Indicator description

Why is it indicative

What does the data show

Data

Indicator description

The indicator summarises what is known about the extent and condition of estuaries by type. Estuary types include hypersaline lagoons, barred estuaries, open estuaries, drowned river valleys, river-mouth estuaries, and some marine inlets and bays (Edgar et al. 1999). Estuarine habitat types include salt marshes, sandy beaches, rocky shores and intertidal mudflats.

Please note: Because of the way the environmental information is structured and the arbitrary nature of the boundaries in reality, there is considerable overlap between this indicator and the very closely related Extent and Condition of Marine and Coastal Habitats Indicator. Therefore, seagrass beds, soft-sediment bottoms, algal beds and subtidal reefs that occur in estuaries cannot be separated from the rest of the coastal and marine areas and a consolidated report can be found in the related indicator. Conversely, all saltmarsh data, whether estuarine or not, are reported in this indicator.

Why is it indicative

Biological diversity, or biodiversity, is a fundamental measure of aquatic environmental health. Biodiversity can be measured at a range of levels - from the molecular and genetic to ecosystem and landscape (Saunders et al. 1998). In Tasmania, there is no complete Statewide assessment of estuarine biodiversity at any level, so, instead, it is necessary to rely on surrogates and partial assessments. For this indicator the extent and condition of estuaries by type is used as a surrogate. Different estuarine types reflect different ecological and environmental conditions and, therefore, different components of biological diversity.

What does the data show

Indicator reliability comment: there is no systematic collection of data to support this indicator at present, and so we have no comparative data and little certainty about either condition or trends.

• Evidence is emerging of massive losses of shell (mollusc) species over the past 150 years in shallow, sheltered estuarine waters of the south-east (Samson & Edgar 2001). The losses were previously undetected, highlighting the risk of the 'sliding baseline syndrome' whereby changes that occur over generations are not noticed and the new environmental conditions are thought of as 'normal' (Dayton et al. 1998).
   
• In 1999, 24 of the 90 Tasmanian mainland catchments were considered pristine, with little human impact within the catchment. These catchments were nearly all distributed in the south and west of the State and on Cape Barren Island. A small number of catchments were severely impacted by urban development and large-scale land clearance, and many others were moderately affected by human impacts.
   
• Species richness varies with geographic location for both macrofauna and fishes, with highest numbers of species occurring in the Furneaux Group, north-eastern Tasmania and south-eastern Tasmania.
   
• Currently, the condition of Tasmanian saltmarshes is not well known. The level of uncertainty means that the risks associated with the management of the saltmarshes are high.
   

Data

Estuarine habitat - extent

Estuaries form a transitional zone between the land and marine environments, often without clear boundaries. They exhibit a wide range of physical and chemical characteristics that can change rapidly over short time periods. Because of this, any definition of estuaries is problematic and precise measurements of extent are not possible. However, they are very important, unique environments and the majority of human activity takes place, or impacts, on estuaries.

Edgar et. al. (1999) defined an estuary as 'a semi-enclosed or periodically closed coastal body of water in which the aquatic environment is affected by the physical and chemical characteristics of both fluvial [freshwater]and marine systems'. Using this definition Edgar et. al. defined 111 estuaries around Tasmania. They then gathered information about the estuaries including catchment extent, catchment geology, rainfall, size of the estuary, seaward barrier characteristics, tidal regime, salinity levels and biological data. The estuaries were classified into nine groups, or types. It is notable that the groups do not clearly match the IMCRA bioregions. The groups are detailed in the following table.

Extent and number of Tasmanian estuaries by type

Area in square kilometres

Estuary type	Example	Number	Extent(1)	Extent(2)
Barred, low salinity estuaries	Scamander	25	72.6	0.3
Open estuaries	Little Swanport	29	154.4	1.2
Marine inlets and bays	Georges Bay	24	321.7	11.3
Hypersaline lagoons	Sellars Lagoon	8	90.7	4.9
Large mesotidal river estuaries	Mersey	15	598.1	2.4
Mesotidal drowned river valley	Tamar	1	12,147.3	97.9
Microtidal drowned river valley	Derwent	4	7028.6	122.0
Large open microtidal river	Pieman	4	1844.1	4.1
Barred river	Wanderer	1	424.4	1.0

1. Catchment and drainage extent in square kilometres

2. Estuary extent in square kilometres

Source: Edgar et al. 1999

The LIST maps show 635 estuarine water bodies with a total extent of 1,248 km². Note that the number of estuaries includes sub-estuaries and tidal lagoons and river mouths.

Estuarine habitat - condition

Estuaries have extremely high densities of plants and animals and are among the most productive habitats on earth. Estuarine seagrass beds produce as much per day as the fastest growing forests and grasslands on land (Edgar 2001). This elevated productivity is partly explained by the high levels of nutrients delivered from the land and rivers. Many bird species, including migratory species, can be seen taking advantage of the rich food sources.

All the following findings are from Edgar et al. 1999 except where noted.

Biodiversity

• Startling evidence is emerging of a catastrophic loss of shell (molluscan) species over the last 150 years in the shallow, sheltered estuarine waters of the south-east (Samson & Edgar 2001). Species numbers in dated core samples dropped massively through time while, in most cores, the fraction of mud increased significantly. Further work in preparation by the same authors suggests that while there are many factors at work - including siltation and pollution - the pattern of losses seems to match the rise, and then collapse, of the oyster and scallop dredging industries remarkably well. The lack of previous detection of such large losses of local biodiversity highlights the lack of knowledge about estuarine ecology.
   
