Influence of biomass and water velocity on light attenuation of Cladophora glomerata L. (Kuetzing) in rivers

The attenuation of photosynthetically active radiation (PAR) through the filamentous alga Cladophora glomerata was investigated both in situ and in an experimental flow tank under natural and artificial lighting using a fiber-optic sensor. Over 35 separate experiments were completed to characterize downward light attenuation and water velocity effects. Measured extinction coefficients ranged from 4.3 to 150.7 m⁻¹ for biomass samples ranging from 40 to 253 mg chlorophyll-a m⁻² (41 to 381 g ash free dry weight m⁻²) at shear velocities from 0 to 51.8 cm s⁻¹. An exponential relationship between light attenuation coefficient (K_alg) and algal biomass thickness (δ) was found (lnK_alg = −1.1831 lnδ + 8.3789; r² = 0.41, p < 0.001). Unexpectedly, chlorophyll-a (Chla) and ash free dry weight (AFDW) areal density were poor predictors of K_alg. No significant correlations (p > 0.01) were found amongst any of the other experimental variables (e.g., shear velocity, Reynolds number, etc.). Cladophora mat thickness (δ) and volumetric biomass (Chla m⁻³) provided the greatest explanation of downward light attenuation in flowing water, reducing irradiance in proportion to how closely the filaments are pressed together. Observed δ generally decreased with increasing shear velocity, albeit not predictably. Results have important implications on aquatic systems where macroalgae generate variable light regimes on understory aquatic species.