The metabolic signature of small headwater streams: Natural variability and the response to forestry

A strong connection to the landscape means that small headwater stream metabolism is easily disturbed by land management practices such as forestry. A current landscape ecology framework, the hierarchical patch dynamics paradigm, was applied to examine the heterogeneity of small headwater stream metabolism and how it is affected by anthropogenic disturbance. Metabolism was examined at several scales to elucidate specific metabolic components and processes, including microbial community structure, bacterial carbon productivity, cellulose decomposition potential, algal accumulation, and ecosystem respiration and productivity. Firstly, in-stream metabolic patch dynamics was examined throughout a calendar year. Small headwater streams displayed very low metabolic rates dominated by heterotrophic processes. Predictable spatial and temporal patterns in population-level, community-level, and whole system-level metabolisms were observed, shaped by temperature, hydrology, and the physical and chemical properties of patches. Secondly, an inundation experiment was conducted to examine metabolic patch dynamics in a small headwater landscape. Inundation resulted in increased metabolic response and a change in the metabolic community profile of the terrestrial patch. It is suggested that regular, above-bank flows are likely to transfer the complexities of in-stream patch dynamics into the terrestrial environment, exemplifying the strong connection between headwater streams and the surrounding landscape. Thirdly, the impact of forestry on stream metabolism was examined by a survey of small headwater streams 2-5 years after logging. Forestry stimulated autotrophy and lead to increased metabolic rates. Methods that incorporate temporal variability, such as cellulose decomposition potential, provided a strong assessment of forestry impact and are recommended as robust indicators of disturbance. Fourthly, the potential recovery of stream metabolism from the impacts of forestry was examined by a survey of small headwater streams 2-15 years after logging. Only autotrophic processes displayed recovery over time, with a sustained change in heterotrophic form and function. It is suggested that, together, these surveys demonstrate a lack of resistance and resilience to forestry disturbance in the key metabolic processes of small headwater streams. Finally, through the description of metabolic patterns a metabolic 'signature' of small headwater streams is presented. The metabolic signature provides a characterisation of small headwater stream metabolism and contributes to our understanding of small headwater stream ecology

Can C4 plants contribute to the aquatic food webs of subtropical streams?

1. Recent stable isotope studies have revealed that C4 plants play a minor role in aquatic food webs, despite their often widespread distribution and production. We compared the breakdown of C3 (Eucalyptus) and C4 (Saccharum and Urochloa) plant litter in a small rain forest stream and used laboratory feeding experiments to determine their potential contribution to the aquatic food web. 2. All species of litter broke down at a fast rate in the stream, although Urochloa was significantly faster than Eucalyptus and Saccharum. This was consistent with the observed higher total organic nitrogen of Urochloa compared with the other two species. 3. The breakdown of Urochloa and Saccharum was, however, not associated with shredding invertebrates, which were poorly represented in leaf packs compared with the native Eucalyptus. The composition of the invertebrate fauna in packs of Urochloa quickly diverged from that of the other two species. 4. Feeding experiments using a common shredding aquatic insect Anisocentropus kirramus showed a distinct preference for Eucalyptus over both C4 species. Anisocentropus was observed to ingest C4 plant litter, particularly in the absence of other choices, and faecal material collected was clearly of C4 origin, as determined by stable isotope analysis. However, the stable carbon isotope values of the larvae did not shift away from their C3 signature in any of the feeding trials. 5. These data suggest that shredders avoid the consumption of C4 plants, in favour of native C3 species that appear to be of lower food quality (based on C : N ratios). Lower rates of consumption and lack of assimilation of C4 carbon also suggest that shredders may have a limited ability to process this material, even in the absence of alternative litter sources. Large scale clearing of forest and vegetation for C4 crops such as sugarcane will undoubtedly have important consequences for stream ecosystem function.Griffith Sciences, Griffith School of EnvironmentFull Tex

Metabolic patch dynamics in small headwater streams: exploring spatial and temporal variability in benthic processes

