Ecological assessment of streams

Project D210 Urbanization and the ecological function of streams

Outcomes

A wide range of ecological indicators was broadly correlated with the gradient of urban density as estimated by catchment imperviousness. Streams with more densely urban catchments were more degraded according to many indicators.
They transported more phosphorus, dissolved organic carbon and salt.
They supported greater stands of algae on their bottoms, and total primary productivity in the streams was greater, both in itself and compared to total respiration.
The species of algae and invertebrates living in the streams changed with increasing urbanization: many species sensitive to disturbance were lost.

Independent of the general correlation with urbanization, most indicators were very strongly, independently explained by drainage connection.

In the table below the direction of correlations with urban density are indicated (+ means a higher value with greater urban density). The last two columns show if imperviousness (IMP) or drainage connection (CON) are correlated with each indicator independently of all the other urban indicators more than might be expected by chance. Other urban indicators are those that are broadly correlated with the rural-urban gradient (imperviousness, connection, density of unsealed roads, elevation and longitude).

		Sig ind. correlation
	Urban gradient correlation	IMP	CON
Water quality
Electrical Conductivity	+		*
[Dissolve Organic Carbon]	+		*
[Filterable Reactive Phosphorus]	+		*

Biological composition
Macroinvertebrate species richness	-		*
EPT richness	-		*
SIGNAL score	-	*	*
Macroinvertebrate compositional similarity	yes		*
IBD Diatom trophic score	-		*
Diatom compositional similarity	yes		*

Biological function
Benthic algal biomass accrual	+		*
Production: Respiration	+		*

These findings suggest that 'effective imperviousness' (just those impervious areas that are directly connected to the stream) is likely to be a good predictor of the condition of a stream. Most of the indicators assessed were strongly explained by effective imperviousness (EI) in a very similar way. They all had a strong relationship with EI, (increased degradation with increased EI) up to a threshold level beyond which they changed little. Water quality concentrations and indicators using algae all reached a threshold at very low levels of EI (1-5%), while thresholds for indicators using macroinvertebrates were higher (6-14% EI) (See figure below).

This figure shows the relationships between effective imperviousness and a selection of indicators used in the study. The lines in each graph are the best-fit model ? linear degradation to a threshold

Now, there were no formal stormwater treatment measures in any of the catchments studied here. All of the impervious surfaces in this study that were considered to be 'not connected' (and apparently had very little effect on the ecology of the streams) either drained to surrounding land or to vegetated/earthen channels. The main difference between these sorts of indirect drainage systems and pipes is that they would intercept surface flow from small rain events. For big storms, they would behave much like pipes. This suggests that if stormwater pipes can be replaced by drainage systems that encourage infiltration and retention of water from small rain events, then the effect of impervious surfaces on streams can be greatly reduced. We are proposing that the most important aim for stormwater management aimed at protecting stream ecology is to keep the frequency of surface flow near the frequency that would have occurred before urban development. This aim can be achieved using a number of techniques that are part of 'water sensitive urban design' or 'low impact design'.

For the Little Stringybark catchment in the northern Dandenong Ranges area, we calculated that surface runoff would occur from a forested 600 sq m block from a 15mm storm. This table outlines the sorts of impacts to Little Stringybark Creek (as an example) that would be expected without urban land use and with a moderate degree of urban land use (>10% total imperviousness) under two scenarios: one using conventional stormwater drainage, and one using low-impact design to intercept up to the 15mm storm. In this scheme, the small rain events do the bulk of the ecological damage to the stream.

	Conventional urban design:	Low-impact design:	Pre-urban land:
Size and frequency of rain event	all impervious surfaces drained by pipes or sealed drains directly to stream	runoff from impervious surfaces up to a 15mm rain event retained	no impervious surfaces
No effective rainfall (<1mm: typically 67% of days)	Lowering of water table resulting in low flow levels. High nutrient concentrations. Highly variable, but usually low turbidity. High algal biomass, resulting in large diel fluctuation in O₂. High risk of any pollutant spill in the watershed reaching the stream. Low diversity of invertebrates and fish	Plentiful stream baseflow of high quality water fed by subsurface flows. Good quality habitat for many types of microbes, plants and animals.
Small-moderate rain events (>1 to 15mm per day: typically occur on 29% of days, spread throughout the year)	Moderate to large increase in discharge: larger events causing movement of substrata and erosion of unarmoured banks. Inputs of stormwater with high nutrients, suspended sediments and toxicants. Frequent hydraulic disturbances interacting with toxic effects to stress or kill the more sensitive animal species. Frequent nutrient pulses stimulate growth of filamentous algae and eutrophic diatoms.	No surface runoff, all rainfall not evaporated reaches the stream by subsurface flows Beneficial augmentation of subsurface-fed baseflow Negligible physical disturbance associated with slightly higher flows following larger events.
Large rain events (>15 mm per day: typically occur on 4% of days, mostly during the wet season)	Very large flood causing major incision and bank erosion, also associated with large inputs of nutrients, sediments and toxicants. Catastrophic loss of all but the hardiest of invertebrates, smothering and scouring of algal growth, major loss of habitat	Major flood causing movement of substrata and erosion of unarmoured banks, associated with inputs of nutrients, sediments and toxicants. Loss of many invertebrates, particularly of sensitive species, but species adapted to annual flooding likely to re-colonize.	Elevated flows causing movement of substrata following larger events. Elevated nutrient, suspended sediments concentrations. Temporary loss of many invertebrates, but species adapted to annual flooding will re-colonize.

D210 project