These decreases were interpreted as evidence of regulatory controls to reduce nitrogen in¯ux as excess inorganic nitrogen becomes available. The lack of a strong decreasing trend in the present experiment may be a result of the overall low loading of ammonium on the nutrient-treated microatolls. The addition of ammonium during the ENCORE experiment occurred only during the period in which the microatolls were ponded (3 to 4 h each low tide). The rapid uptake of ammonium in the microatolls during this period resulted in high concentrations of ammonium that only persisted for a couple of hours during each low tide (Koop et al. 1995). Counter to this explanation is the observation that P. damicornis growing in microatolls that received ammonium had low growth rates and higher rates of mortality than those growing in control microatolls (Hoegh-Guldberg 1999). This may indicate that the ammonium concentrations of the ENCORE project were higher than might be considered ``healthy'' for P. damicornis, and hence might be expected to induce biochemical controls on the in¯ux of nitrogen. These eects on growth and mortality, however, may also be related to secondary eects (e.g. great rates of algal overgrowth or disease infestation) as opposed to primary eects on coral/algal metabolism.