The influence of spectral light quality on growth and reproduction in the seagrass Halophila ovalis

Seagrass meadows provide crucial ecosystem services but are globally threatened. Much of the seagrass loss to date has been due to activities such as dredging, flooding and eutrophication, which alter both the quantity and... [ view full abstract ]

Seagrass meadows provide crucial ecosystem services but are globally threatened. Much of the seagrass loss to date has been due to activities such as dredging, flooding and eutrophication, which alter both the quantity and quality of light reaching seagrasses. Although there is a significant body of work on the effects of reduced light quantity, there is a poor understanding of how altered light quality affects seagrasses. Terrestrial angiosperms are sensitive to changes in light quality but seagrasses have received little attention in this area, even though dredging is known to shift light quality towards the less-photosynthetically useful wavelengths (yellow-green). Aquarium-based experiments were conducted to determine whether seagrasses respond to shifts in light quality. Adult Halophila ovalis ramets were grown under monochromatic light treatments (blue, green, yellow and red) with a control of full spectrum light. All treatments received the same quantity of light. Photosynthetic characteristics, pigment content, carbohydrate reserves, biomass and growth measures were used to assess their response. There were no significant differences (P>0.05) in the leaf pigment compliment across treatments, possibly due to the high variability of the pigment concentrations. There was a significant effect of light quality on flowering. While there were no statistically significant differences in total flowering intensity across treatments, the sex ratio was affected in green treatments, with 60% more male flowers compared to controls. Furthermore, the highest proportion of flowers to nodes were present in the red light treatments. New shoot production per day declined 50% in the blue, yellow and green treatments compared to control light, and plants grown under red light were no different to the controls. These results indicate that this seagrass may be well-suited to growing and reproducing in full spectrum and/or red dominated light, conditions likely to dominate in the shallow estuarine habitat which they were collected from, and that dredging-induced changes in light quality have the potential to affect seagrass flowering.