Lough Ennell: laboratory studies on sediment phosphorus release under varying mixing, aerobic and anaerobic conditions

SUMMARY. 1. The exchange of phosphorus between the epilimnetic (shallow zone) sediment and water column in Lough Ennell was investigated in laboratory experiments using five intact cores. 2. Variations in water mixing, sediment suspension and aerobic–anaerobic oxygen status in the water column and its effects on sediment phosphorus release rates were determined. 3. Experimental results indicated that phosphorus release is possible under both aerobic and anaerobic conditions. Aerobic release (0.025 mg P l−1 over 5 days) was possible up to the point when mass resuspension of sediment occurred. Anaerobic release for the same period and mixing conditions was 0.183 mg Pl−1. 4. The release rate under aerobic conditions at 10°C equates to an internal areal loading of 0.134 g P m−2 yr−1, which is approximately 17% and 30% of the average total phosphorus and orthophosphate loadings respectively for the period 1974–79. 5. The results clearly implicate aerobic inorganic phosphorus release from the epilimnetic sediments as a significant source of this nutrient to the overlying water column and is likely a major factor in the continuing eutrophic status in the lake.