SUMMARY 1. Thirty small upland lakes in Cumbria, Wales, Scotland and Northern Ireland were visited three times between April and August 2000. On each occasion water chemistry was measured and phytoplankton bioassays were performed in the laboratory to assess growth‐rate and yield limitation by phosphorus and nitrogen. In addition, yield limitation of periphyton growth was investigated twice, in situ, using nutrient‐diffusing substrata. 2. Over the whole season the percentage frequency of P, N and co‐limitation was 24, 13 and 63%, respectively, for phytoplankton rate limitation and 20, 22 and 58%, respectively, for phytoplankton yield limitation. 3. A clear response of periphyton yield to nutrient additions was found in 75% of all cases and of these, co‐limitation was most common (54%). Average percentage frequency for P and N limitation was 26 and 20%, respectively. 4. Phytoplankton and periphyton showed seasonal changes in nutrient limitation within sites. In particular, co‐limitation became progressively more common as the season progressed. 5. The response of phytoplankton growth rate to ammonium and nitrate addition was identical, but ammonium was a slightly better source of nitrogen than nitrate for phytoplankton yield on 7% and for periphyton yield on 10% of the occasions. However, the magnitude of the effect was small. 6. The concentration of dissolved inorganic nitrogen (DIN) and the molar ratio of DIN to total dissolved phosphorus (TDP), appeared to be the main environmental factors controlling the extent of nitrogen or phosphorus limitation at a given site. Nitrogen limitation was more likely than phosphorus limitation where the DIN was <6.5 mmol m−3 and the ratio of DIN : TDP was <53. Co‐limitation was the most likely outcome at a DIN concentration <13 mmol m−3 and at a DIN : TDP molar ratio <250. Above these values phosphorus limitation was most likely. 7. The relatively high frequency of nitrogen limitation and co‐limitation at higher N : P ratios than previously reported, may result from the inability of nitrogen‐fixing cyanobacteria to thrive in these upland lakes where pH and the concentration of phosphorus tended to be low and where flushing rates tended to be high.

SUMMARY 1. The changes in the vertical distributions of red coloured Planktothrix rubescens and green P. agardhii filaments in Blelham Tarn, English Lake District, were related to vertical profiles of temperature and light attenuation and to continuous records of the surface irradiance and windspeed, from August 1999 to October 2000. 2. The potential growth rate of each organism was calculated from the irradiance and temperature at 0.5 m depths and hourly intervals throughout the year, using algorithms determined from growth rates in culture. The analyses indicated that there was sufficient irradiance for growth, integrated over the 24‐h cycle, at depths down to the metalimnion where the Planktothrix populations stratified in summer. The compensation depth for growth by P. rubescens reached a maximum of 9.3 m in spring and midsummer, and fell to a minimum of 1.6 m in midwinter; the corresponding values for P. agardhii were 7.9 and 0.5 m. 3. The mixed depth (zm) exceeded the critical depth for growth (zb) by P. rubescens (the condition preventing population increase) on only 3 days of the year; for P. agardhii, however, zm exceeded zb on 31 days, contributing to its faster decline. The stratified population of P. rubescens was the major cause of light attenuation during the summer of 2000, and resulted in competitive exclusion of P. agardhii. 4. The calculated growth rates integrated over the depth of the water column in Blelham Tarn equalled, or exceeded, the measured changes of the populations during periods when they were increasing, during summer and autumn. Close agreement between the two values was found for much of the year when allowance was made for dilution of the lake population by rainfall over the watershed. During periods of rapid decline, of P. agardhii in September 1999, P. rubescens in December 1999 and both in July–August 2000, additional losses (e.g. by chytrid parasitism and grazing) are invoked.

1. Thirty small upland lakes in Cumbria, Wales, Scotland and Northern Ireland were each visited once during June and July 2000. From each lake, samples of surface sediment epilithon, macrophytes and total dissolved nitrogen (TDN) were collected for nitrogen stable isotope analysis. As part of a wider programme, samples were also collected for chemical analysis and bioassays. 2. Considerable variation was found in d 15 N values in all measured nitrogen compartments. Some regional variation was evident but was generally weak. Sediment and epilithon d 15 N were positively correlated with d 15 N of TDN, suggesting that baseline nitrogen isotope ratios influence those in some organic matter compartments in the lakes. 3. Sediment d 15 N was higher when inorganic nitrogen concentration in the water was low, possibly reflecting reduced isotope fractionation under these conditions. However, this was not the case for epilithon or macrophytes. Sediment d 15 N values were also negatively related to annual nitrogen deposition. 4. Sediment, epilithon and macrophyte d 15 N values all showed significant relations to nutrient limitation in the lakes as determined by algal bioassays. We suggest that sediment d 15 N might be developed as a simple integrating measure of the degree of nitrogen limitation in lakes.

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