Publications from CRCFE project D210

Effective water quality management of streams in urbanized basins requires identification of the elements of urbanization that contribute most to pollutant concentrations and loads.  Drainage connection (the proportion of impervious area directly connected to streams by pipes or lined drains) is proposed as a variable explaining variance in the generally weak relationships between pollutant concentrations and imperviousness.  Fifteen small streams draining independent sub-basins east of Melbourne, Australia, were sampled for a suite of water quality variables.  Geometric mean concentrations of all variables were calculated separately for baseflow and storm events, and these, together with estimates of runoff derived from a rainfall-runoff model were used to estimate mean annual loads.  Patterns of concentrations among the streams were assessed against patterns of imperviousness, drainage connection, unsealed (unpaved) road density, elevation, longitude (all of which were inter-correlated), septic tank density and basin area.  Baseflow and storm event concentrations of dissolved organic carbon (DOC), filterable reactive phosphorus (FRP), total phosphorus (TP) and ammonium (NH4+), along with electrical conductivity (EC), all increased with imperviousness and its correlates.  Hierarchical partitioning showed that DOC, EC, FRP and storm event TP were independently correlated with drainage connection more strongly than could be explained by chance.  Neither pH nor total suspended solids concentrations were strongly correlated with any basin variable.  Oxidized and total nitrogen concentrations were most strongly explained by septic tank density.  Loads of all variables were strongly correlated with imperviousness and connection.  Priority should be given to low-impact urban design, which primarily involves reducing drainage connection, to minimize urbanization-related pollutant impacts on streams.

(output of CRC FE project D210 - Urbanization and the Ecological Function of Streams)

Urbanization has long been recognized to alter the hydrology, water quality and channel form of waterways.  Recently, consideration of urban impacts on waterways has expanded to include assessment of the aquatic biota, generally focusing upon the macroinvertebrate fauna. This study compares the impacts of urbanization on the structure of stream benthic diatom communities in 16 first- and second-order streams in the east of Melbourne, Australia.  Relationships between the physical elements of urbanization, water quality and diatom communities were examined using multivariate analyses with compositional similarity, and univariate analyses with selected diatom species and indices,.  There was a strong negative correlation between urban density  and the diatom indices of water quality.  The element of urbanization most strongly related to the measured decline in the diatom community was drainage connection.  Electrical conductivity was the most influential water quality variable.  It is hypothesized that, in addition to increasing electrical conductivity, drainage connection may be impacting the diatom community by increasing the delivery of phosphorus during small storm events.  The study suggests that reducing directly piped drainage connection using infiltration and retention is a logical step in the mitigation of urban impacts upon receiving streams.  A corollary part of the project indicated strong potential for using overseas diatom indices in studies on water quality in southeastern Australia.

(output of CRC FE project D210 - Urbanization and the Ecological Function of Streams)

Abstract

1. Urbanisation is an important cause of eutrophication in waters draining urban areas.  We determined whether benthic algal biomass in small streams draining urban areas was explained primarily by small-scale factors (benthic light, substratum type and nutrient concentrations) within a stream, or by catchment-scale variables that incorporate the interacting multiple impacts of urbanisation (i.e. variables that describe urban density and the intensity of drainage or septic tanks systems).
2. Benthic algal biomass was assessed as chlorophyll a density (chl a) in 16 streams spanning a rural-urban gradient, with both a wide range of urban density and of piped stormwater infrastructure intensity on the eastern fringe of metropolitan Melbourne, Australia.  The gradient of urban density among streams was broadly correlated with catchment imperviousness, drainage connection (proportion of impervious areas connected to streams by stormwater pipes), altitude, longitude and median phosphorus concentration.  Catchment area, septic tank density, median nitrogen concentration, benthic light (photosynthetically active radiation) and substratum type were not strongly correlated with the urban gradient. 
3. Variation in benthic light and substratum type within streams explained a relatively small amount of variation in log chl a (3-11% and 1-12%, respectively) compared to between site variation (39-54%).
4. Median chl a was positively correlated with catchment urbanisation, with a large proportion of variance explained jointly (as determined by hierarchical partitioning) by those variables correlated with urban density.  Independent of this correlation, the contributions of drainage connection and altitude to the explained variance in chl a were significant. 
5. The direct connection of impervious surfaces to streams by stormwater pipes is hypothesised as the main determinant of algal biomass in these streams through its effect on the supply of phosphorus, possibly in interaction with stormwater-related impacts on grazing fauna.  Management of benthic algal biomass in streams of urbanised catchments is likely to be most effective through the application of stormwater management approaches that reduce drainage connection.

(output of CRC FE project D210 - Urbanization and the Ecological Function of Streams)

Abstract

Urbanization is a looming global threat to in-stream biodiversity, but the best approaches to mitigation are unclear.  This paper asks if the protection of in-stream biota, in particular macroinvertebrate assemblages, is dependent on the sequestration of catchments from urbanization, or if protection in urbanized catchments can be achieved through better drainage design.  In-stream macroinvertebrate assemblage composition was assessed for 16 catchments spanning a rural-urban gradient.  Catchment imperviousness and drainage connection (the proportion of impervious area directly connected to streams by stormwater pipes), together with other possible driving factors, were assessed as explanatory variables of macroinvertebrate assemblage composition.  The proportion of variance explained independently and jointly by each variable was assessed by hierarchical partitioning.  Assemblage composition was strongly explained by the gradient of urban density (i.e. a large proportion of variance was jointly explained by variables correlated with the urban gradient: imperviousness, connection, longitude and elevation).  However, drainage connection was the strongest independent correlate.  Most sensitive taxa were absent from sites with >20% connection.  Thus the connection of impervious surfaces to streams by pipes is a more likely determinant of taxa loss than the impervious areas themselves.   Low-impact urban design approaches that reduce drainage connection are postulated as the most effective management solution to the protection of stream biota in urban catchments.

