Estimating nutrient budgets in tropical estuaries subject to episodic flows.

Most Australian estuaries are subject to riverine discharge regimes that are highly episodic. This characteristic poses difficulties for estimating nutrient budgets of such systems based on sampling regimes that do not resolve the discharge variation and the changes in nutrient distributions that they cause. This paper presents a method for calculating nutrient budgets in estuaries having episodic hydrology. The method utilises a simple hydrodynamic transport model that is calibrated using measured salinities and which is used to describe the transport properties of the estuary as they respond to river discharge. Using this transport model, the temporal variation in nutrient concentrations within the estuary can be resolved between sampling surveys even when the discharge events are of short duration. An inverse method is then applied to calculate internal fluxes of nutrients from measurements obtained on successive sampling surveys. The approach is demonstrated through an application to the Fitzroy Estuary in Queensland, Australia.

Divergence of carbon dioxide fluxes in different trophic areas of Taihu Lake, China.

Carbon dioxide partial pressures (pCO2) and CO2 fluxes on air-water interface in different trophic-level areas of Taihu Lake were calculated and corrected using alkalinity, pH, ionic strength, active coefficient, water temperature and wind speed on the basis of the data sets of monthly sampling in 1998. The mean values of pCO2 in the hypertrophic, eutrophic, and mesotrophic areas are 1807.8 +/- 1025.8 (mean +/- standard deviation) microatm, 416.3 +/- 207.8 microatm, and 448.5 +/- 194.0 microatm, respectively. A maximum and minimum pCO2 values were found in the hypertrophic (4053.7 microatm) and the eutrophic (3.2 microatm) areas. There was about one magnitude order of difference in mean CO2 fluxes between the hypertrophic area (27.3 +/- 17.4 mmol/(m2 x d)) and the eutrophic (1.99 +/- 4.50 mmol/(m2 x d)) and mesotrophic (2.22 +/- 4.31 mmol/(m2 x d)) areas. But there was no significant difference between eutrophic and mesotrophic areas in pCO2 and the flux of CO2. In respect to CO2 equilibrium, input of the rivers will obviously influence inorganic carbon distribution in the riverine estuary. An exponential relationship between the pCO2 values and chlorophyll-a concentrations was obtained (r = 0.8356, n = 60) in eutrophic bay. Results suggested that lake ecosystems, also may be considered as unique aggregation, which can contain and be patient of different components that have their relative independence so long as its size enough to large. A productive lake, though it has positive fluxes of CO2 to atmosphere during the most of time, is a huge and permanent sink of carbon in terrestrial ecosystems through receiving a great quantity of carbon materials via rivers, precipitation, and biological production.