Light, Nutrient, Algae Thresholds For Tropical Seagrass: An Ecohydraulic Growth Model

Seagrass is an important component of nature-based shore protection to reduce waves and currents, yet many coastal predictions still assume static vegetation or split growth drivers in separate models. We present a process-based model that integrates three canopy-level drivers in one framework: photosynthetically active radiation (PAR), temperature, and dissolved nutrients such as nitrogen and phosphorus. Algal competition is represented as epiphyte shading that filters canopy light. Model outputs are above- and below-ground biomass, shoot density, and allocation, controlled by a small, interpretable parameter set chosen for identifiability and fast calibration.

Method benchmarking reproduces temperate seasonal envelopes using settings consistent with Carr (2012) for seasonal dynamics and Kenov et al. (2013) for nutrient limitation, and calibrated against the Virginia Coast Reserve Long-Term Ecological Research datasets. Tropical applicability is assessed under Singapore conditions with continuous-flow mesocosms of Cymodoceae rotundata and Halodule uninervis across light and nutrient treatments; temporal trajectories of biomass and tissue nitrogen are compared to model predictions and cross-checked with literature percent cover and density where available.

For coastal-scale application, the module is coupled to Delft3D FM via Python BMI, where seagrass density and canopy traits are mapped to bed roughness and drag, and hydrodynamic fields are linked to biological components at each step. Analyses first defined a baseline seasonal pattern from light and temperature alone, then quantified additional changes when the limiting nutrient and epiphyte shading were active. This contrast yields operational threshold bands and identifies habitat types where control flips from light or temperature to nutrient limitation. The result is a screening-level workflow to test attenuation reliability, prioritise nutrient management versus physical light restoration, and support scenario design for hybrid nature-based solutions in tropical coasts.