Christy Susan Fellows

Coupling Nutrient Uptake and Energy Flow in Headwater Streams

Nutrient cycling and energy flow in ecosystems are tightly linked through the metabolic processes of organisms. Greater uptake of inorganic nutrients is expected to be associated with higher rates of metabolism [gross primary production (GPP) and respiration (R)], due to assimilatory demand of both autotrophs and heterotrophs. However, relationships between uptake and metabolism should vary with the relative contribution of autochthonous and allochthonous sources of organic matter. To investigate the relationship between metabolism and nutrient uptake, we used whole-stream and benthic chamber methods to measure rates of nitrate–nitrogen (NO3–N) uptake and metabolism in four headwater streams chosen to span a range of light availability and therefore differing rates of GPP and contributions of autochthonous carbon. We coupled whole-stream metabolism with measures of NO3–N uptake conducted repeatedly over the same stream reach during both day and night, as well as incubating benthic sediments under both light and dark conditions. NO3–N uptake was generally greater in daylight compared to dark conditions, and although day-night differences in whole-stream uptake were not significant, light–dark differences in benthic chambers were significant at three of the four sites. Estimates of N demand indicated that assimilation by photoautotrophs could account for the majority of NO3–N uptake at the two sites with relatively open canopies. Contrary to expectations, photoautotrophs contributed substantially to NO3–N uptake even at the two closed-canopy sites, which had low values of GPP/R and relied heavily on allochthonous carbon to fuel R.

ENDOGENOUS AND EXOGENOUS CONTROL OF ECOSYSTEM FUNCTION: N CYCLING IN HEADWATER STREAMS

Allochthonous inputs act as resource subsidies to many ecosystems, where they exert strong influences on metabolism and material cycling. At the same time, metabolic theory proposes endogenous thermal control independent of resource supply. To address the relative importance of exogenous and endogenous influences, we quantified spatial and temporal variation in ecosystem metabolism and nitrogen (N) uptake using seasonal releases of 15N as nitrate in six streams differing in riparian-stream interaction and metabolic character. Nitrate removal was quantified using a nutrient spiraling approach based on measurements of downstream decline in 15N flux. Respiration (R) and gross primary production (GPP) were measured with whole-stream diel oxygen budgets. Uptake and metabolism metrics were addressed as z scores relative to site means to assess temporal variation. In open-canopied streams, areal uptake (U; microg N x m(-2) x s(-1)) was closely related to GPP, metabolic rates increased with temperature, and R was accurately predicted by metabolic scaling relationships. In forested streams, N spiraling was not related to GPP; instead, uptake velocity (v(f); mm/s) was closely related to R. In contrast to open-canopied streams, N uptake and metabolic activity were negatively correlated to temperature and poorly described by scaling laws. We contend that streams differ along a gradient of exogenous and endogenous control that relates to the relative influences of resource subsidies and in-stream energetics as determinants of seasonal patterns of metabolism and N cycling. Our research suggests that temporal variation in the propagation of ecological influence between adjacent systems generates phases when ecosystems are alternatively characterized as endogenously and exogenously controlled.

Sources of carbon fuelling production in an arid floodplain river

Dryland rivers are characterised by highly pulsed and unpredictable flow, and support a diverse biota. The present study examined the contribution of floodplain sources to the productivity of a disconnected dryland river; that is a waterhole, after a major overland flood event. Rate measures of productivity were combined with stable isotope and biomass data on the food web in the waterhole and floodplain. The present study estimated that 50% of the fish carbon in the waterhole after flooding was derived from floodplain food sources. In the few months after retraction of the river to isolated waterholes, the large biomass of fish concentrated from the flooding decreased by 80%, most likely as a result of starvation. Based on the development of a carbon budget for the waterhole, mass mortality is hypothesised to be the cause of the high rates of heterotrophic production in the waterhole. The present study suggests that floodplain inputs are important for fuelling short-term production in waterholes, but via an unconventional pathway; that is, fish mortality. The episodic nature of flooding in dryland rivers means that changes in flow regimes, such as water regulation or abstraction, will reduce flooding and hence floodplain subsidies to the river. This is likely to have significant impacts on river productivity.

Measures of nutrient processes as indicators of stream ecosystem health

To better understand how freshwater ecosystems respond to changes in catchment land-use, it is important to develop measures of ecological health that include aspects of both ecosystem structure and function. This study investigated measures of nutrient processes as potential indicators of stream ecosystem health across a land-use gradient from relatively undisturbed to highly modified. A total of seven indicators (potential denitrification; an index of denitrification potential relative to sediment organic matter; benthic algal growth on artificial substrates amended with (a) N only, (b) P only, and (c) N and P; and δ15N of aquatic plants and benthic sediment) were measured at 53 streams in southeast Queensland, Australia. The indicators were evaluated by their response to a defined gradient of agricultural land-use disturbance as well as practical aspects of using the indicators as part of a monitoring program. Regression models based on descriptors of the disturbance gradient explained a large proportion of the variation in six of the seven indicators. Denitrification index, algal growth in N amended substrate, and δ15N of aquatic plants demonstrated the best regression. However, the δ15N value of benthic sediment was found to be the best indicator overall for incorporation into a monitoring program, as samples were relatively easy to collect and process, and were successfully collected at more than 90% of the study sites.

