Two 13·1 m2 ponds at Auchincruive, Scotland, were used to treat the diluted liquid phase of separated piggery slurry, in order to identify the climatic and pond operational parameters which influence biomass production and nutrient removal in these systems at a constant areal loading rate. The ponds were operated from April to November at 0·12, 0·24 and 0·34 m depth as batch-fed reactors. Average 5-day biochemical oxygen demand (BOD5) loading was 6·24 g m−2 d−1 and the ponds were mixed at a mean surface velocity of 0·20 m s−1. Dry matter, chlorophyll, optical density (OD560), NO3, NO2, NH4, urea and total phosphorus were determined daily. Temperature, pH, dissolved oxygen and incident irradiance were monitored continuously. Correlation and multiple regression analyses were used to determine significant interactions between environmental factors, biomass production and nutrient removal. Both chlorophyll a and optical density were accurate predictors of dry matter biomass. All measures of pond biomass were positively correlated with elapsed time, surface daily irradiance, daylength and pH, but negatively correlated with pond depth. Significant correlations between pH and daily irradiance, maximum dissolved oxygen and forms of nitrogen (nitrite or nitrate) suggested that the final pond pH represents an equilibrium between alkalization by photosynthesis and acidification by nitrification. Total nitrogen removal was influenced by biomass, elapsed time, temperature and daily irradiance, but not by either pH or depth. The concentration of ammonium nitrogen (NH4-N) was inversely correlated with temperature, biomass, depth, daily irradiance and daylength. Nitrification was found to occur with nitrate concentration showing a strong negative correlation with daylength, reflecting an increase in nitrifying activity by the pond biomass throughout the season. Nitrate concentrations were positively correlated with elapsed time, but negatively correlated with biomass, temperature and daily irradiance. Phosphorus removal was influenced by elapsed time and biomass concentration. Removal of biological and chemical oxygen demand (COD) at the completion of the batch runs was 96% and 78·6% respectively.