Light Availability and Diurnal Growth of a Green Macroalga (Caulerpa-Cupressoides) and a Seagrass (Halophila-Decipiens)

The effects of daily light period on diurnal growth patterns of a green macroalga [Caulerpa cupressoides v. lycopodium f. elegans (J. Agardh) Weber-van Bosse] and a seagrass (Halophila decipiens Ostenfeld) were investigated in Salt River submarine canyon in the US Virgin Islands in summer 1984. The daily light period, in which quantum irradiance exceeded the light saturation point for photosynthesis of the macroalga and seagrass, was manipulated in situ using lamps and shades. Plant growth was measured every 6 h for 7 d under natural and experimental daily light periods. C. cupressoides grew at the same rate day and night. H. decipiens grew more during the day than at night, a pattern that persisted under continuous light and dark treatments, indicating endogenous control of diurnal growth. Growth vs daily light period curves indicate that C. cupressoides grew faster than H. decipiens in short daily light periods, consistent with the observation that the macroalga penetrates to deeper water than the seagrass in Salt River canyon. Overall growth (day + night) of H. decipiens was unaffected in lengthened light periods and reduced in shortened light periods. Chlorophyll content of C. cupressoides was not correlated with light availability, while that of H. decipiens was positively correlated. The alga and seagrass had different diurnal growth patterns but similar overall growth responses to daily light periods. This study shows that diurnal growth patterns are probably under endogenous control, while overall growth is a response to in situ light conditions.

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