqueous solution. In a fish tank, TIC is distributed across three chemical forms: dissolved carbon dioxide (CO₂*), bicarbonate ions (HCO₃-) and carbonate ions (CO₃2-). These species exist in a dynamic pH-dependent equilibrium described by the following reaction chain:

CO₂(aq) + H₂O⇌ H₂CO₃ ⇌ HCO₃- + H+ ⇌ CO₃^2- + 2H+

Under typical operating conditions, bicarbonate is dominant and constitutes the majority of the inorganic carbon content. Changes in pH, however, alter the relative distribution of the three species, with direct consequences for the CO₂ levels in the water and thus fish health and operations. See figure 1 for an illustration of the equilibrium between the three carbon compounds in water.

It is important to distinguish TIC from total alkalinity (TA). These are two parameters that are closely related, but which measure fundamentally different properties of the water:

• TIC quantifies the total moles of inorganic carbon per unit volume, regardless of chemical form:

TIC = [CO₂*] + [HCO₃-] + [CO₃^2-]

• TA quantifies the solution’s acid-neutralising capacity, expressed in charge equivalents:

TA = [HCO₃-] + 2[CO₃^2-] + [OH^-] − [H+]

The difference between the two reveals the relative balance between dissolved CO₂ and carbonate:

TA − TIC ≈ [CO₃^2-] − [CO₂*]

In practical terms, TIC represents the total carbon content in the water, while TA represents the system’s buffering capacity, that is, its resistance to pH changes. Both are essential for a complete understanding of carbonate chemistry. TIC is the more direct indicator of carbon available for degassing and for the potential CO₂ concentrations that can be achieved under varying pH conditions.

The significance of TIC in aquaculture

Many aquaculture facilities appear stable based on CO₂ and pH, while at the same time containing a hidden carbon reservoir that can trigger rapid and critical CO₂ spikes. This reservoir is described by total inorganic carbon (TIC).

In intensive land-based fish farming - whether in recirculating aquaculture systems (RAS) or flow-through facilities - maintaining stable and controlled water chemistry is a prerequisite for animal welfare and economically profitable operations. The concentration of dissolved CO₂ is a primary stress factor for fish. Elevated CO₂ triggers physiological stress responses, increases the respiration rate, and in critical situations creates a positive feedback loop in which fish-generated CO₂ further increases the system’s CO₂ levels, causing a drop in pH and the release of more CO₂.

What makes TIC particularly relevant is that dissolved CO₂ represents only one fraction of the total inorganic carbon in the system. The remaining carbon, bound as bicarbonate and carbonate, constitutes a buffered reservoir that can be released as free CO₂ when pH falls. This means that even if instantaneous CO₂ readings appear acceptable, a system with high TIC levels is more vulnerable to rapid CO₂ spikes following any disturbance, whether biological, chemical or operational.

Important dynamics to take into account:

• pH drop → CO₂ increase: When pH falls, the carbonate equilibrium shifts towards the dissolved CO₂ form. In a system with high TIC, this can release significant amounts of CO₂ and quickly reach concentrations that are harmful to fish.

• Degassing strategy: Carbon can only leave the water in its dissolved form (CO₂*). Bicarbonate and carbonate must therefore first be converted before degassing can take place. A system with high TIC will therefore require a deliberate pH strategy, where careful real-time monitoring of pH and CO₂ is key to restoring acceptable TIC levels

• Stress-induced amplification: Fish stress, regardless of the original cause, increases metabolic rates and CO₂ production. In a high-TIC environment, this worsens the chemical imbalance and creates compounded risks for the biomass.

TIC monitoring therefore gives operators a tool that not only provides a snapshot of current CO₂ levels, but also a measure of the system’s latent CO₂ potential under adverse conditions.

TIC in the total carbon accounting

Carbon in closed production facilities occurs both as total organic carbon (TOC) and total inorganic carbon (TIC). TOC originates, among other things, from uneaten feed, faeces and microbial biomass, while TIC is formed through fish respiration, microbial mineralisation and chemical pH control. Together, these make up total carbon (TC). TOC is reduced through mechanical filtration and TIC is reduced through degassing. For operators and operations managers, it is important to understand how carbon moves between organic and inorganic forms, and that good management therefore requires an overview of both fractions.

In land-based aquaculture facilities, process water is discharged into a natural recipient, a river, fjord or municipal treatment plant. Environmental permits set annual limits on the quantity and composition of substances that may be discharged, with carbon being an important part of this accounting. Historically, TIC has been the most difficult fraction to track, which creates gaps in the total carbon accounting and exposes operators to the risk of unknowingly approaching or exceeding permitted limit values.

Accurate TIC monitoring will help close this gap. By integrating real-time TIC data with TOC measurements across all inlet, recirculation and discharge streams, operators can see a complete carbon mass balance. This provides a tool for understanding carbon production and its relationship with biomass growth and feed intake, and thereby optimising production within the permitted annual quota. TIC therefore simplifies documentation for regulatory reporting.

As environmental standards are tightened and production licences in vulnerable areas become increasingly competitive, documented carbon management will gain growing strategic importance.

Model-based TIC estimation in AquaWARE

Concept and driver for development

As part of this work, a model-based method has been developed and implemented in AquaWARE - decision-support software based on sensor data. The purpose is to address an important gap in current water quality management: the lack of a robust, operational estimate of total inorganic carbon that takes the entire carbonate system into account, and not only measured dissolved CO₂.

Figure 2 a+b: Synthetic TIC sensor values shown in 2a and 2b; temperature, salinity, pH, CO₂ and lye dosing in the AquaWARE-integrity platform. Characteristic drops in the values indicate automatic calibration of gas sensors to ensure correct measurements.

