Effects of light deprivation on the survival and recovery of the seagrass Halophila ovalis (RBr) Hook

Survival and recovery of the seagrass Halophiln ovalis (R.Br.) Hook during and after light deprivation was investigated to assist in the interpretatio n of recent losses of Halophiln spp. in Queensland, Australia. Light depriv ation experiments were conducted in outdoor aquaria and in situ at two wate r depths. Halophiln ovalis plants were deprived of light for a maximum of 3 0 days, and recovery processes were investigated for up to 18 days followin g 15 days of light deprivation. Measurements of H. ovalis biomass, storage carbohydrate concentrations, chlorophyll a+b concentrations, stable carbon isotopes ratios (delta(13)C) 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 comp lete plant death occurred after 30 days. During the recovery period, biomas s continued to decline for a short duration of time before stabilising. Sug ar concentrations declined rapidly for the first 2 days of light deprivatio n before stabilising, then increased rapidly during the recovery period, Ch lorophyll a+b concentrations were sensitive to very small differences in li ght availability: concentration decreased in total darkness, remained uncha nged at 0.1% of surface irradiance and increased at 0.5% of surface irradia nce. Photochemical efficiency of photosystem II (F-v/F-m) remained unchange d during the light deprivation and recovery periods. The lack of response i n delta(13)C during light deprivation indicated the cessation of carbon fix ation. Decreased sugar utilisation after 2 days of light deprivation indica ted a reduction in respiration and growth. Starch concentrations did not ch ange during light deprivation, suggesting the inhibition of starch utilisat ion 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 t o a build-up of the phytotoxic end products of anaerobic respiration. Overa ll, H. ovalis has a very limited tolerance to light deprivation when compar ed to larger species of seagrass. Consequently, the persistence of this spe cies in coastal marine environments may be dependent upon the occurrence an d duration of transient light deprivation events. (C) 1999 Elsevier Science B.V. All rights reserved.