Importance of Flood Flows to the Productivity of Dryland Rivers and Their Floodplains

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Final Report
Prof P.M. Davies, Prof S. E. Bunn and Ms F. Balcombe
Environment Australia, 2003

Assessing the Importance of Phytoplankton

Visiting Professor Stephen Hamilton of Michigan State University measured open-water primary production and respiration rates at three Cooper Creek sites during the dry. Open-water production by phytoplankton is of interest because even though water-column rates of production may be lower than in the 'bath-tub ring' of benthic algae along the shore, there is likely to be much more open-water area in the waterholes. These measurements were made by enclosing a water column of 80-cm depth within large, clear plastic cylinders and measuring vertical profiles of temperature and dissolved oxygen over the course of the diel cycle. The mesocosm design isolated an open water parcel from oxygen production and consumption along the edges of the waterhole, while maintaining natural patterns of thermal stratification and mixing. An approximate mass balance for dissolved oxygen, after accounting for gas exchange of the atmosphere, allows estimation of the photosynthetic rate of phytoplanktonic algae in the uppermost water column where light is sufficient. Benthic respiration is effectively isolated from the upper water column by diurnal thermal stratification, and respiration by suspended organisms can be accounted for by examination of the nocturnal decrease in dissolved oxygen in the surface water. Preliminary results indicate that phytoplankton production was quite low compared with most lakes of the world, which can probably be explained by the high inorganic turbidity that precludes photosynthesis in most of the water column. Buoyancy-regulating algae were occasionally observed to form surface scums but their overall primary production does not appear to be large based on the mesocosms.

In-situ mesocosms used in the assessment of open-water production by phytoplankton.

In-situ mesocosms used in the assessment of open-water production by phytoplankton.