Simon A. Townsend

The effect of three fire regimes on stream water quality, water yield and export coefficients in a tropical savanna (northern Australia)

The effects of three fire regimes—(1) burning early in the dry season (June), (2) burning late in the dry season (September) and (3) not burning (protected from wildfires)—on the water quality, water yield and export coefficients of three intermittent streams, which flow between December and June, have been examined in a tropical savanna in northern Australia. The study was conducted over a three year period in Kakadu National Park, and employed a comparative catchment approach though without any pre-treatment data. The canopy cover, density of riparian vegetation, litter- and ground-cover of the catchment burnt early in the dry season (catchment E, stream E) and the unburnt catchment (catchment U, stream U) were similar. Fires lit late in the dry season (catchment L, stream L) however resulted in tree mortalities, and a lower canopy cover (50% less), riparian tree density (80% less) and litter cover, and increased amounts of bare ground; thereby increasing catchment Ls susceptibility to erosion. This resulted in episodic runoff events from catchment L in November and December, before continuous wet season flow. These events, absent in catchments E and U, featured high concentrations of total suspended sediment (TSS), volatile suspended sediment (VSS), N, P, Fe and Mn up to 10 times those measured later in the wet season. During continuous wet season flow between December and June, baseflow water quality of the three streams were similar. Storm runoff concentrations for N and P were also similar, however stream L storm runoff concentrations of TSS, VSS, Fe and Mn were 2–5 times higher than those measured in streams E and U. Despite this, only the export coefficients for TSS from catchment L (average 61 kg ha−1) were significantly higher (average 2.4 times) than catchment E and U coefficients. This was attributed to the overwhelming influence of stream volume, relative to concentration, in determining stream load and hence catchment export coefficients (load/catchment area). The apparently negligible impact of the fire regimes on VSS, N, P, Fe and Mn export coefficients, and also the overall low sediment export coefficients for the three catchments which were up to 100 times less than that reported for other tropical environments, were ascribed to the low catchment slopes (average 0.5%), low soil fertility, maintenance of a protective surface gravel lag, the negligible impact of the fire regimes on water yield, and the sometimes lengthy (maximum 6 months) period between burning and runoff.

The seasonal accrual and loss of benthic algae (Spirogyra) in the Daly River, an oligotrophic river in tropical Australia

The hierarchy of factors that control the growth and biomass of Spirogyra sp. was examined for an 18-km reach of the Daly River in the wet/dry tropics of northern Australia. On an annual temporal scale, hydrological disturbances control Spirogyra. Over the wet season (typically December-April), frequent runoff events prevent the colonisation and growth of Spirogyra in the Daly River. This is followed, however, by a lengthy period (typically May-November) without hydrological disturbances and river velocities that favour benthic algal growth. In 2001, Spirogyra became visible in mid-May, then grew to reach a maximum biomass in early August of 28 mg m -2 of chlorophyll a. The standing crop of Spirogyra was primarily determined by the availability of gravel substrate and the velocity and shear stress at the river-bed. Photosynthetically available radiation (200-800 μEm -2 s -1 ) reaching the river-bed should not have limited algal growth, though self shading within the Spirogyra mats may have been important. Although the growth rate of Spirogyra was probably limited by nutrients, the maximum biomass was constrained by autogenic sloughing. The biomass of Spirogyra steadily declined to half its maximum in early October despite favourable river velocities, most likely a result of nutrient limitation. Spirogyra was then removed from the river by the first major runoff event of the wet season in late October. The hierarchy of factors that control benthic algal biomass in the Daly River are the same as in lower latitudes, though the long period of favourable river velocities when smaller scale, proximate factors (e.g. nutrients, shear stress) control biomass should be noted.

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

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.

Factors contributing to a fish kill in the Australian wet/dry tropics

Abstract Factors which contributed to the death of 5000 fish, comprising 18 species, and accompanying water quality changes in Donkey Camp Pool, a part of the Katherine River system in the Australian wet/dry tropics, are discussed. The water quality of the pool was modified by the first run-off of the 1987–1988 wet season. A similar run-off event 9 days later caused significant water quality changes. Colour, turbidity, iron and manganese were at least an order of magnitude greater and coliform concentrations several orders of magnitude higher than typical dry season values. Furthermore, the pool was stratified with low surface dissolved oxygen concentrations and anoxic conditions at depth. These conditions remained until the pool was completely flushed by a large run-off event 11 days later. The fish kill was primarily related to natural causes associated with low dissolved oxygen concentrations. The pool water was displaced with cool run-off from a tributary of the Katherine River which carried a substantial organic load and had a high oxygen demand. It is concluded that the low dissolved oxygen concentrations were the major cause of the fish kill with possible additional harmful effects from toxic humic compounds. The event highlights the significant impact storm run-off can have on the quality of receiving waters in the Australian wet/dry tropics.

