SUMMARY: During summer stratification ammonia is released from the profundal sediments of Blelham Tarn (English Lake District). The quantity of ammonia released exceeds the consumption of nitrate in the hypolimnion. Nitrate dissimilation may be a component in the generation of ammonia, but only during early summer when nitrate is still available. The remainder of the ammonia arises largely from the deamination of proteins, amino acids and urea. Population estimates of bacteria which produced ammonia from nitrate, amino acids and urea were of the same order of magnitude. Numbers of bacteria which produced ammonia from nitrate increased with sediment depth, and urea decomposers were more numerous in the profundal (deep water) sediments. While nitrate was available in the water column, surface sediments converted nitrate almost exclusively to nitrogen gas. After depletion of the nitrate, the release of ammonia from washed sediment particles was largely microbiological, whereas there was a significant chemical component to the release from intact sediment cores. Chemical binding of ammonia by the sediments was demonstrated and this hindered calculations of inorganic nitrogen metabolism based on changes in water chemistry. Trace additions of C-labelled protein, amino acids and urea to sediments showed that urea was turned over the most rapidly, but more reliable estimates of available protein in the sediments are required before decomposition rates can be treated with confidence.


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