1. To gain a better understanding of the heterotrophic nature of small headwater streams in forested landscapes we explored the spatial and temporal variability of in-stream organic matter processes. Three methods were used to measure the benthic metabolism of different in-stream habitats in seven streams throughout a calendar year. This allowed us to analyse the contribution of various metabolic habitats (i.e. sediment, leaf litter, cobbles) to in-stream metabolism during a natural flow regime. Furthermore, it allowed us to define in-stream patchiness based on functional rather than structural elements. 2. Bacterial growth, measured using a leucine assay, displayed a quadratic relationship over time with a peak in warmer months and consistently higher bacterial growth in fine depositional (3.00�710.64 mg C m)2 day)1) than coarse gravel (38.84�582.85 mg C m)2 day)1) sediments. 3. Community metabolism, measured using dissolved oxygen chambers, showed distinct diel patterns and consistently greater net daily metabolism in leaf packs ()261.76 to )24.50 mg C m)2 day)1) than ï¬ne depositional sediments ()155.00 to )15.56 mg C m)2 day)1). Coarse gravel sediments ()49.55 to )16.88 mg C m)2 day)1) and cobble habitats ()151.98 to 55.38 mg C m)2 day)1) exhibited the lowest metabolic rates. Modelled whole-stream metabolism was highly variable among streams and temporal patterns appeared driven by temperature and the relative contribution of patch conï¬guration as a function of flow. 4. Cellulose decomposition potential showed higher rates of microbial activity in fine depositional compared to coarse gravel sediments (30.5 and 29.1 kg average cotton tensile strength loss respectively), though there were higher rates of thread loss indicative of macroinvertebrate activity in gravel compared to depositional sediment (21% and 13% average thread loss respectively), with a slight quadratic trend. The high variability among habitats, streams and over time in this integrative measure may be explained by variability in local microbial activity as well as the potential for macroinvertebrates to contribute across patches. 5. There were strong relationships among benthic processes and habitat structure, nutrient status, stream temperature and flow. Different habitats had distinct metabolic character- istics and these characteristics appear to influence stream food webs and biogeochemical cycling depending on the relative abundance of habitats. Generally, within habitat variability was less than among habitat variability and among stream variability was less than temporal variability. Hence, in terms of the spatial and temporal heterogeneity of benthic processes, these small headwater streams showed predictable metabolic patterns. However, there were few correlations between differing measures of benthic metabolism at the same patch and this suggests that caution should be taken when attempting to infer the rates of one level of metabolic activity (e.g. whole community metabolism) based on another (e.g. bacterial productivity)

Forest clearance increases metabolism and organic matter processes in small headwater streams

Small headwater streams are abundant components of the riverine landscape where critical biochemical processes occur that provide clean water, energy, and nutrients to downstream reaches. Disturbance to these systems as a result of human land use has the potential to affect downstream health. Rates of metabolism and organic matter processing were measured in 22 small forested headwater streams in 2 regions of Tasmania, Australia, to evaluate the effects of forestry disturbance. Twelve of these streams had been subjected to recent clearfell-burn-and-sow (CBS) harvest. Benthic metabolism was measured in small in situ chambers (production ranged from −2 h−1 and respiration from −2 h−1), whole-system metabolism was estimated based on relative habitat abundance (gross primary production ranged from −2 d−1 and daily respiration from 0.003 to 0.072 g C m−2 d−1). Algal growth potential was measured on nutrient diffusing pots (chlorophyll a ranged from 2), and cellulose decomposition potential was assessed with a cotton-strip assay (cotton tensile strength loss ranged from 17.8% to 38.3% in 28 d). Sometimes an increase in the variability of response is a consequence of disturbance, but in our study, the difference between forested streams and clearcut streams was a significant increase in the mean values of all functional variables. The degree of response depended on the underlying geology (broad-scale spatial variability) of the streams. Current management practices for small headwater streams in Tasmania do not protect instream processes from forestry disturbance in the short-term (i.e., 2–5 y), and we suggest that an investigation of long-term response is warranted

Variation in stream organic matter processing among years and benthic habitats in response to forest clearfelling