Interdisciplinary foundations: reflecting on interdisciplinarity and three decades of teaching and research at Griffith University, Australia

Interdisciplinarity is widely practised and theorised. However, relatively few studies have reflected on university‐wide attempts to foster the concept. This article examines interdisciplinary teaching and learning at Griffith University, Australia. It reflects on the foundations of interdisciplinarity at the university and situates them within the broader context of innovations in worldwide practice; it draws from the literature on interdisciplinarity to traverse the broad understandings of the term; it discusses the Griffith University innovations implemented in support of the concept; and, it reports on the likely outcomes of current methods designed to improve interdisciplinary practice. Whilst challenging barriers to interdisciplinarity continue to exist, compounded by varied conceptions of what interdisciplinarity entails, positive learning and research outcomes have been accomplished at the university from its interdisciplinary foundations, which also provide a platform to go forward.

Do climatic or institutional factors drive seasonal patterns of tourism visitation to protected areas across diverse climate zones in eastern Australia

Abstract Seasonality in tourism is a regular and predictable cycle of visitation across a year. Although seasonality in visitation is extremely common and is known, in principle, often to be driven by temporal changes in a range of natural and institutional factors, the relative importance of different individual pressures has yet to be quantified for any large-scale geographical areas. To assess the relative importance of natural versus institutional factors in driving tourism seasonality, data on visitation patterns were collated from 23 protected areas across six Koppen climate zones in eastern Australia. Analyses sought to determine the degree to which climatic variables (such as mean monthly rainfall and minimum and maximum temperatures) explained visitation patterns, and to understand how these relationships could assist in the prediction of tourism futures. Climate was the principal force driving seasonal patterns of visitation in equatorial, tropical, desert, grassland and temperate zones, whereas visitation to alpine/sub-alpine areas was driven by a complex array of natural and institutional factors. Tourism seasonality was driven mostly by institutional factors only in the sub-tropical climate zone. These analyses suggest that seasonal characteristics of current visitation could be used to predict the degree to which changes in climate and/or institutional arrangements, such as school holiday periods, might influence tourism opportunities in protected areas in eastern Australia.

Water quality in two Australian dryland rivers: spatial and temporal variability and the role of flow.

Water quality, along with hydrology, plays an important role in the spatial and temporal dynamics of a range of ecological patterns and processes in large rivers and is also often a key component of river health assessments. Geology and land use are significant drivers of water quality during flow periods while during periods of no-flow, local-scale factors such as evaporation, groundwater influence and the concentration and precipitation of compounds are important. This study explored the water quality changes in two Australian dryland rivers, the Cooper Creek (Lake Eyre Basin) and the Warrego River (Murray–Darling Basin), across different hydrological phases over several years. Water quality varied both spatially and temporally; the greatest spatial variability occurred during the no-flow phase, with temporal changes driven by flow. Concentrations of major anions and cations also varied spatially and temporally, with an overall cation dominance of calcium and magnesium and an anion dominance of bicarbonate. This bicarbonate dominance contrasts with previous data from inland lentic systems where sodium chloride was found to dominate. Such extreme spatial and temporal variability hampers successful derivation of water quality guidelines for these variable rivers and suggests such guidelines would need to be developed with respect to ‘flow phase’.

Ecosystem metabolism in a dryland river waterhole

Little is known about ecosystem processes in dryland rivers, despite the global distribution of these systems. Those in Australia are characterised by long periods of no flow in which they persist for many months as series of isolated, often turbid, waterholes. We assessed benthic and pelagic primary production, respiration, and bacterial production in one of these waterholes to determine the metabolic balance of the waterhole and resolve the relative importance of autochthonous and allochthonous sources of organic carbon. Despite a photic zone depth of only 0.25 m, three lines of evidence suggested that autochthonous sources of organic carbon were important for fuelling bacterial production under no-flow conditions: the metabolic balance of the waterhole was not indicative of large allochthonous inputs; rates of gross primary production were great enough to meet a substantial fraction of estimated bacterial carbon demand; and pathways for allochthonous carbon to enter the waterhole were limited. These results suggest that models of lake metabolism based on temperate ecosystems can be expanded to include dryland river waterholes, which group with eutrophic lakes owing to their high levels of inorganic nutrients, low allochthonous inputs and autotrophic metabolic balance.

Spatial and temporal variation in the ecological stoichiometry of aquatic organisms in an urban catchment

Abstract Urban land use has increased dramatically over the past few decades. Urban streams are distinguished from forested or agricultural streams, in particular, by their more variable and unpredictable hydrologic pattern. The resulting high variability in nutrient loading is likely to alter the elemental composition of primary producers and, ultimately, to change the elemental composition of other foodweb components. Ecological stoichiometry is a useful framework for improving our understanding of the mass balance of multiple key elements in ecosystems. To this end, the C∶N∶P of key foodweb components were measured in Oxley Creek, an urban catchment in southeastern Queensland, Australia. Ten stream reaches were sampled to explore the spatial variation of C∶N∶P of abundant taxonomic groups across the catchment. Four of these sites were sampled weekly (for 8 wk) to examine temporal variation in elemental composition. Our results suggested that spatial and temporal variation in elemental composition of primary producers and some animal taxa were highly dependent on local (i.e., site) conditions. This local dependence makes determination of catchment-wide drivers of stoichiometric variability difficult, but our results do suggest that site-based influences in urban streams can generate substantial variability in the C∶N∶P content of in-stream biota. In the context of other studies that have been undertaken principally in forested streams, this application of ecological stoichiometry promises to further our understanding of the effects of urbanization on stream food webs and the stability of elemental flow.