Report No DWI0630

Mar 1988

Summary of main conclusions

  1. Slight changes in the method used to prepare sediment slurries can result in large changes in the measured nitrifying activity. This makes comparisons between studies, using different methods, extremely difficult.

  2. Methods to study sediment nitrification processes which do not disrupt preformed substrate gradients within the sediment provide the most reliable rate estimates.

  3. On an annual basis the nitrate load to Grasmere lake from the catchment was 157 kg N/ha compared to nitrate produced by nitrification within the lake of 80 kg N/ha. If these separate nitrate loads were calculated on a monthly basis their relative imp ortance to the total nitrate load to the lake showed considerable variation.

  4. Rates of nitrification were greatest in the surface sediments of the littoral zone. However, in terms of nitrate produced within the lake the water column had most influence as the volume which could support nitrification was 700 times greater than the active volume of sediment.

  5. Increased eutrophication of lakes resulted in increased rates of nitrification in the deeper waters and surface sediments of the littoral zone. Such effects were not observed in profundal sediments possibly due to inhibition by unknown mechanisms.
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