Short-term nitrogen and phosphorus release during the disturbance of surface sediments: a case study in an urbanised estuarine system (Gold Coast Broadwater, Australia)

Understanding the effects of sediment disturbances on nutrient loadings is important for the management of estuarine settings. This study investigated the initial influence of sediment disturbance on water column nutrient concentrations in a shallow estuarine setting within the Gold Coast Broadwater, using a laboratory-based approach. Undisturbed sediment cores (200 mm Ø × 330 mm length, plexiglass) were incubated before and after being subjected to a disturbance event, to investigate the effect on the immediate and subsequent short-term water column nutrient concentrations. Sediment NH4 ⁺ bio and PO ₄ ^3- bio concentrations ranged from 150 to 478 and 1.50 to 8.56 nmol g _-1 dry wt, respectively. Water column NH4 ⁺ concentrations underwent the greatest increase (>1000% or approx. 14 times greater) immediately following disturbance, with mean effluxes increasing by >300%. Thereafter, water column NH4 ⁺ concentrations and efflux rates declined to near initial pre-disturbance concentrations. Water column NH4 ⁺ concentrations accounted for 0.58%-5.50% of the depth-integrated sediment NH4 ⁺ bio concentration, indicating mobilization of the sediment bound exchangeable NH4 ⁺ . The observed changes in PO ₄ ^3- concentrations and fluxes were much lower in comparison to those observed for N-species. Following disturbance, increases in the water column PO ₄ ^3- concentration accounted for 7.16%-8.22% depth-integrated sediment bioavailable PO ₄ ^3- at +1 and +2 hours, and 5.65% at +7 hours, respectively. These results provide important insight into the potential implications of disturbance events, such as vessel activities and dredging operations, within the case study region, providing information for potential management options and relevant water quality concerns.