The goal is to develop a synthetic TIC sensor, that is, a software-based estimator that calculates TIC from high-resolution measurements of pH, temperature, salinity and alkalinity. These data are delivered by a real-time monitoring system. The approach is based on the mathematical relationships in carbonate equilibrium and is developed for continuous use in operations, without the need for separate laboratory-based TIC analyses in real time.

Functional content

The TIC sensor has been developed to:

• Estimate instantaneous TIC from available sensor data.

• Model pH scenarios: given a measured TIC, calculate the expected free CO₂ concentration under user-defined or system-detected pH conditions. This is to enable proactive risk assessment of the situation before a critical event occurs.

• Detect early signals of imbalance in the carbonate system and flag conditions where rapid CO₂ increases may become likely.

• Integrate with operational dashboards and deliver actionable results rather than purely chemical parameters.

High-resolution, continuous monitoring data, such as from AquaSENSE, is a prerequisite for reliable TIC calculation, as the distribution of carbonate in its various chemical forms is highly sensitive to small changes in pH and temperature.

Operational value

From an operational perspective, a reliable TIC sensor can provide value in several areas:

Risk reduction: A validated TIC estimate enables operators to quantify the buffered CO₂ potential in the water at any given time. This translates directly into early action, by adjusting degassing, degassing capacity or feeding plans before the fish are exposed to harmful CO₂ concentrations.

Optimisation of degassing facilities: Knowledge of TIC enables operators to size and operate degassing more precisely. In high-TIC scenarios, the need for degassing increases non-linearly with changes in pH. The synthetic sensor provides the data needed to anticipate, rather than react to, these events.

Increased production and biomass optimisation: Chronic sublethal CO₂ stress reduces feed utilisation and growth rates. By maintaining tighter control of the carbonate system, operators can keep the fish under conditions that optimise physiological performance and reduce the risk of mortality.

Carbon mass balance and regulatory compliance: As described in section 3, TIC is the central component in the total carbon mass balance required for environmental emissions reporting. The synthetic sensor provides the continuous, high-resolution TIC data needed to track inorganic carbon components across all discharge streams, integrate them with TOC measurements, and produce audit-ready records for regulatory submission. This directly supports compliance with annual carbon quotas and strengthens the operator’s position in negotiations on licence renewal and expansion.

From reactive monitoring to smarter water treatment

The development of TIC sensors is part of an important transition from reactive, threshold-based monitoring to more predictive and model-supported water treatment. TIC is a key variable in this transition because the parameter links ongoing sensor readings to the underlying chemical state of the system.

When real-time estimation of TIC is combined with pH modelling, the operator gains a better basis for assessing not only what is happening now, but also what may happen if conditions change. This is particularly important in land-based systems, where high biomass densities and closed water volumes can make chemical shifts both rapid and severe.

Such an approach is relevant across species and facility types, and it points towards more data-driven management of water quality in aquaculture. It is particularly important to have control of TIC prior to crowding operations, when biomass is often high and the fish are exposed to stress.

Conclusion

Total inorganic carbon is a key parameter that is currently only to a limited extent included in the operational management of water quality in land-based aquaculture. Today, its full significance is often overlooked in favour of simpler single-species measurements such as dissolved CO₂ or pH alone. An understanding of TIC and its relationship to buffer capacity, degassing potential and pH-driven equilibrium shifts gives fish farmers, researchers and technology developers a more complete and predictive picture of water chemistry.

Equally important is TIC’s role within the broader total carbon accounting of a production facility. As the inorganic fraction of the system’s carbon inventory, TIC is an integral component of environmental emissions accounting, and accurate quantification is increasingly necessary for compliance with annual carbon quotas. Producers that can track, report and actively manage their total carbon footprint will have a structural advantage as regulatory requirements tighten and production licences in vulnerable areas become more difficult to obtain.

The FHF project’s development of a synthetic TIC sensor represents a practical and commercially important step towards closing the knowledge gap regarding the risks associated with carbon accumulation in water. By delivering accurate, continuous TIC estimates through existing sensor infrastructure, the system enables proactive water quality management, improved biomass protection, increased operational efficiency and verifiable environmental compliance. This is of direct relevance to producers, research institutions, authorities and the wider aquaculture industry.

References

• Professor Håkon Dahle, UiB. Has shared knowledge and insights regarding microbiology and immunology. https://www4.uib.no/finnansatte/H%C3%A5kon. Dahle

• N. Metzl et al., 2024: «A synthesis of ocean total alkalinity and dissolved inorganic carbon measurements from 1993 to 2022: the SNAPO-CO₂-v1 dataset». Earth Syst. Sci. Data, 16, 89-120, 2024

• L. Jafari et al., 2024: «A descriptive study of carbon dioxide production and removal in full-scale RAS for Atlantic salmon (Salmo salar L.) post-smolt: a comparison of two different measurement methods for CO₂». Aquacultural Engineering 107 (2024) 102442

• Katrin Adamczyk et al., 2009: «Real-time observation of carbonic acid formation in aqueous solution». Science 18 December 2009, vol 326

• G. Munhoven 2013: «Mathematics of the total alkalinity - pH equation - pathway to robust and universal solution algorithms: the SolveSAPHE package v1.0.1». Geosci. Model Dev., 6, 1367-1388, 2013

• Andre Meriac et al., 2026: «Stress respiration and changes in CO₂ concentrations affect the risk of H2S toxicity - insights from model calculations». Aquacultural Engineering, 114 (2026) 102732

• The article can also be downloaded here