Seagrass lives only where enough light reaches the leaves. Make the water murkier or the leaves dirtier and watch the deep edge of the meadow retreat.
Sweeps the chlorophyll level from 0 to 30 µg/L with the other settings as they are and compares the model depth limit with the Beer-Lambert rule that the limit scales as 1 / Kd.
The science
Light in water. Irradiance falls exponentially with depth, I(z) = I0 e-Kd z (Beer-Lambert), with Kd = Kw + kchl Chl + ktss TSS: clear water 0.04 per m, plus a term for chlorophyll (phytoplankton, µg/L) and one for suspended sediment (mg/L). The Secchi depth is about 1.7 / Kd.
Epiphytes. Algae growing on the blades block light: leaf light is I0 e-Kd z e-κ E with E the epiphyte load in mg per cm². Nutrients feed them and snails graze them: dE/dt = r (Emax n/(n+Kn) - (1+g G) E), so the steady load is E* = Emax n/(n+Kn) / (1+gG).
Photosynthesis. Gross rate P = Pmax tanh(I/Ik), respiration R constant. Over a day with I(t) = Isurface sin(π t / 12 h) the net daily production is the integral of P minus 24 h of R. The relative growth rate is μ(z) = a · (net production).
Meadow. Shoot density S in each depth band follows dS/dt = S (μ (1 - S/Smax) - m), steady state S* = Smax (1 - m/μ). The depth limit is where μ = m.
Honest simplifications. Relative, illustrative rate constants (Pmax, Ik 180, R 0.12 of Pmax, a, m 0.01 per day) in the right order of magnitude, not fitted to a particular species; no tides, waves, sediment sulphide, temperature, salinity, rhizome stores or seed bank beyond a small floor; no carbon accounting (the older catalogue page on seagrass carbon covers that); chlorophyll and sediment are independent sliders here although nutrients drive both in nature; blades drawn about twice life size and the slope is exaggerated.
Models built with Blender (seagrass tufts, sea floor, fish, snails, rocks) and rendered with Three.js.