Estuaries

Assessing estuarine resource condition

Estuarine ecosystems are highly complex and dynamic environments. Since estuaries are at the bottom end of catchments, their condition can be affected by activities occurring within the upstream freshwater catchment. Where the condition of catchments, rivers or estuaries is poor, there are likely to be additional impacts on the marine receiving waters and coastal areas.

There are more than 100 estuaries in Victoria. Most of Victoria’s estuaries are brackish mouths of rivers and streams that flow directly into the ocean or into large marine bays (such as Port Phillip Bay, Western Port and Corner Inlet). The definition of estuaries also includes coastal barrier lagoons (such as the Gippsland Lakes) and some coastal inlets (such as Anderson Inlet).

Many of Victoria’s estuaries close intermittently because of sand bar formation at the estuary entrance. This usually occurs during periods of low freshwater inflow. Intermittently closed estuary entrances are a natural feature of the Victorian coastline and can be critical to the ecology and physical form of estuaries.

It is vital to understand the links between the values of estuaries, threats to those values and how management activities can reduce threats and improve environmental condition of estuaries. A consistent method for assessing the environmental condition of Victorian estuaries has been developed: the Index of Estuary Condition (IEC).

The IEC assesses estuarine resource condition for the purposes of:

reporting on the condition of estuaries across Victoria
assisting the prioritisation of management investment among estuaries
providing a baseline for assessing long-term changes in estuary environmental condition.

The IEC consists of 5 themes: physical form, hydrology, water quality, flora and fish. The first state-wide IEC assessment for Victorian estuaries was produced in 2021.

Assessment of Victoria’s estuaries using the Index of Estuary Condition: Results 2021

Inaugural report of the condition of Victoria’s estuaries

Assessment of Victoria’s estuaries using the Index of Estuary Condition: Results 2021 provides a snapshot of the environmental condition of all 101 large estuaries in Victoria. Estuaries are diverse, complex and highly valued systems that support many environmental, recreational, cultural and economic values. Improved knowledge of estuary environmental conditions and threats will be used to guide state and regional estuary management to benefit communities and the environment.

The Index of Estuary Condition (IEC) was tested, refined and applied over a decade to allow the first consistent and rigorous assessments of all estuaries across Victoria. Similar condition indices are also used for Victoria’s rivers and wetlands: Index of Stream Condition and Index of Wetland Condition, respectively. Five important aspects were assessed for IEC: Hydrology, Physical Form, Water Quality, Flora and Fish.

Assessment of Victoria’s estuaries using the Index of Estuary Condition	2021 results show
The most common environmental condition of Victoria’s estuaries was moderate	32% of Victoria’s estuaries had moderate condition
The estuaries with excellent condition were located along the Gippsland coastline, downstream of forested landscapes	13% of Victoria’s estuaries had excellent condition
Estuaries with good condition were spread along Victoria’s coastline, downstream from landscapes without intensive land use	25% of Victoria’s estuaries had good condition
The estuaries with poor or very poor condition were downstream of landscapes with intensive agriculture and/or urbanisation	25% of Victoria’s estuaries had poor condition, whilst 5% had very poor condition
Most of Victoria’s estuaries had minimally developed Physical Form (i.e. without engineered shorelines and barriers within estuaries), but at least moderately altered Hydrology owing to upstream storages altering inflow of freshwater to estuaries	70% of Victoria’s estuaries had minimally modified Physical Form, but 69% had at least moderately modified Hydrology
Water quality was good or excellent in most Victorian estuaries but poor or very poor in those downstream of landscapes with intensive agriculture and/or urbanisation	54% of Victoria’s estuaries had excellent or good Water Quality, whilst 25% had poor or very poor Water Quality
The condition of Flora and Fish in most Victorian estuaries was at least moderate.	The condition of Flora was at least moderate for 89% of Victoria’s estuaries and the condition of Fish was at least moderate for 76% of Victoria’s estuaries

State-wide condition assessments will be rotated among estuaries, wetlands, and rivers, with each waterway type assessed approximately every decade. This monitoring frequency reflects that changes to the environmental condition of waterways are likely to be slow at broad spatial scales. However, there may be more rapid changes in response to new threats or management interventions at individual estuaries. In the periods between IEC assessments, management decisions may be refined with updated information from targeted monitoring of key aquatic values and threats in specific estuaries.

DELWP are grateful for all contributors to the development of IEC, which included representatives from the Victorian Government, Catchment Management Authorities, universities and citizen scientists via EstuaryWatch.