Condition of Leschenault
The Regional Estuaries Initiative supported the ongoing water quality, seagrass and macroalgae monitoring of the Leschenault Estuary since 2016 with monitoring continuing through Healthy Estuaries WA.
Below is the latest data about the condition of Leschenault Estuary.
For more detail on the condition of Leschenault Estuary, read Leschenault Estuary (Derbal Elaap): condition of the estuary 2016-19.
Seagrass in Leschenault Estuary
Seagrass meadows are an important part of estuarine ecosystems, providing habitat and food for birds, fish and crustaceans. They contribute to good water and sediment quality by consuming nutrients and oxygenating the water and are estimated to provide $12 million per year in ecosystem services to WA estuaries. Seagrass meadows also store carbon and release oxygen – making them a strong ally in the fight against climate change.
Seagrass condition and distribution throughout an estuary can provide important information about the overall health of the estuary and the quality of water entering it from streams, creeks, rivers and drains.
Three species of seagrass occur in the Leschenault Estuary. Halophila ovalis (left image) is dominant, and is generally found throughout the estuary basin. Ruppia megacarpa (centre image) is often observed along the eastern shoreline. Zostera muelleri (right image) is found near the Cut, where the waters are more marine.
Halophila ovalis
Ruppia megacarpa
Zostea muelleri
Seagrass distribution in the Leschenault Estuary
- In the 1980s and ’90s, seagrass was widely distributed in the estuary, particularly in the northern area, and absent only in a small area of deep water in the centre of the estuary.
- In April 2009, seagrass extended across 1,741 hectares – about 69 per cent of the estuary area.
- In 2014, there was a substantial loss of seagrass, which prompted monitoring to start in 2015.
- Seagrass is slowly recovering but is yet to return to the extent reached in 2009.
2023-24 monitoring
In February 2024, seagrass was estimated to extend across 1,437 hectares, about 57 per cent of the estuary area. Areas close to The Cut and along the eastern shoreline north of Australind continue to have the densest seagrass cover. Compared with 2023, there was a slight decrease in seagrass extent, specifically in the central and northern basins.
Seagrass cover declined slightly across the estuary, particularly in meadows near The Cut. However, some areas in the northern basin showed an increase in seagrass cover. Ruppia was the most dominant species and has continued to expand its range in the southern basin near The Cut. Halophila was present across most areas of the estuary yet was absent from the northern basin. Zostera showed a small decrease in its distribution but remains localised close to The Cut.
The abundance of small organisms growing on seagrass leaves (epiphytes) was low on average; however, the recovering meadows in the northern basin had medium to high abundance of epiphytes.
Overall, seagrass in the Leschenault Estuary was in a stable and good condition in February 2024. However, high nutrient inputs and increasing macroalgae abundances in several areas of the estuary could negatively impact seagrass health in the future.
Macroalgae in Leschenault Estuary
Macroalgae (or seaweeds) are a vital and natural part of many estuarine and marine ecosystems. They provide important habitat and food for many creatures, as well as perform essential ecological functions like oxygenating water and absorbing nutrients. However, an overabundance of macroalgae can become a problem, and often indicates too many nutrients in the water – so they can be indicators of water quality. Assessing macroalgae in estuaries forms part of our integrated understanding of ecosystem health.
As part of the Department of Water and Environmental Regulation’s (the department) seagrass monitoring, an annual snapshot survey occurs during the summer months in the Leschenault Estuary. Previous studies on the distribution and types of macroalgae have characterised seasonal patterns in abundance and distribution. These studies, combined with the department’s snapshots, can provide an idea of some (but not all) of the species that can be found in this estuary.
Macroalgae are generally grouped based on their appearance or pigment in their tissue. Some of the macroalgae found in the Leschenault Estuary include:
Red algae (Rhodophyta)
- Chondria sp
- Gracilaria sp
- Laurencia sp
Brown algae (Ochrophyta)
- Caulocystis sp
- Dictyota sp
- Hormophysa sp
- Sargassum sp
- Sirophysalis sp
- Sphacelaria sp
Green algae (Chlorophyta)
- Acetabularia sp
- Caulerpa sp
- Chaetomorpha sp
- Lamprothamnium sp (Charophyte)
- Cladophora sp
- Rhizoclonium sp
- Ulva sp
Macroalgae distribution 2016 to 2023
Macroalgae has been consistently abundant in the northern basin, which is where we typically find the greatest cover. It is possible that dense macroalgae growth in this part of the estuary is smothering seagrass and restricting seagrass recovery following
observed loss between 2009 and 2015.
To a lesser extent, macroalgae has also increased in the south of the estuary near Vittoria Bay close to the mouth of the Preston River. Declining water quality in the Preston River may be contributing to increasing macroalgae abundance, and further deterioration could pose a risk to the health of the estuary. Macroalgae have remained absent from large sections of the central basin over time.
Macroalgae over time
The increasing extent of macroalgae over time is often an indication of declining estuary health. A change from the ecosystem being dominated by seagrass to macroalgae or
cyanobacteria, alongside decreased biodiversity, could be a risk in the Leschenault Estuary. Seagrass recovery in the northern basin of the estuary has been slow and highlights the importance of continuing work to improve water quality and reduce nutrients that enable excessive growth of macroalgae.
Salinity and oxygen concentrations (2016-19)
Brunswick River
The estuarine part of the river is often stratified in terms of salinity. When stratified, bottom water oxygen concentrations are consistently low, with the lowest oxygen concentrations recorded during the dry season.
Collie River
The salinity differences between surface and bottom waters in both wet and dry seasons show that the waters are rarely flushed with freshwater from the rivers and stratification is persistent. Like the Brunswick River, bottom water oxygen concentrations were consistently low in 2016–19, especially in the dry seasons.
Preston River
Surface waters were fresh to brackish. Unlike the Brunswick and Collie River systems, the waters in the Preston had a healthy oxygen concentration. This is probably because the river is shallow and does not have excessively high organic loads (which can consume oxygen).
Estuary basin
During the monitoring period, there was a small decrease in winter salinity averages in the estuary basin to below seawater levels, showing the influence of the freshwater river inputs. However, the estuary is strongly influenced by tidal exchange and in summer the limited freshwater input to the northern basin leads to hypersaline conditions. The oxygen status of this system was good in both seasons.
Parkfield drain
This area is often hypersaline, particularly during the summer (up to 1.5 times as salty as marine water), due to high evaporation rates and poor flushing. Because of the shallow depth of this system, only surface oxygen concentrations were measured, and showed elevated values all year. While high oxygen concentrations are generally good, excessive concentrations in day-time values can also indicate algal bloom activity.
