Marine and Freshwater Research Marine and Freshwater Research Society
Advances in the aquatic sciences
RESEARCH ARTICLE

An analysis of primary production in the Daly River, a relatively unimpacted tropical river in northern Australia

I. T. Webster A D , N. Rea B , A. V. Padovan C , P. Dostine C , S. A. Townsend C and S. Cook C
+ Author Affiliations
- Author Affiliations

A CSIRO Land & Water, GPO Box 1666, Canberra, ACT 2601, Australia.

B Faculty Indigenous Research and Education, Charles Darwin University, Darwin, NT 0909, Australia.

C Department of Infrastructure, Planning and Environment, PO Box 30, Palmerston, NT 0831, Australia.

D Corresponding author. Email: ian.webster@csiro.au

Marine and Freshwater Research 56(3) 303-316 https://doi.org/10.1071/MF04083
Submitted: 3 May 2004  Accepted: 14 December 2004   Published: 3 June 2005

Abstract

In this paper, the dynamics of primary production in the Daly River in tropical Australia are investigated. We used the diurnal-curve method for both oxygen and pH to calculate photosynthesis and respiration rates as indicators of whole-river productivity. The Daly River has maximum discharges during the summer, monsoonal season. Flow during the dry season is maintained by groundwater discharge via springs. The study investigated how primary production and respiration evolve during the period of low flow in the river (April–November). The relationship between primary production and the availability of light and nutrients enabled the role of these factors to be assessed in a clear, oligotrophic tropical river. The measured rate of photosynthesis was broadly consistent with the estimated mass of chlorophyll associated with the main primary producers in the river (phytoplankton, epibenthic algae, macroalgae, macrophytes). A significant result of the analysis is that during the time that plant biomass re-established after recession of the flows, net primary production proved to be ~4% of the rate of photosynthesis. This result and the observed low-nutrient concentrations in the river suggest a tight coupling between photosynthetic fixation of carbon and the microbial degradation of photosynthetic products comprising plant material and exudates.

Extra keywords: algae, heterotrophic, macrophyte, oligotrophic, photosynthesis, phytoplankton, respiration.


Acknowledgments

The authors are grateful for suggestions for manuscript improvement from Phillip Ford, George Ganf, Peter Pollard, Sue Vink and two anonymous reviewers. Funding support for this project was provided by Environment Australia through the Environmental Flows Initiative within the National River Health Program and with assistance from the Northern Territory Department of Infrastructure Planning and Environment, Darwin.


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Appendix 1. Evaluating the potential impact of groundwater input on the diurnal-curve analysis for oxygen

Here we consider the impact that groundwater input as a spring will have on the respiration rates calculated using the diurnal-curve method for a single station. After expanding the derivative on the left hand side of Eqn 1 and after substituting for the re-aeration term (Eqn 4), Eqn 1 can be rewritten as:

EA1

where U is the flow speed of the river. The solutions developed for this equation have assumed horizontal uniformity in the oxygen concentrations, so that the spatial derivative in the oxygen concentration is zero. However, if there is a large input of spring water with an oxygen concentration significantly different from that in the river, then there will be a zone downstream of the input where the river adjusts to the deficit (or possibly excess) oxygen concentration introduced by the spring. If the deficit (or excess) oxygen concentration is significant at the measurement site, then the calculated respiration rate will be in error.

First, we average Eqn A1 over 24 h. Assuming that average oxygen concentration does not change significantly from one day to the next, then O/∂t ~ 0. If we further assume that the spring has an insignificant input to the dissolved oxygen, Eqn A1 becomes:

EA2

where the overbar represents averaging over 24 h. Now, consider the oxygen concentration downstream from a spring that has caused a significant alteration in dissolved oxygen concentration. Denoting this oxygen concentration as O′, then the equivalent equation to Eqn A2 for O′ becomes:

EA3

Subtracting Eqn A2 from A3 yields:

EA4

which has the solution:

EA5

where Δ = OO′ is the amount by which mean oxygen is depressed owing to the presence of the spring at distance x downriver from the spring and Δ0 is the oxygen depression in the river just downstream from the spring. Note that in Eqn A4, the length scale for oxygen-concentration adjustment in the river appears as HU/δ. For the Daly River during the study, this distance is calculated to be 25 km. We use the observed similarity of conditions along the river channel over this distance to support the validity of Eqn 1.

Rearrangement of Eqn A3 yields:

EA6

so that the respiration term calculated using the fitting procedure, which has not explicitly considered downstream variation in dissolved oxygen concentration, is really:

EA7

The respiration corrected for spring input would be:

EA8

where use has been made of Eqn A4 to calculate the spatial derivative in oxygen concentration. Thus, if the effect of the spring were to depress the local oxygen concentration (positive Δ0) then the respiration calculated would be too high unless the correction is made.

Cook et al. (2003) inferred spring volumes into the Daly River for October 2001 using changes in tracer concentrations. The largest inputs to the river were from springs ~60 km upstream from the measurements, with an estimated volume of 10 m3 s-1, and from a second set ~28 km upstream, with an estimated volume of 3 m3 s-1. For assessing the impact of the springs on calculated respiration rates, we assume that U = 0.47 m s-1 (median discharge), H = 1.5 m (median water depth), δ = 2.4 m day-1 (see Fig. 5) and that the daily averaged oxygen concentration in the river upstream of the springs is O = 0.21 mol m-3. The daily averaged concentrations measured by the Hydrolab varied between 0.19 and 0.23 mol m-3 during the study and these will be affected by the presence of the springs, but this impact is estimated to be ~0.005 mol m-3 for both cases considered in the following.

Assuming the extreme case that the spring oxygen concentration is zero, application of Eqn A5 would suggest that the springs 60 km upstream of the measurement site would cause the areal respiration in the river to be overestimated by ~0.017 mol m-2 day-1. The corresponding calculation for the springs 28 km upstream from the measurement site also yields 0.017 mol m-2 day-1 as the estimated error. Thus, with zero oxygen concentrations in the springs, the total impact of the spring inflow on estimated areal respiration rate would be ~0.03 mol m-2 day-1. Oxygen concentrations in two of the springs ~28 km upstream of the Hydrolab site were measured to be 0.18 and 0.20 mol m-3; that is, fairly similar to the river, so that the respiration error assuming zero oxygen concentration in the springs may very well be considerably overestimated.



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