Light climate and energy flow in the seagrass canopy of Amphibolis griffithii (J.M. Black) den Hartog

Absorption of light and radiation use efficiency (RUE) were measured in a dense stand of the seagrass Amphibolis griffithii in Warnbo Sound, a temperate marine embayment in southern Western Australia. Total light intercepted by the canopy was measured and compared with dry weight leaf production, under both summer and winter conditions. RUE was found to be higher in winter (1.56 g MJ(-1)) than summer (1.01 g MJ(-1)). These values are very similar to values measured for annual crop plants and emphasise the value of applying theory developed for terrestrial crop plants to seagrasses. Canopy extinction coefficients were 0.93 ml(-1) in winter and 0.44 m(-1) in summer. There were large differences in hours above saturating irradiance (H-sat) between the top (H-sat = 5 h 14 min) and base (18 min) of the canopy in winter. Energy flows in A. griffithii suggest that this species is highly susceptible to short-term perturbations in incident irradience during the winter period as the energy stored within the rhizomes is small relative to daily respiratory demands.