Abstract:
Coastal benthic sediments play an important role in regulating water column nutrient concentrations and primary production via nutrient regeneration and exchanges at the sediment-water interface. This study aimed to characterize the diffusive benthic fluxes of NH4 + , NO3 – , PO4 -3, and O2 in some of the most common shallow sedimentary habitats (fine and coarse sands, seagrass beds, and unvegetated patches within the seagrass beds) along the Bulgarian coast, and their changes under organic loading, through a laboratory experiment. Nutrient fluxes were dominated by a release of NH4 + to the water column in all sediment types, and a parallel uptake of NO3 – by the sediments; both fluxes increased under organic loading, possibly indicating stimulation of nitrate reduction within the sediments. The PO4 -3 fluxes were smaller, and the sediments mostly acted as a source for phosphorus under organic loading. O2 was taken up from the overlying water in all treatments and sediment types, and this flux increased under organic loading, probably in relation to the decomposition of the organic matter and spontaneous chemical oxidation of sulphide ions, released during sulphate reduction within the sediments. The study contributes towards the understanding of nutrient cycling and the role of the benthic compartment in Black Sea coastal soft-bottom habitats.