(output of CRC FE project D210 - Urbanization and the Ecological Function of Streams)

Abstract

Restoration of streams degraded by urbanization has usually been attempted by enhancement of instream habitat or riparian zones. Such restoration approaches are unlikely to substantially improve instream ecological condition because they do not match the scale of the degrading process. Recent studies of urban impacts on streams in Melbourne, Australia, on water chemistry, algal biomass and assemblage composition of diatoms and invertebrates, suggested that the primary degrading process to streams in many urban areas is effective imperviousness (EI), the proportion of a catchment covered by impervious surfaces directly connected to the stream by stormwater drainage pipes. The direct connection of impervious surfaces to streams means that even small rainfall events can produce sufficient surface runoff to cause frequent disturbance through regular delivery of water and pollutants; where impervious surfaces are not directly connected to streams, small rainfall events are intercepted and infiltrated. We, therefore, identified use of alternative drainage methods, which maintain a near-natural frequency of surface runoff from the catchment, as the best approach to stream restoration in urban catchments and then used models of relationships between 14 ecological indicators and EI to determine restoration objectives. Ecological condition, as indicated by concentrations of water-quality variables, algal biomass, and several measures of diatom and macroinvertebrate assemblage composition, declined with increasing EI until a threshold was reached (EI = 0.01–0.14), beyond which no further degradation was observed. We showed, in a sample catchment, that it is possible to redesign the drainage system to reduce EI to a level at which the models predict detectable improvement in most ecological indicators. Distributed, low-impact design measures are required that intercept rainfall from small events and then facilitate its infiltration, evaporation, transpiration, or storage for later in-house use.

(output of CRC FE project D210 - Urbanization and the Ecological Function of Streams)

Abstract

A decision-making framework (DMF) for stormwater management based on a stochastic watershed model has been developed by the CRC for Catchment Hydrology.  The DMF predicts stormwater flows and loads of pollutants (total suspended solids, phosphorus, nitrogen and gross pollutants) exported from urbanized catchments with varying types and levels of stormwater abatement techniques.  The utility of the DMF is limited by the lack of prediction as to the effects of these changes on the ecology of receiving waters.  This paper places the DMF within a conceptual model of stormwater physico-chemical impacts on stream ecosystems.  Emerging methods are identified and described for inserting predictive models of ecological indicators (community composition) into the DMF, and to include the assimilative capacity of the stream ecosystem as part of the 'treatment train'.  Current research using the catchment ecosystem approach to quantify in-stream biogeochemical processes is described, and areas of research need are identified, such as the effects of the spatial arrangement of catchment urbanization on stream ecology and the interactions between engineering solutions, stream condition and human behaviour

(output of CRC FE Mk II project D210 - Urbanization and the Ecological Function of Streams - and CRC CH project 4.1 - Stormwater Pollutant Sources: Pathways and Impacts)

Abstract

The general hypothesis that catchment urbanization explained the distribution of the threatened, stream-dwelling amphipod, Austrogammarus australis (listed under the Victorian Flora and Fauna Guarantee Act 1988), was tested using several surveys of 58 sites in streams draining the Dandenong Ranges on the eastern fringe of Melbourne, Victoria, Australia.  More specifically, four catchment-scale elements of urban land, hypothesized as sources of stress to receiving streams, were separated: catchment imperviousness, drainage connection (proportion of impervious areas connected to streams by stormwater pipes), density of unsealed roads and density of septic tanks.  The degree to which each attribute independently and jointly explained the occurrence of A. australis was assessed using hierarchical partitioning of logistic regression analyses.  Drainage connection independently best explained the occurrence of A. australis, pointing to stormwater drainage design as the priority area of catchment management for the conservation of the species.  The separation of urban land use into attributes that characterize likely stressor sources provides a useful framework for assessing and prioritizing the most appropriate management actions to minimize urban-related stresses to aquatic biota.

(output of CRC FE project D210 - Urbanization and the Ecological Function of Streams)

Abstract

The term ‘‘urban stream syndrome’’ describes the consistently observed ecological degradation of streams draining urban land. This paper reviews recent literature to describe symptoms of the syndrome, explores mechanisms driving the syndrome, and identifies appropriate goals and methods for ecological restoration of urban streams. Symptoms of the urban stream syndrome include a flashier hydrograph, elevated concentrations of nutrients and contaminants, altered channel morphology, and reduced biotic richness, with increased dominance of tolerant species. More research is needed before generalizations can be made about urban effects on stream ecosystem processes, but reduced nutrient uptake has been consistently reported. The mechanisms driving the syndrome are complex and interactive, but most impacts can be ascribed to a few major large-scale sources, primarily urban stormwater runoff delivered to streams by hydraulically efficient drainage systems. Other stressors, such as combined or sanitary sewer overflows, wastewater treatment plant effluents, and legacy pollutants (long-lived pollutants from earlier land uses) can obscure the effects of stormwater runoff. Most research on urban impacts to streams has concentrated on correlations between instream ecological metrics and total catchment imperviousness. Recent research shows that some of the variance in such relationships can be explained by the distance between the stream reach and urban land, or by the hydraulic efficiency of stormwater drainage. The mechanisms behind such patterns require experimentation at the catchment scale to identify the best management approaches to conservation and restoration of streams in urban catchments. Remediation of stormwater impacts is most likely to be achieved through widespread application of innovative approaches to drainage design. Because humans dominate urban ecosystems, research on urban stream ecology will require a broadening of stream ecological research to integrate with social, behavioural, and economic research.

(output of CRC FE project D210 - Urbanization and the Ecological Function of Streams)