We assessed rates of organic matter (OM) processing in coarse gravel and fine benthic sediment, along with water temperature, in four clearfell harvested and two undisturbed headwater streams flowing through wet eucalypt forest in southern Tasmania, Australia. Clearfell forestry in Tasmanian wet eucalypt forest involves felling of all timber followed by a high intensity regeneration burn to provide a receptive mineral seedbed for seedling growth. Bacterial carbon production and cellulose decomposition potential (together referred to as OM processing) were measured seasonally 3–5 years before and 2–4 years after harvesting in each stream. We employed a staircase design (staggered harvesting treatments) within a multiple before–after control–impact design to distinguish harvesting effects from natural variation. Clearfell harvesting raised the yearly mean water temperature by between 0.25 °C and 0.94 °C, and raised the maximum water temperature by between 0.84 and 1.6 °C. Rates of cellulose decomposition were not significantly correlated with sediment temperature but bacterial carbon production showed weak, significant correlations with temperature in fine (r = 0.20, P = 0.01, n = 137) and coarse gravel sediment (r = 0.39, P < 0.001, n = 137). The response in OM processing to clearfell harvesting differed between years and among benthic habitats. In coarse gravel habitat, there was a significant decrease in rates of cellulose decomposition potential in the 2nd and 4th year after harvesting, and a significant decrease in bacterial carbon production in the 3rd year after harvesting. However, we found a significant increase in rates of bacterial carbon production of fine sediment habitat in the 2nd and 4th year after harvesting. The contrasting response of OM processing between habitats indicates that habitat-specific changes occur after clearfell harvesting, which inhibit attempts to quantitatively predict downstream cumulative effects. Scaling up the habitat-specific responses will not only require estimates of the relative abundances of the distinct habitats, but may also require research into how different spatial configurations of habitats may affect reach- and catchment-scale estimates of OM processin

Land use affects temporal variation in stream metabolism

Stream metabolism (gross primary production and ecosystem respiration) is increasingly used to assess waterway health because mean values are responsive to spatial variation in land use, but little is known about how human land use influences the temporal variability of stream metabolism. We investigated daily variation in dissolved O2 (DO) concentrations and calculated mean and within-season variation in gross primary production (GPP) and ecosystem respiration (ER) rates at 13 stream sites across a landuse intensity gradient in the Auckland region, New Zealand, over 9 y. Based on generalized linear mixed models, mean daily GPP (0.1-12.6 g O2 m-2 d-1) and ER (1.8-29.6 g O2 m-2 d-1) and seasonal variation in stream metabolism were significantly related to landuse intensity with higher variability associated with higher values of a landuse stress score. Overall, mean daily rates and day-to-day variation in GPP and ER were greatest in summer and least in winter. We recommend summer monitoring over a minimum 5-d period to assess stream health. Our results show that human land use affects the mean and the temporal variability of DO and stream metabolism. This finding has important consequences for characterizing in-stream processes and the resilience of stream ecosystems. Only long-term temporal monitoring provides the data needed to assess fully how streams function

Greater phosphorus uptake in forested headwater streams modified by clearfell forestry

Clearfell, burn and sow (CBS) forestry can potentially alter stream environments by increasing available light and the input of woody debris. However, little is known about how CBS forestry affects in-stream processes such as nutrient uptake. We evaluate whether short-term (2–7 years) environmental changes (e.g. light availability and woody debris) associated with CBS forestry lead to differences in nutrient uptake metrics. To do this, we measured in-stream uptake of soluble reactive phosphorus (SRP) and ammonium (NH4) in three old growth (OG) and four CBS-affected headwater stream reaches. The abundance of fine woody debris and light availability were significantly greater in CBS-affected than in OG reaches. Uptake velocities varied from 0.0880 to 0.951 mm min−1 for NH4 and from 0.0383 to 1.06 mm min−1 for SRP across all sites. The mean uptake of SRP, but not NH4, was significantly greater (i.e. higher uptake velocities and lower uptake lengths) in CBS-affected than in OG reaches. These results suggest that CBS forestry altered the stream environment enabling greater SRP uptake relative to OG reaches. Our findings highlight the tight linkage between headwater streams and their surrounding terrestrial environment, which has direct implications for catchment-scale biogeochemical processes.No Full Tex