Phosphate uptake by intertidal algae in relation to zonation and season

The removal of phosphate from ambient seawater by whole plants of five species of fucoid algae, collected from the east coast of N. Ireland in 1988 and 1989, was followed over 6-h periods. A transient uptake pattern was observed for Pelvetia canaliculata (L.) Dcne. et Thuret, Fucus spiralis L., F. vesiculosus L. and F. serratus L., consisting of an initial period of high uptake, followed by a phase of zero uptake and then a period at an intermediate rate. Ascophyllum nodosum (L.) Le Jolis had a constant slow rate of uptake over 6 h. The initial uptake rate of F. spiralis was significantly greater than that of any other species. Phosphate uptake over a 2-h period was measured at concentrations ranging from that of ambient seawater to 25µg-at. l⁻¹ for whole plants of F. spiralis and F. serratus, using a large scale batch method. A small scale batch method was used for whole plants of P. canaliculata and sections of the other four species investigated. Uptake abilities of the algae at low concentrations of phosphate were compared using the parameter V₁ (the uptake rate at 1µg-at. l⁻¹) and at high concentrations using V_max, the maximum uptake rate. These kinetic parameters of uptake were calculated using a method that avoids bias and permits statistical evaluation of the results. The fucoid algae studied could be divided into two distinct groups on the basis of their abilities to take up phosphate from seawater. P. canaliculata and A. nodosum had low values of V₁ in winter, which were also correlated with their positions on the shore and did not vary between winter and summer. The Fucus species had higher values of V₁ in winter, which were also correlated with their positions on the shore. In summer, however, V₁-values for these species decreased and no longer correlated with their shore heights. The V_max-value for F. spiralis was higher in winter than in summer but was signifcantly greater than that of any other species at all times of year. The ecological significance of V_max is discussed in relation to nutrient limitation and the possible occurrence of patches of high nutrient concentration in the intertidal environment.