Magela Creek system, Northern Australia. II. Material budget for the floodplain

Abstract

A materials budget was estimated for the Magela Creek system during the 1982-83 wet season. This tropical system in northern Australia consists largely of a well-defined creek (Magela Creek contributes approximately 50% of the total inflow to the floodplain) flowing into an extensive wetlands area and then into the East Alligator River. Intensive sampling of creek water, rainfall and water flowing from the system provided the data base for the budget calculations.

The annual transport of both dissolved and particulate matter by Magela Creek (area 600 km²) is very low, even when compared with other low-relief tropical systems. The annual load transported during 1982-83 was 1260 t (21 kg ha^-1) of dissolved salts and 2330 t (39 kg ha^-1) of particulate matter. Rainfall appeared to contribute all the sodium, potassium and chloride, and part of the calcium (c. 30%) and magnesium (25%) transported during the 1982-83 wet season by Magela Creek. Most of the manganese (c. 60%) (and probably iron) was contributed from weathering processes occurring in the catchment. Only small amounts of the trace metals copper, lead, zinc and uranium were transported by the creek. During the 1982-83 wet season, more trace metals were contributed in rainfall than transported from the catchment by the creek. However, this is probably atypical and resulted from dust particles that had entered the atmosphere in greater numbers due to the extended dry season. The vast bulk of the nutrients (total P 93%, NO₃^- N 86%, NH₄⁺ N 98%) added to the catchment by rainfall was removed by the catchment, probably via uptake by the vegetation. Consequently, the creek transported only very small amounts of nutrients to the floodplain. An input-output budget for the Magela floodplain was calculated. The uncertainty in the net amounts deposited or released from the floodplain was estimated using a new quantitative method developed for this purpose. The uncertainties in the net values estimated were high, ranging from around 30% for bicarbonate to 500% for uranium. These data suggest that the Magela floodplain is a net source of the major ions (sodium, potassium, calcium, magnesium, chloride, sulfate and bicarbonate) and also of iron, and a net sink for suspended solids, nutrients (total phosphorus, nitrate and ammonia) and manganese. The floodplain also appears to be a net sink for the trace metals copper, lead, zinc and uranium.