Metabolism in a groundwater-fed river system in the Australian wet/dry tropics: tight coupling of photosynthesis and respiration

Abstract The temporal pattern of river metabolism was estimated for high-order rivers (5–7th) in the Daly watershed, tropical Australia, during the dry season (May–October) when discharge was supplied predominantly by groundwater. Rates of photosynthesis (P) and respiration (R) were calculated at 4 sites using the open-channel method based on a model of the rivers O2 budget and measured diurnal cycles of dissolved O2 concentrations and temperatures. The rivers were shallow (average depth  =  0.8 m), clear (1–2 NTU), and had low concentrations of nutrients (≤15 µg/L soluble N and P at most sites) and generally open canopy. At the reach scale, P was limited by light with no evidence of light saturation. An increase in primary producer biomass over the dry season probably underpinned an approximate doubling of P at the 4 sites over the dry season, but increased water temperatures would have contributed, too. P (0.1–4.6 g O2 m−2 d−1) in the Daly watershed was similar to rates in a shaded tropical Puerto Rican stream and some temperate rivers but was lower than in nutrient-enriched temperate rivers. We surmise that most P resulted in production of dissolved organic C (DOC), rather than growth of primary producer biomass, which was nutrient limited. R exceeded P (P/R ≈ 0.5), and increased approximately linearly with P (r2  =  0.79–0.99) over the dry season with no statistically significant difference among sites. The similar environmental setting of the 4 sites underpinned their similar temporal pattern of metabolism. Bacterial metabolism of photosynthetically produced DOC (PDOC) could partially explain the tight coupling of R and P but could not account for the rivers overall net heterotrophy. The priming effect of bacterial degradation of labile PDOC to increase the mineralization of recalcitrant DOC (e.g., humic acids) provides an explanation for the rivers heterotrophy and tight coupling between P and R.

Multiple approaches to microbial source tracking in tropical northern Australia

Microbial source tracking is an area of research in which multiple approaches are used to identify the sources of elevated bacterial concentrations in recreational lakes and beaches. At our study location in Darwin, northern Australia, water quality in the harbor is generally good, however dry‐season beach closures due to elevated Escherichia coli and enterococci counts are a cause for concern. The sources of these high bacteria counts are currently unknown. To address this, we sampled sewage outfalls, other potential inputs, such as urban rivers and drains, and surrounding beaches, and used genetic fingerprints from E. coli and enterococci communities, fecal markers and 454 pyrosequencing to track contamination sources. A sewage effluent outfall (Larrakeyah discharge) was a source of bacteria, including fecal bacteria that impacted nearby beaches. Two other treated effluent discharges did not appear to influence sites other than those directly adjacent. Several beaches contained fecal indicator bacteria that likely originated from urban rivers and creeks within the catchment. Generally, connectivity between the sites was observed within distinct geographical locations and it appeared that most of the bacterial contamination on Darwin beaches was confined to local sources.

Implications of water extraction on the low-flow hydrology and ecology of tropical savannah rivers: an appraisal for northern Australia

Balancing the freshwater needs of humans and ecosystems is a fundamental challenge for the management of rivers worldwide. River regulation and water extraction can affect all components of the natural flow regime, yet few studies have investigated the effects on the low-flow end of the hydrograph. Low-flow periods are hydrologically distinctive and ecologically important, varying in nature among climatic zones. Tropical savannah rivers are characterized by highly seasonal and predictable flow regimes, but with high interannual variation in the magnitude, timing, and duration of low flows. Many tropical savannah rivers are relatively intact, especially in northern Australia, but many are now receiving increasing attention for water-resource development through surface- and groundwater extraction. We identified the hydroecological effects of water extraction on 3 phases of the seasonal flow regime: the wet–dry transition, dry season, and dry–wet season transition for perennial and intermittent rivers in tropical savannah climates. We propose a conceptual model and 7 predictions that describe the ecological implications of dry-season water extraction in tropical savannah river systems worldwide. The predictions address: 1) connectivity, 2) availability of in-stream habitat, 3) dry-season persistence of in-channel refugia, 4) water quality during dry–wet and wet–dry transition periods, 5) decoupling of wet- and dry-season flows, and the cumulative effects on 6) groundwater-dependent species and 7) whole-ecosystem shifts. We used northern Australia as a case study to review the current level of evidence in support of these predictions and their potential ecological consequences, and used this review to propose key priorities for future research that are globally applicable.

