Element and nutrient mass-balances in a large semi-arid riverine lake system (the Lower Lakes, South Australia)

[摘要] Executive Summary: We undertook an analysis of historical water quality data (including nutrients and major ions, pH and bicarbonate) for the River Murray, Australia, and its terminal lake system, Lake Albert and Lake Alexandrina (the Lower Lakes) spanning the period 1979 to 1997.This was undertaken in order to elucidate the role of the Lower Lakes in the retention, transformation, and delivery of salts and nutrients to the Coorong, and how this varies with flow regime. The partial pressure of CO2 (pCO2) was highest in the river and relatively low at a site in the Lower Lakes.Values of pCO2 fluctuated above and below atmospheric values, indicating that the lakes alternated between periods of net autotrophy and heterotrophy, which was primarily determined by water column turbidity.The average pCO2 in the Lower Lakes was 460 ìatm, indicating that overall they were net heterotrophic, but to a lesser extent than other lakes with comparable Dissolved Organic Carbon (DOC) loading. We suggest the DOC entering the Lower Lakes is of a relatively refractory nature owing to long water residence times within the highly controlled river system. The Lower Lakes were consistently a sink for Filterable Reactive P (FRP), Total P (TP), nitrate+nitrite (NOx) and Si, and an overall source of Total Kjeldahl N (TKN).Measured rates of P retention agreed well with a mass balance derived from previous measurements of sedimentation rate and sediment P content for the lakes.Rates of P retention within the Lower Lakes could be described well by a model and parameters used for North American and European systems under flow conditions representing the upper 75% of river discharge.Under low flow conditions, the retention of P was variable and difficult to predict accurately.Input and output of total N were in close balance. However, there was a substantial loss of N to the sediments, suggesting that N2 fixation by cyanobacteria within the lakes made up the deficit. The Lower Lakes serve as a significant modulator of material entering the Murray Estuary, converting inorganic nutrients into an organic form and substantially increasing the N:P ratio of material entering the lakes (from ~12 to ~30 on a molar basis). Given that the material exported from the Lower Lakes was predominantly in an organic form, one would expect the initial metabolic response of the receiving coastal and estuarine waters to be heterotrophic.Thus, rather than a rapid algal bloom being associated with waters released from the Lower Lakes, one would expect an initial response in zooplankton and bacterial growth, followed by a reassimilation of the released nutrients into algal biomass.The relative enrichment in N over P that occurs in the Lower Lakes is also fortuitous because coastal waters are generally considered to be limited by N rather than P.In this context we note that the volume of water released from the Lower Lakes over the past decade has been very low owing to drought and high rates of water extraction from the river system.This has most likely resulted in substantially reduced loads of organic material entering the adjacent lagoonal system (the ‘Coorong’) and coastal waters.The impact of reduced loads on the local food-webs remains to be determined. There was a considerable input of salts to the Lower Lakes from an unknown source, amounting to 42% of the total input from the Lower Murray for Na+.We suggest this salt either originates from saline groundwater, or possibly leakage from the ocean through the barrages.In general the ratio of the major ions to chloride in the Lower Murray is slightly more elevated than in seawater. This is also reflected in the Lower Lakes, possibly supporting the hypothesis that the dominant supply of salts to the Lower Lakes is from the Lower Murray and its associated groundwater, rather than from seawater leakage.Further work is required to follow up the observations and hypotheses made here and we make the following suggestions for future research: ·To improve the hydrological flux estimates for the Lower Lakes, especially during low flow conditions; ·To measure the bioavailability and composition of dissolved organic carbon entering and leaving the Lower Lakes, and how this changes over flow regime and water source; ·To measure internal nutrient cycling processes such as nitrogen fixation and denitrification to help constrain the nitrogen budget for the Lower Lakes; To identify the sources of salt, and to understand salt cycling between the Lower Lakes and surrounding floodplain and lacustrine wetland systems.

[发布机构] CSIRO

[学科分类] 地球科学(综合)