Problems with the use of terracotta clay saucers as phosphorus-diffusing substrata to assess nutrient limitation of epilithic algae

1. We examined the diffusion properties of terracotta clay saucers, of type s often used as components in phosphorus-diffusing substrata for investigat ing nutrient limitation of epilithic algae. 2. Laboratory experiments showed that phosphorus diffusion was low and inco nsistent (0.06-2.6 mg P day(-1)) through clay saucers filled with agar cont aining orthophosphate. Similarly, in situ release of phosphorus from two ty pes of terracotta clay saucers (Australian and Italian) was variable (2-8 m g P day(-1); 5-25%) under three flow regimes (0, 0.1 and 0.3 m s(-1)) over 30 days, with most phosphorus being released during the first day. Clogging of pores by agar appears to prevent the diffusion of phosphorus through th e terracotta clay saucers. However, the two types of terracotta clay saucer also irreversibly sorbed large quantities of phosphorus (40-140 mg P). 3. Individual saucers can have markedly different physical and chemical pro perties both within and among terracotta types. Terracotta saucers also con tain large quantities of iron, calcium and aluminium that are known binding agents for phosphorus. Variability in saucer composition and diffusion pro perties makes treatments difficult to replicate and weakens comparisons bet ween studies that have used terracotta with different clay composition and manufacture. 4. We recommend that phosphorus-diffusing substrata should not be construct ed from terracotta clay components.