THE EFFECT OF WATER-FLOW ON PHOTOSYNTHETIC PROCESSES OF THE ALGA ULVA-LACTUCA L

Photosynthetic rates of aquatic macrophytes are affected by the diffus ion of carbon through the boundary layer which decreases with increasi ng flow velocities. This is shown by a significant increase in photosy nthetic rates of Ulva lactuca fronds exposed to friction velocities (u ) between 0.0 and 0.3 cm s-1. No further increase in photosynthetic r ates is observed at u values between 0.3 and 1.0 cm s-1. Stagnating f low conditions at saturating light levels caused a 29 % reduction in p hotosynthetic rates. Experiments in a closed system indicate that Ulva may, utilize internal carbon sources during periods of stagnation. On a subtidal rock directly exposed to wave action, Ulva lactuca is not exposed to such conditions while in a wave flushed intertidal pool sta gnation occurs during very short time intervals based on in situ measu rements of u levels on fronds of this species using a microprocessor- controlled, autonomous recorder and hot-film sensors.