Ammonium, nitrite, nitrate and phosphate fluxes across the sediment-water interface in a tropical lagoon

Abstract

The sediments of Chelem Lagoon, on the coast of the Gulf of Mexico, north of the Yucatán Peninsula, were studied measuring the nutrient concentrations in the interstitial water. The average values were: ammonium 459 ± 281 µM, peaking up to 1,045 µM; nitrite 1.8 ± 1.8 PM, with a maximum of 5.3 µM; nitrate 8.2 ± 10.3 µM, reaching 34.7 µM, phosphate 0.4 ± 0.4 µM, with a maximum of 1.1 µM. Fluxes were estimated with Fick’s first law equation. In al1 cases, the mean flux of the 14 samples from the lagoon were from the sediment to the water column; these values were 182 µmol/m²h for ammonium, 0.5 µmol/m²h for nitrite, 1.9 µmol/m²h for nitrate and 0.03 µmol/m²h for phosphate. In order to determine denitrification rates, incubations of the sediments were carried out under anaerobic conditions with water from the lagoon enriched with nitrate; denitrification was found to be active in five of the 14 samples, with the highest rate in sample No. 8 (16.07 µmol/m²h), which had the greatest content of organic matter (9.2%) and the highest nitrate flux (from the water to the sediment). The calculation of the net flux in the Chelem area showed an important supply from the sediment to the water column of the four nutrients studied, greatest in ammonium.