Effect of environmental forcing on the fate of nutrients, dissolved organic matter and heavy metals released by a coastal wastewater pipeline

Discharges of nutrients, urea, dissolved organic matter and heavy metals by a sewage underwater pipeline are analysed in comparison to environmental conditions in a shallow coastal zone. Variable thermo-haline stratifications of the water column and currents in upper (2.62-34.97 cm s(-1)) and deeper (0.83-10.91 cm s(-1)) layers drive vertical diffusion and lateral transport of wastewaters. Loads of reactive phosphorus (0.13 tons d(-1)) and ammonium (1.62tonsd(-1)) by the pipeline are not negligible compared to the major river loads in the gulf. High concentrations of urea (<= 11.51 mu mol N dm(-3)) were found in the area of wastewater release. Ammonium uptake (6.14-534 nmol N dm(-3) h(-1)) strongly exceeded nitrate uptake (0.19-138 nmol N dm(-3) h(-1)), indicating that discharges of ammonium by the pipeline are actively assimilated by plankton community even at low levels of light. Distribution of Zn (<= 27.7ppb), Cu (<= 25.6ppb), Cd (<= 0.80ppb) and Pb (<= 13.5ppb) in the water column and the measurement of their complex-forming capacity in seawater did not indicate a persistent perturbation of the pelagic environment due to heavy metals.