Fluxes of nutrients and suspended solids at the marine boundary of a tropical macrotidal estuary, Darwin Harbour – observations and modelling

Many Australian estuaries characterised as slightly or moderately disturbed ecosystems have macronutrient (N & P) budgets that are dominated by exchanges at the marine boundary. This ensues from a continental landscape that is... [ view full abstract ]

Many Australian estuaries characterised as slightly or moderately disturbed ecosystems have macronutrient (N & P) budgets that are dominated by exchanges at the marine boundary. This ensues from a continental landscape that is generally impoverished in these nutrients. Although Darwin Harbour, as a tropical macrotidal estuary, is a type of system that is not well represented in Australian studies, previous semi-quantitative analyses have suggested that the same nutrient regime prevails.
In a late wet-season study (Feb – Apr 2015), a set of observations were made in Darwin Harbour under spring-tide conditions to resolve definitively the fluxes of macronutrients and suspended solids at the marine boundary. Water-quality measurements were made hourly over a full tidal cycle, as an enhancement of a regular sampling of the IMOS National Reference Station in outer Darwin Harbour. Results from four depths in the water column show that concentrations of dissolved inorganic nutrients were directly correlated with the oscillation in tidal amplitude: nitrate + nitrite, 5.9–14.0 µg/L; dissolved reactive P, 1.3–2.7 µg/L; dissolved reactive Si, 118–138 µg/L — with a lag of about 2 h. Dissolved organic forms of N and P (29.4–64.4 µg/L and <0.3–2.5 µg/L, respectively) did not show a clear pattern. Whereas, suspended solids, particulate organic C and particulate N (5.3–50.9 mg/L; 142–1170 µg/L; 23.0–123 µg/L, respectively) were at peak concentrations in the water column during maximum tidal currents at ebb and flow.
ADCP currents at key cross-sections in Darwin Harbour have also been obtained under like tidal conditions. These results reveal the spectrum of tidal currents and the volume exchanges in the estuary over a spring tidal cycle.
Early modelling results indicate that the harbour equilibrates with the boundary conditions and that increases in nutrient concentrations in the inner harbour arise from either or both of localised inputs and evapo-concentration.