Ecological responses and adaptive stream rehabilitation: application to degraded rural streams

"June 2005".

Project Number: Ecological responses and adaptive stream rehabilitation - CRCFE Project B240.

MDFRC item.

31 pages.

Excessive sedimentation and the formation of sand slugs is a widespread, and highly damaging form of disturbance in many catchments of eastern and south-western Australia, notably those where erodable uplands have been cleared and grazed. A major impact of sand slugs is to greatly reduce instream physical and biotic diversity, and to exacerbate the effects of floods and droughts. There is a now a keen interest in restoring sand slugged streams. This interest stimulated an attempt to test experimentally the efficacy of a restoration strategy using designed timber structures to restore habitat diversity. Prior to commencing the experiment a number of preliminary studies on the history of catchment land-use, the geomorphology and hydrology of the streams, and patterns of abundance and diversity of fish and invertebrates were carried out. Structures made from river redgum railway sleepers were built and installed in Castle and Creightons Creeks by the Goulburn Broken CMA in May 2001. The design of the structures arose from recommendations based on flume experiments on patterns of scour around wood structures carried out by CRCCH. In each Creek there were 3-control sites (0 structures added), 3 1-structure sites (1 structure added) 3 4-structure sites (4 structures added). At each site invertebrates were sampled twice, and fish thrice prior to the installation of the structures, and on 2 occasions (invertebrates) and 7 occasions (fish and habitat) following restoration. Severe drought in 2002/3 and 2004/5 caused both creeks to dry up in the sand slug sections. From May 2001 to June 2005 only 2 structures became non-functional due to burial (4-sleeper site) or bed degradation (1-sleeper site) in Creightons Creek. The structures were clearly capable of withstanding floods, and successfully produced small scour pools immediately downstream of the structures. The structures also accumulated high loadings of organic material (leaves and branches) and this, together with the scour pools, successfully increased local habitat diversity. The invertebrate fauna was dominated by oligochaetes, chironomids, crustaceans (cladocerans and ostracods) and coleopterans (predominantly elmids). Despite the changes in habitat, invertebrates showed no clear response to restoration in spite of a steady increase in abundance across the sampling period (2001-2004) - a trend that may be related to hydrological change. In contrast, the fish showed a clear, rapid and positive response to the increase in habitat structure. Most notably, mountain galaxias increased in abundance almost 3-fold at the 4-structure sites in Creightons Creek. Other species of fish, river blackfish and southern pygmy perch showed a weaker and less emphatic response. With the onset of drought however, local fish populations were driven to extinction in the sand-slug by stream drying and lack of refugia. Populations in the headwaters and in chain-of-ponds section below the sand-slug provided limited resistance and resilience to this disturbance. The above finding highlights the importance of including both residential and refugial habitats in the planning of restoration at the catchment level. Sand slugs have a significant impact on the microbial community structure and function in Creighton’s Creek. The sand-impacted section of Creightons Creek has very low primary production compared to other streams in Australia and overseas, which is likely to be due to the abrasiveness of sand. This is supported by very low chlorophyll-a and protein concentrations on benthic sediments. Chlorophyll a and protein in sediments from the non-impacted section of the Creek were substantially higher. The respiration rates are comparable with other streams and with low production rates this means Creightons Creek is a strongly heterotrophic system, requiring allochthonous inputs of carbon to fuel the ecosystem. When dissolved organic carbon is introduced into the system it is rapidly assimilated. The rate of assimilation between the sand impacted area is statistically lower than the non-impacted reach. The two-station method was unsuccessful in the sand-impacted portion Granite Creeks, due to the environmental conditions (i.e. reach travel times were too fast to allow measurable changes between probes) and the limitations of the instrumentation. The one-station method however was applied to Granite creeks with success, but no significant differences were detected between treatment (sleeper) and control sites. In high flow, microbial activity (measured by FDA hydrolysis and enzyme mapping devices) is higher in the hyporheic zone than in surficial sediments, whereas in low flow, the opposite is true (possibly due to an anoxic hyporheic zone in low flow). Sediment respiration, measured by horizontal sediment reactors, occurs in 'hotspots' and is more dependent on intra-site spatial variability than the effect of the sleepers or flow. The enzyme mapping devices show that the microbial community can be very localized, and this variability in activity can be detected down to the millimetre scale. It is suspected these 'hotspots' and carbon concentrations vary in a similar pattern. DOC concentrations in the hyporheic zone reflected stream water concentrations, but were highly variable. The hyporheic sediment, therefore, is not a storage zone for carbon, as it is for nitrogen and phosphorus. The most upstream sleeper accumulates more CPOM than the other sleepers in low/moderate flow, and the carbon concentrations in the hyporheic zone surrounding this sleeper reflect this. The enzymatic fingerprint from the hyporheic samples surrounding the first sleeper saw increases in the activity of all six enzymes. The most abundant/active enzymes are the non-specific esterase and leucine aminopeptidase, which suggests the carbon inputs are from animal waste and algal detritus. The brown/black coloured coating on benthic sediment in summer was identified by Paul Leahy from the EPA to be either diatoms or an algal mat. It appears that sediment instability is the primary stressor on the system, which hinders the success of the sleepers. This result emphasizes that when monitoring a small stream rehabilitation project, it's important to study the health of the ecosystem rather than the stressor that is being eliminated (i.e. habitat homogeneity in the case of the sleepers). For example any effect of introduced structures on microbial community structure are only transitory.