MEPS 165:283-292 (1998)  -  doi:10.3354/meps165283

Photosynthetic acclimation of Ulvarigida and U.rotundata arranged in canopies was assessed by combining laboratory and field experiments. Changes in the light field caused by the self-shading of Ulva thalli resulted in an instantaneous effect on the photosynthetic parameters. Photosynthetic efficiency decreased from the top of the canopy to the bottom as the photosynthetic active radiation/photosynthetic usable radiation ratio (PAR/PUR) increased. In contrast, the light compensation point showed an opposite pattern. Other photosynthetic parameters, light-saturated rate of photosynthesis and dark respiration, were unaffected. Both species acclimated to light gradients within the canopies, both in the laboratory and in the field, by changes in capacity of light capture and in photosynthetic performance: total chlorophyll concentration and absorptance decreased in the upper Ulva thalli, while the C:N ratio showed an opposite pattern. With regard to photosynthetic performance, area-normalized photosynthesis rate under saturating and subsaturating light increased towards the bottom of the canopies, while dark respiration was higher in the upper surface of the mats. The photoacclimation of canopies in which the thalli were randomly mixed 2 times per day showed a highly variable response: U. rigida thalli, submitted to similar mean irradiances and with comparable fluctuations in the light regime, showed high variability in chlorophyll content and growth rate, which, in turn, were not correlated. Canopies of U. rigida were also set up in the field. Total chlorophyll displayed a vertical profile similar to those obtained under laboratory conditions. Growth rate decreased exponentially through the canopy, but remained positive despite the low photon fluence rate reaching the bottom of the mat. Chlorophyll profiles in natural U. rotundata canopies suggested a structured arrangement of thalli in the field. Light attenuation through the Ulva canopies and photoacclimation due to this arrangement should be included in any model of primary production in these systems.

Canopy · PAR/PUR ratio · Photoacclimation · Photosynthesis · Ulvarigida · Ulvarotundata