• The number of macrofaunal species collected at a site primarily correlates with local salinity and biomass of submerged plant material, particularly seagrass.
   
• Species richness varies with geographic location for both macrofauna and fishes, with highest numbers of species occurring in the Furneaux Group, north-eastern Tasmania and south-eastern Tasmania.
   
• The low number of species recorded from estuaries along the western Tasmanian coast seems to reflect low macrofaunal productivity in that region, though the environmental conditions prevailing at the time of collection were relatively unusual and more work needs to be conducted to confirm the results.
   

Naturalness Index of estuaries

• The consequences for estuarine health of the land uses activities-such as those described in the Indicator-are assessed in a report on Tasmanian estuaries. This summary is taken from the survey of the condition of Tasmanian estuaries and their conservation significance using ecological and physical attributes, population and land use (Edgar et al. 1999). The full report is available from the Tasmania Aquaculture and Fisheries Institute. For management response purposes, such as identifying conservation priorities, please refer to this source report.
   
• A total of 24 out of the 90 Tasmanian mainland catchments were considered to be pristine, with little human impact within the catchment. These catchments were nearly all distributed in the south and west of the State and on Cape Barren Island. A small number of catchments were severely impacted by urban development and large-scale land clearance, and many others were moderately affected by human impacts. The highest levels of land clearance, population and urban development were found in catchments along the south-east, east and north coasts of Tasmania.
   
• The extent to which estuarine catchments and estuarine drainage areas are affected by anthropogenic change was assessed using a Naturalness Index (NI). This index increases from a base of 1 in pristine areas without cleared land or urban areas, and is weighted by a factor of 5 for cleared areas and a factor of 20 for urban areas. It is based on the SoE Landsat Satellite image classification of land cover types derived from images captured between 1992 and 1994.
   

Intertidal habitat - extent and condition

Extent of intertidal habitat types - estuarine only

hectares

IMCRA Bioregion	Rocky Shore	Sand	Mudflat	Unknown	Total
Otway	1	0	1	4	6
Central Bass Strait	n.a.	n.a.	n.a.	n.a.	n.a.
Twofold	n.a.	n.a.	n.a.	n.a.	n.a.
Flinders	0	583	0	2	585
Boags	32	4,019	4,221	37	8,309
Freycinet	12	671	1,529	38	2,250
Bruny	167	3,219	830	47	4,263
Davey	129	645	155	47	976
Franklin	68	1,633	49	46	1,796
Macquarie	n.a.	n.a.	n.a.	n.a.	n.a.
Totals	409	10,770	6,785	221	18,185

Source: The LIST 2001

On-shore coastal habitat - extent and condition

Saltmarsh

Saltmarshes are areas vegetated by herbs, grasses or low shrubs, bordering sheltered saline water bodies (Wong et al. 1993). They represent an important ecosystem with a specialised flora and fauna, and provide roosting locations for migratory bird species. It should be noted that there is a continuum from coastal to brackish to freshwater marshlands, which makes precise mapping difficult. Sarcocornia quinqueflora, also known as Beaded Glasswort and Samphire, is a marker species of Tasmanian saltmarshes as it occurs in almost all saltmarsh habitats.

Estimates of the extent of coastal saltmarsh habitat derived from TasVeg 2000 indicate that by far the most saltmarsh is found in the Boags and Freycinet Bioregions, followed by Bruny and Flinders. Note that no mapping is available for Franklin, Twofold and Macquarie Bioregions. The alternative data source-the LIST map layers-underestimates the amount of saltmarsh, reporting less than half that shown by TasVeg2000.

Extent of coastal saltmarshes grouped by IMCRA bioregions

hectares

IMCRA Bioregion	Saltmarsh
Otway	55
Central Bass Strait	Not relevant
Twofold	n.a.
Flinders	519
Boags	2,311
Freycinet	2,182
Bruny	938
Davey	62
Franklin	n.a.
Macquarie	n.a.
Totals	8,067

Source: Tasveg 2000-2001

Currently, the condition of Tasmanian saltmarshes is not well known with limited reports available. In south-east Tasmania, Gouldthorpe (2000) found that drainage, road works and grazing definitely has 'substantial and significant effects' on saltmarsh ecosystems with effects measured on soil moisture, conductivity, acidity, organic soil content, percent of bare ground and invertebrate species composition. He notes that many landowners display a clear understanding of the impacts of stock on saltmarsh and are fencing them out.

The maps of Kirkpatrick and Glasby (1981) showing the distribution of vegetation species in saltmarshes across Tasmania could act a basis for change analysis. The authors present a relationship between the various vegetation communities and the environmental variables of salinity and waterlogging. The report is a potential basis for studying trends in condition of saltmarshes in Tasmania.

Statewide surveys of the biodiversity of the animals (fauna) of Tasmanian saltmarshes indicate the richest areas are located in the sheltered bays of the south-east, the inlets of the north coast and the huge coastal wetlands of the north-west. The surveys found relatively few species on the west coast and the upper part of the east coast (Richardson et al. 1997).