Sediment Nitrogen Trapping in a Mangrove Lagoon

Quantification of the inputs, transformations and outputs of nitrogenous species in the sedimentary environment of Joyuda lagoon, a mangrove-lined coastal lagoon on the west coast of Puerto Rico, indicates a nitrogen mineralization rate equivalent to 6•5% of the total input to the sediment. Sedimentation of particulate matter provides a nitrogen flux of c . 855 μmoles N m-2 h-1. Nitrogen fixation in the sediments provides an additional 22 μmoles N m-2 h-1. Ammonium diffusion to the water column removes 50 μmoles N m-2 h-1 from the mineralized fraction, while only 5 μmoles N m-2 h-1 are removed from the system via denitrification. Overall, the data presented here indicate that oxidative bacterial metabolism (mineralization and nitrification) in Joyuda Lagoon is severely limited. Our results lend support to the hypothesis which describes the role of tropical coastal lagoons as nutrient and carbon traps, probably a requisite for the well-being of seagrasses and coral reefs prone to degradation by uncontrolled nutrient inputs.