FACTORS AFFECTING SURFACE ALKALINE PHOSPHATASE ACTIVITY IN THE BROWN ALGA FUCUS SPIRALIS AT A NORTH SEA INTERTIDAL SITE (TYNE SANDS,SCOTLAND)1

Surface alkaline phosphatase activity (APA) was measured in the tips of Fucus spiralis L. thalli over an approximately 1-year period, using small plants from upper littoral rock pools at a site on the east coast of Scotland. Maximum APA (4.4 μmol p-NP·g dry wt^?1·h^?1) occurred in April, coincident with a sharp increase in tissue N:P ratio of the tips and the probable period of most rapid growth. Enzymatic activity was not correlated with any of the physical and chemical factors (temperature, pH, external phosphate, salinity) measured in the pools or nutrient concentrations (C, N, P) in the apical part of the thallus. No obvious correlation was found between APA and the position of a plant on the shore. However, APA was significantly higher in plants emersed at low tide than in plants submerged during the whole tidal cycle. There was a significant positive correlation between APA and tissue P within a single thallus, with the highest value at the tip (which includes the meristematic zone). Changes in APA of Fucus spiralis appear to result from a complex interaction of biotic and environmental factors, including stress by emersion, growth period, rapid changes in tissue nutrient content and N:P ratio, and regzon of thallus.