Ammonification rates and 210Pb in sediments from a lagoon under a wet tropical climate: Marica,Rio de Janeiro state,Brazil

The ammonification rates in surficial sediments of the Marica lagoon (Rio de Janeiro, Brazil) were estimated using two methods:

(1)

A direct evaluation of the ammonia productivity in sediments by means of an incubation procedure: the ammonia production rate is deduced from the difference in the ammonia concentrations in interstitial waters which were extracted directly after sampling and after incubation from two samples collected at the same point.

(2)

Since ammonia results essentially from the destruction of organic nitrogen, the ammonia productivity will be obtained by evaluating the nitrogen destruction rate: the amount of organic N mineralized is assumed to correspond to NHinf4sup+produced. The organic N destruction rate can be calculated by means of the matter balance equation provided the organic nitrogen profile and the sediment accumulation rate are known.

The sedimentation rate, evaluated by means of the ²¹⁰Pb method, differed in various areas of the lagoon. It was 0.35 cm y^–1 in the open area of the lagoon (at 1.6 m in water depth), near the mouth of the Mombuca river (site M2); and 0.27 to 0.28 cm y^–1 at 1.6 to 1.7 m at a greater distance of the river mouth (site M4). In a confined part of the lagoon (Z1), surrounded by reeds (Typha dominguensis), the rate is lower: 1 to 1.4 mm y^–1 in the upper level (from 0 to 4 cm) where the organic matter amount reaches 60 to 80% (dry w.), and 1.1 to 1.2 mm y^–1 deeper (below 5 cm) where organic matter corresponds to less than 40% of the sediment.