Phytoplankton composition and constraints to biomass in the middle reaches of an Australian tropical river during base flow

Under high flows, the biomass of riverine phytoplankton can be constrained by short transport times and advective losses. However, under slower flows and longer transport times, secondary factors and sometimes their interaction with flow may constrain phytoplankton biomass. To contribute to a wider understanding of the riverine conditions that constrain phytoplankton biomass, we tested the hypothesis that phytoplankton of the Daly River (tropical Australia) was constrained by transport time during dry-season base flow. The river is virtually undisturbed, with oligotrophic nutrient concentrations during the dry season. The most frequently occurring taxa were planktonic, rather than benthic, and dominated by the dinoflagellate Peridinium inconspicuum which has r-strategist traits that favour rapid growth in a nutrient-deficient environment. Our hypothesis was not supported because increased downstream loads of Chlorophyll a and the domination of P. inconspicuum inferred phytoplankton net growth. Instead, phytoplankton biomass was more likely to be nutrient-limited, although transport time may limit phytoplankton growth over some reaches and for specific taxa. The present study demonstrated that even in the fast-flowing middle reaches of a river (~0.4 m s–1), a population of phytoplankton can be sustained.

Benthic Algal Resilience to Frequent Wet-Season Storm Flows in Low-Order Streams in the Australian Tropical Savanna

Abstract: Storm-flow disturbances are frequent during the wet season of Australian tropical savannas. We examined benthic algal resistance and resilience in open-canopy streams in the Daly River watershed. Storm flows occurred every 2 to 3 d at 1st- to 4th-order sites, with sharp rises and falls and relatively long periods of shallow, low-turbidity base flow. At a 5th-order site, storm-flow duration was longer and base flows were deeper and more turbid. We hypothesized that: 1) storm flow would dislodge benthic algal biomass, 2) baseflow biomass would be low, 3) taxon richness would be low, and 4) algal composition would be dominated by resistant algae with prostrate or erect growth forms or by fast growing colonizing algae. Hypothesis 1 was supported. Storm flows dislodged ~93% of epilithic biomass. Support for hypothesis 2 was equivocal. At the 5th-order site, sand mobility prevented establishment of benthic algae until seasonal flows receded. At the other sites, epilithon net growth rates were much greater than rates in some temperate streams. Benthic biomass was typical of temperate oligotrophic streams, but maximum biomass was typical of mesotrophic streams. We attributed the relatively rapid growth and high biomass to warm water temperatures (mean = 29°C, maximum = 36°C), high incident light, rapid algal nutrient uptake, loss of grazing invertebrates caused by storm flows, and physical impediments to fish access. Hypothesis 3 was not supported. Mean taxon richness was high because of the occurrence of rare taxa. Hypothesis 4 was not supported. Epilithic algal biomass was dominated by resistant filamentous chlorophytes. Epilithic algal resistance was similar to resistance in higher latitude streams, but resilience was greater. Epilithic algae potentially could supply autochthonous C to the Daly River and other tropical aquatic food webs.

Context dependency of top-down and bottom-up effects in a Northern Australian tropical river

Top-down and bottom-up forces (consumer and resource limitation, respectively) influence biomass of primary producers and primary consumers in natural food webs. Few investigators experimentally examine both in concert, especially in the tropics. Tropical systems probably are more sensitive than temperate systems to eutrophication and other disturbances, such as destruction of riparian canopy cover, because of wide windows of ecological opportunity. We experimentally examined the relative importance of top-down and bottom-up effects at 2 sites in the Edith River in the Australian wet–dry tropics during baseflow conditions. We used large fish-exclusion cages to examine top-down effects of fish, and we used nutrient-diffusing substrates to manipulate bottom-up effects of nutrients. Macroinvertebrates were significantly more abundant in fish-exclusion cages than in open control areas at both sites. At 1 site, chlorophyll a on rock substrate was 1.5× greater and ash-free dry mass was 1.4× greater in fish-exclusion than in open cages. Chlorophyll a was >3× greater in the high-N and -P nutrient treatment than in the unamended control at both sites. Top-down effects on the algae on nutrient-diffusing substrates were detected at only 1 site. Bottom-up factors seem to be relatively more important in controlling primary producer biomass compared to top-down factors, with no evidence of a trophic cascade. Bottom-up and top-down effects on macroinvertebrates were strong and consistent, whereas top-down effects on algae were context dependent and varied depending on benthic habitat, current speeds, and community structure. These results indicated spatial variation at the scale of tens of meters.