Lower Nambeelup Brook 2

The condition of the Lower Nambeelup Brook 2 site (site code: MR31NAMB2, site reference: 6140263) was assessed between 23 and 24 November 2022. Results are compared with a previous assessment in 2017.

Healthy Rivers assessments are conducted using standard methods from the South West Index of River Condition (SWIRC), which incorporates field and desktop data from the site and from the broader catchment. Field data collected include the following indicators, assessed over about a 100 m length of stream:

• Aquatic biota: fish and crayfish community information (abundance of native and exotic species across size classes, general reproductive and physical condition)
• Water quality: dissolved oxygen, temperature, specific conductivity, and pH (logged in situ over 24 hours) as well as laboratory samples for colour, alkalinity, turbidity and nutrients
• Aquatic habitat: e.g. water depth, substrate type, presence of woody debris and detritus, type and cover of macrophytes and draping vegetation
• Physical form: channel morphology, bank slope and shape, bioconnectivity (barriers to migration of aquatic species), erosion and sedimentation
• Fringing zone: width and length of vegetation cover within the river corridor and lands immediately adjacent, structural intactness of riparian and streamside vegetation
• Hydrology: measures of flow (velocity) at representative locations (compared against data from stream gauging stations within the system)
• Local land use: descriptions of local land use types and activities (compared against land use mapping information for the catchment)

The MR31NAMB2 site is about 600 m upstream of a site assessed in November 2017 (MR31NAMB1). The site was moved because of issues with site access and tidal influence.

Other departmental data: The Lower Nambeelup Brook 2 site is about 2.5 km downstream of the Department of Water and Environmental Regulation’s (the department) flow gauging station known as Kielman (site reference: 614063), which has been in operation since 1990.

Search the site code or site reference in the department’s Water Information Reporting (WIR) system to find data for this site and nearby sampling points (flows, surface water quality, groundwater monitoring, the department's meteorological data). See also the Bureau of Meteorology website for additional meteorological data for the area.

The Lower Nambeelup Brook 2 site is on Nambeelup Brook on the southern Swan Coastal Plain, about 65 km south of Perth.

The catchment lies entirely on the coastal plain and much of the upper catchment is drained by deeply incised channels to enable grazing on the low-lying land. The assessed site is near the bottom of the catchment, about 1 km from where the brook discharges into Black Lake. The site is in a low-gradient section of the brook. Within the site the brook had a U-shaped channel with a convex channel shape and a moderate to steep bank slope (30–80 degree slope).

At the time of sampling, the site was connected channel habitat.

Aquatic habitat quality was near optimal. Woody debris in the channel was abundant and varied, reflecting the near-optimal tree canopy and extensive riparian vegetation at the site. Channel depth was varied, and the substrate was mostly sandy with some gravel and silt. There was minor sand sedimentation through the site and a large plume was produced when sediment was disturbed. Biological substrate cover, consisting of leaves and detritus (which provides food and habitat for macroinvertebrates), was moderately dense across more than half of the site (60–100 percent).

In-stream vegetation was sparse (5 per cent of site area covered) and consisted of some emergent Typha spp. and some floating duckweed (Lemna spp.).

Overhanging banks provided some habitat over a little less than 50 per cent of bank length, and draping vegetation and roots provided habitat along more than half the length of the banks – this mostly consisted of tree roots and exotic grasses.

The image below illustrates the optimal stream shading through the assessment site, with Melaleuca spp. (paperbarks) and a few Eucalyptus rudis (flooded gum) providing shade along 85–95 per cent of the bank length. Tree canopy shading extended over the wetted channel for an average of 6 m from the banks, which provided almost complete shading across the 7 m wide wetted channel.

At the time of sampling, the site was flowing slowly (about 0.1 m/s) and was connected in terms of fish passage. Stream discharge (flow) measured at the department’s Kielman Rd gauging station (614063) was about 0.5 ML/day and flow was decreasing through November 2022.

Based on flow records (since 2005) from the Kielman Road gauging station, flow typically ceases in early summer and resumes in late autumn. In 2022 flow stopped on 17 December and did not start again until 8 June 2023. The road crossing culvert immediately upstream of the site is likely to dry as flow decreases; this would form a barrier to fish movement. The lower section of Nambeelup Brook has permanent water throughout the year due, in part, to tidal influence. Fish were expected to be able to move downstream between the assessment site and the estuary.

The riparian zone (containing species adapted to living at the interface between aquatic and terrestrial environments) comprised a relatively broad and largely intact, mature tree layer, comprising both larger (flooded gum) and smaller (paperbark) trees. The shrub layer was reduced (a few tea trees) and the ground cover layer was largely exotic species (grass, dock and honeysuckle), indicative of historical clearing. Some native ground cover included reeds, as well as Cyperacae spp. and Typha spp. on the banks. Pteridium esculentum (bracken) was more noticeable through the understory moving away from the brook. This riparian vegetation (tree, shrub and ground cover layers) extended about 50 m from the channel on the right bank and up to 150 m on the left bank, reflective of the low-lying (and likely regularly inundated) section of the wider river channel. The image below shows a native tree canopy with paperbarks and some flooded gums standing more than 40 m from the channel, with a mixed ground cover layer of native bracken and exotic grasses.

Channel stability was excellent with minimal extent (0–4 percent of the bank length) and low severity of erosion observed. This is likely because of the low-lying landform and slow-flowing, depositional nature of the brook as it nears the tidally influenced Serpentine lakes, including Black Lake. During periods of high flows, the water most likely spreads out to inundate the riparian and adjacent zone, rather than scouring out the active channel.

Below is a graph of the water quality recorded during the 24-hour assessment.

Water quality over the 24 hours was varied.

