A Classification of Tasmanian estuaries and assessment of their conservation significance using ecological and physical attributes, population and land use

Abstract

Physical attributes of Tasmanian estuaries A total of 111 estuaries of moderate or large size were recognised around Tasmania and associated Bass Strait islands. The catchments of these estuaries were mapped using GIS, and available data on geomorphology, geology, hydrology and rainfall collated for each estuary and catchment area. Tasmanian estuaries were classified into nine groups on the basis of physical attributes that included salinity and tidal data collected during a field sampling program. The nine groups primarily reflected the size of estuaries and their tidal, salinity and rainfall characteristics, and the presence of any seaward barrier. Catchments in the west, northwest and south of the state were characterised by high rainfall and high runoff, while catchments in the east and northeast were relatively dry. Estuaries in northern Tasmania possessed much greater tidal ranges than those on the eastern, southern and western coasts and were all open to the sea, whereas many estuaries in eastern Tasmania and the Bass Strait islands were intermittently closed by sand barriers. Biological attributes of Tasmanian estuaries Baseline information on the abundance, biomass and estimated production of macrobenthic invertebrate species was collected during a quantitative sampling program at 55 sites in 48 Tasmanian estuaries. These data were generally obtained at three different intertidal levels and two shallow subtidal depths at each site, and included information on a total of 390 taxa and over 100,000 individuals. Data on the distribution of 101 fish species, as obtained during surveys of 75 Tasmanian estuaries using seine nets by Last (1983) with some supplementary sampling, were also incorporated into the study. Multivariate analyses indicated that the nine estuarine groups identified using physico-chemical attributes were useful for categorising faunal relationships between estuaries, although considerable variation was found between faunas at sites within estuaries, particularly for marine inlet estuaries and drowned river valleys. Variation in faunal composition between sampling dates at the one site was low. The number of species collected at sites also varied with the estuarine groups identified using physicochemical attributes, with highest numbers of species occurring in marine inlets and small open estuaries. Relatively few species were collected at sites in microtidal river estuaries, barred low-salinity estuaries or hypersaline lagoons, with extremely low numbers collected in the western Tasmanian Wanderer estuary. North East Inlet (Flinders Island) and seagrass beds at the mouth of the Tamar estuary possessed exceptionally high diversity for both fishes and invertebrates. The number of macrofaunal species collected at a site was primarily correlated with local salinity and biomass of submerged plant material, particularly seagrass. Species richness also varied with geographic location for both macrofauna and fishes, with highest numbers of species occurring in the Furneaux Group, northeastern Tasmania and southeastern Tasmania. Low numbers of species were collected on the west coast and, to a lesser extent, south coast, central north coast and King Island. These patterns primarily reflected differences in estuary type between regions rather than concentrations of locally endemic species. Nearly all fish and invertebrate species recorded from Tasmanian estuaries occurred widely within the state and have also been recorded in southeastern Australia. Only 1% of estuarine fish species and <5% of invertebrate species were considered endemic to the state. The general pattern of widespread species' ranges around Tasmania was complicated by the absence of most species from the west coast, a small (<10%) component of species that occurred only in the northeast (particularly Flinders Island), and a few localised species that were restricted to different regions of the state. The low number of species recorded from estuaries along the western Tasmanian coast reflected extremely low macrofaunal productivity in that region. Estimated secondary productivity of west coast estuaries was generally at least one, and up to three, orders of magnitude lower than equivalent estuaries on other coasts. This low productivity was attributed to unusually low concentrations of dissolved nutrients in rivers and dark tannin-stained waters which greatly restrict algal photosynthesis and primary production. The estimated productivity and biomass of macrofaunal communities were found to vary little between sites within an estuary compared to variance between estuaries. By contrast, variance in the density of macrofauna was much greater between sites within an estuary than between estuaries, and variance was relatively low at scales of metres and hundreds of metres at similar tidal heights within a site. The environmental