AbstractWaigani Lake, near Port Moresby, Papua New Guinea and Barton Broad, Norfolk, England are both shallow lakes nutrient‐enriched from sewage effluent disposal. In Waigani Lake phytoplankton biomass varied seasonally with lower levels (100‐200 mg chlorophyll α m−3) during the wet season increasing to over 400 mg chlorophyll α m−3at the end of the dry season. Secchi disc depths varied between 0. 11 and 0. 34 m. Phytoplankton productivity in Waigani Lake was very high throughout the year (range:Amax4,370‐21,000 mg O2m−3h−1) but production was lower during the wet season (range:Amax4,370‐12,700 mg O2m−3h−1). High surface productivity was recorded from August to December except on sampling days when the weather was overcast. Productivity throughout the year declined rapidly with depth. Algal biomass in Barton Broad varied from 3‐10 mg chlorophyll α m−3in winter but increased in spring and was very high in summer (200‐500 mg chlorophyll α m−3). Secchi disc depth varied from 0.21 m in August 1976 to 1.76 m in December. Phytoplankton production in Barton Broad was low in winter (range:Amax247‐1,250 mg O2m−3h−1) but increased markedly in spring and summer with the highest rate (Amax6,850 mg O2m−3h−1) being recorded in August. Surface inhibition was observed during summer except when the weather was overcast. Seasonality in nutrients and phytoplankton in Waigani Lake appear to be related to rainfall. Nutrient concentrations in Barton Broad are more closely related to phytoplankton activity which, in turn, corre