Water temperature was optimal, not exceeding the 25°C threshold¹ and remained within the recommended diurnal temperature range of 4°C (2.97°C).

Dissolved oxygen was suboptimal as it was below the 4 mg/L threshold2 for the entire 24-hour assessment period. The average dissolved oxygen concentration was 2.36 mg/L. Conversely, the diurnal range in dissolved oxygen was small (1.1 mg/L) indicating a system where biological drivers (e.g. algae) and physical drivers (temperature) are not influencing dissolved oxygen over a diurnal cycle. Although the dissolved oxygen concentrations were suboptimal, there was still a diverse and abundant aquatic fauna (see fish and crayfish section below). The majority (98 per cent) of the total catch was recorded in the two fyke nets, indicating the aquatic fauna was highly mobile through the site. Importantly, none of the aquatic fauna recorded at the site were either lethargic or dead which indicates the suboptimal dissolved oxygen conditions were not adversely affecting the fauna. It is likely that oxygen refuges exist in the area, closer to surface and sometimes near aquatic vegetation.

Specific conductivity (which can be used as an indicator of salinity) was classed as marginal (mean value of 0.574 mS/cm) for the 24-hour assessment period. Turbidity was high (12 nephelometric turbidity units [NTU]), and pH (mean value of 6.68) was optimal, based on expectations of natural ranges (6.5–8.0) for freshwater rivers of south-west Western Australia.

Nutrient concentrations were classed as very high; total nitrogen (2.2 mg/L) and total phosphorus (0.25 mg/L) both exceeded guidelines for south-west systems. Nutrient enrichment is likely because of land use in the catchment, but the low diurnal flux in dissolved oxygen would suggest it is not fuelling biological drivers of dissolved oxygen (e.g. algae). This is likely because of the very dark colour of the water (true colour was 860 true colour units [TCU]) preventing light penetrating far into the water, which reduces the capacity for algal photosynthesis in the water column. A true colour value of 860 TCU is particularly high compared with many rivers in the south-west, and this value aligns with the in situ rating of ‘black’ for the degree of tannin staining in the water.

Footnote:

1 An upper limit for temperature is nominally set at 25°C based on the range recorded in sites where south-west native fish species are typically found to occur (Beatty et al. 2013).

2 A lower limit of 4 mg/L is taken from the SWIRC where it is used as a guideline limit for condition scoring. This level aligns with Beatty et al. (2013) where south-west native fish species were shown to typically reside in sites with levels above 4 mg/L.

The species recorded at the Lower Nambeelup Brook 2 site in 2022 are provided in the table below. This table includes a list of all species previously reported in the subcatchment, which provides an indication of species that may occur at the assessment site. As differences in habitat within a subcatchment naturally influence species distributions, and variability in methods between sampling programs can affect the species caught, this list is only indicative.

Five species of fish and two species of crustaceans were recorded during the 2022 sampling. This included three native freshwater fish species (western minnow, western pygmy perch, and nightfish), two native freshwater crustaceans (gilgie and south-west glass shrimp) and one native estuarine-freshwater species (blue-spot goby). Blue-spot gobies can complete their life cycle in both freshwater and estuarine environments but are most commonly found in the lower reaches of rivers where they can move into estuaries. This is consistent with this location, being connected to the estuary via Lake Goegrup and Black Lake. Nightfish were notable in their abundance, with hundreds of individuals recorded, at about 40 per cent of the total catch.

Juveniles were recorded for several native species including western minnow, nightfish, western pygmy perch and gilgies. This indicates suitable habitat quality to support life cycle requirements for these four native species. Compared to the 2017 data from the MR31NAMB1 site 600 m downstream, the diversity was slightly reduced from 10 to seven species of fish and crustacean, but two of those were exotics (goldfish and one-spot livebearer) and one was an estuarine opportunistic species (western hardyhead). Considering the similar diversity of native species and absence of two exotic species, compared with the 2017 sampling, the aquatic fauna reflected a healthier system in 2021. The diversity and abundance of native species (including presence of juveniles) was comparable to that observed in 2017 at the MR31NAMB1 site.

While this healthy diversity of native aquatic fauna seems at odds with the suboptimal dissolved oxygen (below 4 mg/L for the entire 24-hour assessment period) it is interesting to note that 98 per cent of the catch was record in the two fyke nets. This indicates almost all fish recorded at the site were in the process of moving either upstream or downstream. Fish are highly mobile and could have been caught in the fyke nets while moving from upstream winter foraging and breeding habitats towards summer refuge habitat in the lower reaches of the brook and lakes. As mentioned earlier, as none of the fish captured showed signs of poor condition (e.g. lethargy) it appears that the suboptimal dissolved oxygen concentrations were not representative of general water quality, at least in the short term.

One exotic species, the eastern gambusia, was recorded with several juveniles also observed. This indicates an established population, which presents a risk to native fish species through aggressive fin-nipping behaviours, as well as a greater tolerances for suboptimal water quality conditions than south-west native fish species.

Note: collection of fauna from inland aquatic ecosystems across Western Australia requires a licence from the Department of Primary Industries and Regional Development (DPIRD) and the Department of Biodiversity, Conservation and Attractions (DBCA). All species collected must be reported to these agencies as part of licence conditions.

Only fish and freshwater crustaceans (crayfish and shrimp) that typically inhabit river channels are targeted by the standard SWIRC sampling methods; however, other species are sometimes captured or observed. The following species were detected at the site:

• South-western snake-necked turtles (Chelodina oblonga)
• Leeches (Hirudinae)
• Diving beetles
• Juvenile nankeen night heron (see image below). This species feeds mostly on aquatic fauna, especially crustaceans, so the presence of a predator species would seem to indicate a certain degree of ecological health and function

For more information on these and other aquatic species, please see the River Science fauna page.