Rythmic ammonium regeneration and flushing in intertidal sediments of the Sado estuary

Rocha, Carlos

Limnol. Oceanogr., 43(5), 1998, 823-831 | DOI: 10.4319/lo.1998.43.5.0823

ABSTRACT: The effects of exposure to the atmosphere on ammonium cycling in intertidal sediments were examined at a single site in the Sado estuary (Portugal) during a tidal cycle in November 1994. During a 9-h period, covering pre-ebb to postflood, 11 high-resolution vertical profiles of dissolved and sorbed ammonia and water content were collected. In addition, the vertical distribution of temperature was measured, primarily at the beginning and the end of the exposure period. The most dramatic changes occurred at the end of the exposed period when water flooded the sediment; ~75% of the dissolved and sorbed NH4+ pool (44.2 mmol m-2) was flushed into the water column by buoyancy-driven Porewater exchange. Some 64% of the flushed inventory (28.2 mmol m-2) was produced during the exposure period, at an average rate of 4.9 mmol NH4+ m-2 h-1. The build-up of ammonium in the sediment was faster in the sorbed pool than in the dissolved pool. An average first-order carbon mineralization rate of 85.2 mmol C m-2 h-l during exposure was estimated from the ammonification rate. Residence times for the organic carbon and nitrogen pools were also calculated (37.5 and 43.4 d, respectively), giving indication of the rapidity of the turnover of organic matter possible in estuary intertidal sediments.

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