2003, Jenkins, Kim M, Boulton, A

Periodic hydrological connectivity among fragmented floodplain habitats governs the persistence of aquatic biota. In dryland rivers, unpredictable flooding interspersed with low flows and drying drive their "boom and bust" ecology. During drying, aquatic habitats contract and fragment. Flooding connects and expands habitats, triggering productivity booms in waterbirds, fish, and plants. Microinvertebrates form the base of the food web and also flourish after floods, but their colonization pathways remain unknown.To determine the relative contribution of floodwater-transported propagules vs. emergence of in situ resting stages from inundated sediments, we studied short-term recruitment of microinvertebrates after flooding in the dryland Darling River, Australia. Lakes open to transported and emerging in situ microinvertebrates were compared with lake and laboratory microcosms closed to transported microinvertebrates. Floods imported most early colonists to floodplain lakes. In situ emergence from sediments was less important until weeks later but potentially sustained production over a longer period of inundation. Some taxa primarily colonized lakes via emergence whereas others were only transported to lakes. A spatially nested ANOVA approach spanning patches within lakes to among reaches revealed contrasting variability across hierarchical scales for taxonomic richness and densities of microinvertebrate taxa. This was most marked at the reach scale, possibly reflecting seasonal patterns in flooding. Microinvertebrate colonization sequences in the Darling River floodplain rely on connectivity between habitats during flooding. Ecological connectivity was greater at small spatial scales (patches within lakes 100–1000 m, among lakes 104–105 m) than at larger spatial scales (reaches 105–106 m). The strength of connectivity is likely to vary across spatial scales with the magnitude, duration, and rate of rise and fall of floods.

2003, Boulton, Andrew John, Dole-Olivier, MJ, Marmonier, P

To determine the optimal combination of sample volume and replicationrequired to assess hyporheic taxa richness and abundance, 10 consecutive one-L Bou-Rouch pump samples were collected from each of 9 wells inserted in a 3x3m grid at two sites on the Rhône River, France. Variable and non-linear relationships between sample volume and numbers of taxa and individuals demonstrate that hyporheic densities cannot simply be expressed per L for comparison among studies that have collected different sample volumes. Our results imply that an optimal sampling design for rapid assessment of hyporheic biodiversity and abundance would need at least five samples of 3–5 L to provide reasonable levels of precision (20%). At both sites, 6–7 replicates of 5L sufficed to be able to detect a 50% change in mean taxa richness with a power of 0.90 and α = 0.10. Although this sampling exercise should be repeated whenever accurate population estimates are needed, it appears that the spatial variance of the hyporheos at a fine scale (< 9m²) is great enough to obscure detection of potentially large differences when replication or sample volume is small.

2003, Claret, C, Boulton, Andrew John

1. Microbes play key roles in nutrient transformation and organic matter mineralisation in the hyporheic zone but their short-term responses to diel variations in discharge and temperature are unknown. Rates of microbial esterase activity were hypothesised to vary vertically and along a gradient of moisture in a drying sand-bed stream where discharge fluctuated daily in response to evapotranspiration. 2. At 'fully saturated', 'moist' and 'dry' locations in three sites along a drying Australian sand-bed stream, microbial activity at three depths (surface, 10 and 30 cm) was assessed using fluorescein diacetate hydrolysis. Samples were collected in mid-summer in the late afternoon and again at dawn to assess diel variation in hydrolytic activity at each site and depth. Data loggers tracked diel variations in temperature at each depth. 3. Hydrolytic activity was up to 10-fold greater in the surface sediments in late afternoon than at dawn in all habitats, and was correlated with surface sediment temperature. Diel differences in activity were not detected at 10 cm, although daily thermal cycles were evident at this depth. Unexpectedly, activity was marginally higher at dawn at 30 cm in all habitats, perhaps reflecting lags in temperature at that depth. 4. Overall, microbial activity declined with depth, strongly correlated with vertical trends in total organic matter and concentrations of dissolved phosphorus. Particulate organic matter, probably buried during a flood 35 days earlier, appeared largely responsible for these vertical trends. On the other hand, there was little evidence for hydrological exchange between much of the hyporheic zone and the surface stream, implying that processes in the subsurface zone of this stream are effectively isolated during baseflow in mid-summer. 5. Diel cycles of wetting and drying in the moist habitats did not enhance esterase activity relative to the dry or fully saturated habitats. Sediment moisture was not correlated with microbial activity, and mats of senescent algae appeared to inhibit water loss from surface sediments in the moist habitat. In this sand-bed stream, local diel fluctuations in water level appear to have less influence on microbial activity and mineralisation of organic matter in the sediments than occasional floods that bury leaf litter and renew many hyporheic zone functions. Subreach-scale processes seem to be the major driving force of microbial processes and nutrient cycling in this sand-bed river.

2003, Boulton, Andrew John, Moss, GL, Smithyman, D

In remote areas, wild-fires often must be controlled by applying fire-retardants and suppressants dropped from small aircraft. However, impacts of these chemicals on natural stream ecosystems are poorly known. Unintentional aerial application of fire-fighting chemicals (Phos-ChekWD-881 and ForExspan S) onto two small streams during a natural wildfire on Kangaroo Island, South Australia, provided an opportunity to study the short-term effects on water chemistry and aquatic invertebrates. Within 2 weeks of application, samples of water and macroinvertebrates were collected from sites upstream of the application point, within the zone of application, and downstream where burning had been controlled on two streams. Three sites on a reference stream in the same sub-catchment that had been burned by the same fire but without application of fire-suppressants were also sampled. All sites were resampled three months later (within two weeks of the first flushing rains). There were no marked differences in water quality among the sites on the reference stream but in one of the impacted streams where flow had ceased before the fire, dissolved and total phosphorus concentrations were elevated at the site where the fire suppressants were applied. Phosphorus concentrations were reduced 2–3-fold at this site after brief flushing by rain. Conversely, dissolved and total N and P concentrations at the other impacted stream that flowed permanently did not differ among the sites and there was no evidence for persistent changes to water quality from the applied fire-suppressant foams. Taxon richness was higher at the application and downstream sites than at upstream sites in the two impacted streams. There were also no discernible effects of the fire-suppressants on macroinvertebrate assemblage composition or taxon richness within the two streams two weeks after the chemical application or soon after flushing rains. Assemblage composition in the temporary stream was significantly different from that in the reference and the other impacted stream but also appeared unaffected by the fire-suppressants. The lack of impact on resident stream macroinvertebrates may result partly from their inherent high tolerance to the harsh physical and chemical conditions of these streams, many of which typically cease flow in summer.