Concentrations of nitrogen in sandy sediments of a eutrophic estuarine lagoon

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Concentrations of sediment organic nitrogen, dissolved inorganic nitrogen (ammonium, nitrite and nitrate), and dissolved organic nitrogen (DON) in sediments were measured at two sites in a eutrophic estuarine lagoon. One is a shallow aerobic site where macrobenthos are abundant and the other is a deep anaerobic site devoid of macrobenthos. Four species of macrobenthos (Bivalvia: Corbicula japonica, Annelida: Notomastus sp., Neanthes japonica and Oligochaeta sp.) were found in 8 sandy sediment cores collected at a shallow site in three succcessive summers. DON (170-1500 μg atom N l-1) was the major constituent of dissolved nitrogen with 10 times greater concentration than ammonium (55-180 μg atom N l-1) and 1000 times greater than nitrate (0.14-5.9 μg atom N l-1) and nitrite (0.21-1.4 μg atom N l-1). The ammonium content in anaerobic muddy sediments at the deep site (210-350 μg atom N l-1) was higher than in aerobic sandy sediments, whereas DON was higher in aerobic sediments than anaerobic sediments (90-240 μg atom N l-1). In aerobic sediments, depth profiles of DIN were nearly constant whereas DON was mostly highest at the surface. On the other hand, the increase of DON and ammonium was observed where macrobenthos was found. The occurrence of macrobenthos and high content of DON and ammonium content in the layers of sediment may suggest the influence of macrobenthos in the partitioning of nitrogen species through their motion and excretion.

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