TRaCK: Tropical Rivers and Coastal Knowledge

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

TitleTowards understanding the impacts of land management on productivity in the Daly River
Publication TypeReport
Year of Publication2010
AuthorsRobson, B, Schult, J, Smith, J, Webster, I, Burford, MA, Revill, A, Townsend, SA, Haese, R, Holdsworth, D
Date Published11/2010
InstitutionCharles Darwin University
ISBN Number978-1-921576-29-4
KeywordsDaly River, model, nutrients, primary production, sediments, water quality

This report is from a project designed to answer the question, “How will changes in land and water management affect the productivity of the Daly River?”. 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 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, 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.

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.
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.

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.