MAGMA

Key message

Mechanistic TKTD modelling can use standard laboratory studies to evaluate macrophyte growth effects under realistic, time-variable exposure.

Challenge

Standard laboratory tests frequently apply constant exposure concentrations. In surface waters, exposure is typically transient and time-variable (e.g., pulses).

This discrepancy motivates the use of TKTD models in a regulatory context (Tier 2C), which leverage standard laboratory studies to predict effects under time-variable exposure.

MAGMA model

In Witt et al. (2026), we introduce MAGMA - a mechanistic TKTD model that translates laboratory studies into predictions for dynamic exposure profiles, focusing on one key endpoint: growth inhibition of aquatic macrophytes.

MAGMA builds on the regulatorily accepted Lemna TKTD framework and brings the approach to other aquatic plant types - also sediment-rooted species such as Myriophyllum spicatum.

MAGMA supports

• Time-variable exposure scenarios (pulses, peaks, recovery) beyond constant-exposure assumptions.
• Mechanistic interpretation by linking external exposure, internal dynamics, and growth responses.
• Regulatory application as a Tier 2C tool using standard study designs and transparent model evaluation.

Evaluation

MAGMA was calibrated and evaluated using GLP studies with Myriophyllum spicatum, including two herbicides with different modes of action:

• Thiencarbazone-methyl (ALS inhibitor)
• Tembotrione (HPPD inhibitor)

For both constant and pulsed exposure profiles, the model reproduced observed growth inhibition and provided generally conservative predictions, consistent with regulatory protection goals.

Implications

MAGMA illustrates how modelling and bioinformatics can support environmental risk assessment by increasing:

• Realism (explicit representation of time-variable exposure).
• Transparency (clear assumptions and quantitative evaluation of model performance).
• Transferability (adaptation across compounds and aquatic plant types).

It follows the same general idea as established TKTD concepts, but is tailored to primary producers and the endpoint growth.

Outlook

MAGMA is intended to support Tier 2C assessments. Future work will extend the framework to additional exposure scenarios and species to further improve realism in aquatic plant risk assessment.