Contribution of epiphyte load to light attenuation on seagrass leaves is small but critical in turbid waters

Abstract

Quantifying contributors to light attenuation is useful for the management of seagrass meadows. Epiphytic growth on seagrasses can lead to diminished light for the host plant, impairing photosynthesis and growth. Here, we quantify the contributions of the water column and epiphytic load to light attenuation in a Cymodocea rotundata meadow at Chek Jawa, Singapore. Using a modified spectrometer and seagrass mimics (clear polyethene strips) colonised by epiphytes, we determined the relationship between light transmission (400-700 nm) and epiphyte load. Subsequently, we derived the percentage of surface light that reaches the leaf surface (PLL) over a range of epiphyte biomass and water-column light-attenuation coefficients (Kd). Results indicated that the relative contribution to light attenuation by epiphytic biomass was greater in clearer waters (Kd < 0.5) than in turbid waters. As Kd increases, the amount of epiphytic material required to reduce PLL to minimum light requirement (11%) decreases exponentially. At Chek Jawa, the average epiphytic load was 32 mg DW cm-2, which was close to the estimated amount (33 mg DW cm-2) required to reduce PLL to 11% at prevailing turbidity levels. Our findings suggest that high epiphyte load is benign in clear waters, but becomes critical in turbid waters.