Effects of light deprivation on the survival and recovery of the seagrass Halophila ovalis (R.Br.) Hook

Abstract

Survival and recovery of the seagrass Halophila ovalis (R.Br.) Hook during and after light deprivation was investigated to assist in the interpretation of recent losses of Halophila spp. in Queensland, Australia. Light deprivation experiments were conducted in outdoor aquaria and in situ at two water depths. Halophila ovalis plants were deprived of light for a maximum of 30 days, and recovery processes were investigated for up to 18 days following 15 days of light deprivation. Measurements of H. ovalis biomass, storage carbohydrate concentrations, chlorophyll a+ b concentrations, stable carbon isotopes ratios (δ 13C) and chlorophyll a fluorescence parameters ( F 0, F m and F v/ F m) were made during and at the end of the light deprivation and recovery periods. Biomass declined after 3–6 days in the dark and complete plant death occurred after 30 days. During the recovery period, biomass continued to decline for a short duration of time before stabilising. Sugar concentrations declined rapidly for the first 2 days of light deprivation before stabilising, then increased rapidly during the recovery period. Chlorophyll a+ b concentrations were sensitive to very small differences in light availability: concentration decreased in total darkness, remained unchanged at 0.1% of surface irradiance and increased at 0.5% of surface irradiance. Photochemical efficiency of photosystem II ( F v/ F m) remained unchanged during the light deprivation and recovery periods. The lack of response in δ 13C during light deprivation indicated the cessation of carbon fixation. Decreased sugar utilisation after 2 days of light deprivation indicated a reduction in respiration and growth. Starch concentrations did not change during light deprivation, suggesting the inhibition of starch utilisation by anaerobic conditions within the plant. Plant death after 30 days was notably faster than previously reported for other species of seagrass. The rapid die-off may be due to a shortage of available carbohydrates or due to a build-up of the phytotoxic end products of anaerobic respiration. Overall, H. ovalis has a very limited tolerance to light deprivation when compared to larger species of seagrass. Consequently, the persistence of this species in coastal marine environments may be dependent upon the occurrence and duration of transient light deprivation events.