Oxygen conditions on surfaces of coralline red algae

Abstract

Oxygen profiles above coralline algal surfaces were measured with micro-electrodes. At flow veloc~ties of 1 to 3 cm S-' in the mixed layer, the diffusive boundary layer was 0.1 to 0.2 mm on algae which were frequently grazed by limpets and chitons. It extended to 0.5 mm above surfaces on which a microbial film had developed after 16 d without grazing. When the mixed layer above grazed algae was not stirred the diffusive boundary layer reached 2.5 mm. The lowest 0 2 concentration on the algal surface was 11 % of air saturation (Lithothamnion sp. dark, unstirred), and the highest concentration was 397 % of air saturation (Lithothamnion, light, unstirred). With a flow velocity of 1 to 3 cm S-' in the mixed layer, the corresponding range was 53 to 211 % of air saturation. The O2 consumption rate was 0.8 mm01 m-2 h-' 0 2 production rates at light saturation were 3.2 mm01 m-2 h' (Corallina sp.) and 10.4 mm01 m-2 h-' (Lithothamnion). For a Lithothamnion sp. the compensation point was at 10 pm01 quanta m-2 S-', and the photosynthetic O2 production was saturated at 50 to l00 pm01 quanta m-2 S The biofilm which developed in the absence of grazers increased the 0, consumption of the surfaces, but no anaerobiosis was observed on coralline surfaces under any of the conditions tested. Coralline algae (Rhodophyta, Corallinaceae) can be found in the photic subtidal of most rocky sea coasts (Johansen 1981). Despite their slow growth, corallines compete successfully for large proportions of the hard substratum under a wide variety of environmental conditions. Intensive grazing appears to be important for the prevention of overgrowth and thus for the longterm survival of coralline algae in many natural habitats (Steneck 1986). Limpets, chitons, abalone, sea urchins and finfish are among the herbivores capable of keeping the coralline surface clean. The coralline algae provide food and shelter for the newly-settled larvae and juveniles of at least some of