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Towards understanding the impacts of land management on productivity in the Daly River

Robson, Barbara J., Schult, Julia, Smith, Jodiie, Webster, Ian, Burford, Michele, Revill, Andy, Townsend, Simon, Haese, Ralf and Holdsworth, Daniel (2010). Towards understanding the impacts of land management on productivity in the Daly River<br />. Darwin, NT: Charles Darwin University.

Document type: Research Report
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Author Robson, Barbara J.
Schult, Julia
Smith, Jodiie
Webster, Ian
Burford, Michele
Revill, Andy
Townsend, Simon
Haese, Ralf
Holdsworth, Daniel
Title of Report Towards understanding the impacts of land management on productivity in the Daly River
Publication Date 2010
ISBN 978-1-921576-29-4   (check CDU catalogue open catalogue search in new window)
Publisher Charles Darwin University
Place of Publication Darwin, NT
Total Pages 170
Field of Research 300800 Environmental Sciences
Abstract Executive Summary

“At a time of increasing awareness of the value of water across Australia, it is vital that public
debate, policy and management decisions about our tropical rivers and estuaries are informed by
sound science.
[TRaCK (Tropical Rivers and Coastal Knowledge) is] providing the science and knowledge that
governments, communities and industries need for the sustainable use and management of
Australia's tropical rivers and estuaries.” [From the TRaCK website]

This document is the final report for the Daly River component of TRaCK Project 4.3, “Towards understanding the impacts of land management on productivity in the Daly and Flinders Rivers”. The project was designed to improve our understanding of materials fluxes, nutrient and sediment processes and primary production in tropical rivers like the Daly River, Northern Territory, and to build the knowledge and models needed to answer the question, “How will changes in land and water management affect the productivity of the Daly River?”

The project has combined water quality monitoring at a number of sites in the river throughout the wet and dry seasons of 2008 with intensive fieldwork campaigns to measure and assess the properties of sediments, organic material, plants and algae in the river, and modelling to simulate key processes and provide a basis for predicting how the river is likely to respond to changes in flow, nutrient or sediment loads.

The results suggest the following conceptual picture for nutrients cycling, sediments and primary production in the Daly River:

Wet season (December to March)

High flow volumes during the wet season bring >95% of total annual
sediment, nitrogen and phosphorus loads reaching the Daly River. High shear
stresses associated with high flows scour the bed, removing most benthic plant
biomass, though in lower-flow years, some Vallisneria beds survive. Turbidity
is high during the wet season and water residence time is low, so primary
production in the main channel is accordingly low.

Early Dry Season (April to June)

At the end of the wet season, sediments settle out of the water column and
benthic substrate stabilises into alternating stretches of dynamic sand ripples,
stable gravel beds and slightly deeper pools. The water clears and benthic
microalgae (in periphyton) and fast-growing macroalgae (Spirogyra) establish
rapidly, storing nutrients from the water column and keeping them within the
system. Most benthic production occurs in gravel runs and along the edges of
pools, with little production in sand ripples. Phytoplankton contribute little to
total primary production in current conditions.

Total primary production is limited not by light, but by nitrogen and
phosphorus concentrations and transfer of these nutrients across a benthic
boundary layer.

Late Dry Season (July to November)

In the late dry season, flow is sustained entirely by groundwater inflows and hydrological breaks
(i.e. small waterfalls) play an important role in controlling flow and water depth.

Primary production remains limited by nitrogen and phosphorus and much of the measured
photosynthesis does not contribute to production of plant biomass.

With little additional nutrient input from the catchment, the system in the late dry season depends
on nutrients recycled within the benthic community. Spirogyra biomass drops as lower flows cause a
decline in transfer-limited nutrient uptake rates.

Release of nutrients from sediment muds does not appear to contribute significantly to the supply
of nutrients in the system during the dry season: rather, nutrients released from decaying
Spirogyra and periphyton or otherwise recycled within the biotic community drive the growth of
slower- growing benthic algae (Chara or Nitella) and aquatic plants (Vallisneria). Sand ripples may
play an important role in remineralisation of detrital organic matter.

Outputs from this project include a one-dimensional hydraulic model and a preliminary model to simulate changes in biomass of five key groups of plant and algae, presented in Chapter 7. The latter has been applied to two simple scenarios, exploring the possible effects of increased nutrient concentrations and reduced flows.

The results suggest that the Daly River is likely to be particularly sensitive to any increase in nutrient loads that may result from changes in catchment management or land use.

Significant knowledge gaps remain, and are discussed in Chapter 8.

Further work is needed to test the models presented here and to assess the impacts of land use on catchment runoff, sediment and nutrient loads, the impact of grazing on primary production in the river and the likely effects of climate change.
 
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