Desiccation and phosphate uptake by intertidal fucoid algae in relation to zonation

Removal of phosphate from ambient sea-water (c. 1 μg-at PO₄ 1^-1) by four species of fucoid algae subjected to different drying treatments was followed over 2- or 3-h periods. For Fucus spiralis, Ascophyllum nodosum and F. serratus, the rate of uptake of phosphate decreased with increasing loss of water from the thalli, and at least some plants of all three species showed a net loss of phosphate from the thalli over the first 2 h in water following severe desiccation. In Pelvetia canaliculata, however, there was no significant effect of desiccation on the mean rate of phosphate uptake, and no plants released phosphate, even after extreme desiccation (up to 85% of volatile water lost). The time courses of phosphate uptake over 3 h following severe desiccation showed that both the rate and the extent of recovery of uptake after resubmersion increased with the height on the shore at which each species is typically found. P. canaliculata showed no effect of desiccation even in the first 30 min after resubmersion; some plants of F. spiralis lost phosphate during the first 30 min but recovered to the undesiccated rate of uptake after 1·5 h; no plants of A. nodosum showed net uptake within 30 min of resubmersion; whereas all plants of F. serratus lost large amounts of phosphate in the same period. These results suggest that the sensitivity to desiccation of the nutrient uptake mechanism in different species could contribute to the typical pattern of zonation of these species on rocky shores in N.W. Europe. Phosphate uptake was not enhanced by desiccation in any of the species studied, even in plants of F. spiralis which had been incubated in sea-water enriched with nitrate for 24 h prior to the drying treatments.