factor most highly correlated with animal density was the biomass of plant material, while estimated faunal productivity and biomass were most highly correlated with salinity. Threats to estuarine biota Biological resources within most large Tasmanian estuaries are exploited, with unknown consequences for ecosystem structure and function. In addition, nine major indirect threats to Tasmanian estuaries have been recognised: (i) increased siltation resulting from land clearance and urban and rural runoff, (ii) increased nutrient loads resulting from sewage and agricultural use of fertilisers, (iii) urban effluent, (iv) foreshore development and dredging, (v) marine farms, (vi) modification to water flow through dams and weirs, (vii) acidification of rivers and heavy metal pollution from mines, (viii) the spread of introduced pest species, and (ix) long-term climate change. While all of these factors can potentially disrupt ecosystem processes, the magnitude and spatial scale of these threats vary greatly. The first seven indirect threats affect individual estuaries and can be ameliorated by changing management practices, whereas substantial global warming would affect all estuarine ecosystems within the state through changes to water flow, increased water temperatures and sea level rise. The effects of introduced pests are also increasing and uncontrollable at present. Although only four introduced species – the green crab Carcinus maenas, the tanaidacean Sinelobus stanfordi, the bivalve Theora lubrica and the gastropod Potamopyrgus antipodarum – were collected during the present study, the threat posed by these and other species (including the seastars Asterias amurensis and Patiriella regularis, the molluscs Musculista senhousia, Crassostrea gigas, Maoricolpus roseus and Corbula gibba, the polychaete Sabella spallanzani and the ricegrass Spartina anglica) was considered to be extremely high. Amongst the more localised threats to estuaries, siltation, or a correlate thereof, was found to have an extremely widespread effect on Tasmanian estuaries. Estuaries with moderate or high human population densities in catchments consistently possessed muddy rather than sandy estuarine beds and shores. Although no change in number of macrofaunal species was associated with high human population densities and associated transformations from sandflats to mudflats, a pronounced shift in the faunal composition was evident in populated estuaries. These faunal changes were readily detectable using two disturbance indices described here, DIn and DIp, which are suggested to provide useful indicators of estuarine health. Assessment of the conservation significance of Tasmanian estuaries Human population densities within each estuarine catchment and the extent of legislative protection were estimated using GIS, census statistics, dwellings marked on 1:25,000 maps and land tenure data. The catchment areas of all Tasmanian mainland estuaries were also categorised in terms of land and vegetation use using GIS and data derived from satellite images. Satellite data for the Bass Strait islands were not available so estuaries in that region were not similarly examined. 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 effected 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 conservation significance of each Tasmanian estuary was assessed using the nine groups of estuaries identified by physical criteria. Within each of the nine groups, estuaries were ranked by level of anthropogenic disturbance using human population density data, and the estuary with least disturbance assigned the highest conservation rank (Class A). Each of these Class A estuaries was therefore the least disturbed estuary of a particular type, and between them they spanned nearly all of the biological and habitat diversity found within estuaries in the state. Where more than one estuary within a group was found to be 'pristine', the estuary with highest conservation status was identified using data on the percentage of catchment area included within national parks and crown reserves, and data on size of estuary. In addition to the nine representative estuaries found to possess highest conservation value, North East Inlet was also assigned Class A conservation status because it possessed high species diversity and included species not contained in other Class A estuaries. The ten class A estuaries are North East Inlet, Black River estuary, Bryans Lagoon, New River Lagoon, Thirsty Lagoon, Tamar River estuary, Southport Lagoon, Bathurst Harbour, Payne Bay and Wanderer River estuary. We recommend that plants, animals and habitats within the ten Class A estuaries and associated catchments be protected within an integrated system of Tasmanian estuarine protected areas. We also recommend that catchments and aquatic ecosystems of a further 38 estuaries, which were assigned Class B conservation status on the basis of minimal anthropogenic impacts, be quarantined from future developments, and existing impacts reduced wherever possible.