DESCRIPTION AND ASSESSMENT OF THE NATURE CONSERVATION VALUES OF BENJE-BENJENUP LAKE AND ADJACENT BUSHLAND 1.0 INTRODUCTON Lake Benje-Benjenup and its adjacent bushland are located 17 km north north east of Esperance on the south east coast of WA. The lake is within unvested reserve no. 14563 of 149 ha gazetted in 1913 for ‘water’. The lake has maximum dimensions of 1.2 x 1.0 km and an area of approximately 90 ha. The adjacent bushland is to the east of the lake on Esperance location 650, which is privately owned by N & P Blumann and part of the farming property ‘Olimarena’. Approximately 85 ha of 130 ha of remnant bush is the subject of this assessment. ‘Benje-Benjenup’ is of aboriginal derivation, the meaning is unrecorded by Department of Land Information; the name was first used for the lake on a plan drafted by surveyor R Brazier in 1894. The regional setting of the lake is in Coramup Creek catchment, a sub-catchment of the Lake Warden system of southward flowing drainages and high value coastal wetlands including Lakes Warden, Woody and part of Lake Mullett which are RAMSAR wetlands. However, the lake appears to function as an internally draining system independent of the larger catchment; it is fed by brackish seepages along the shoreline and a saline creek of approximately 6 km length entering from the north west. Benje-Benjenup Lake is described as a ‘permanent saline lake’ in fair condition, requiring significant management intervention in the biodiversity audit of WA’s Biogeographical Regions (May & McKenzie 2002). Threatening processes are listed as agricultural land clearing, changed hydrology and pollution. The adjacent bushland has been with the present owners since 1994; in the 1960s it was ‘logged’ i.e. chained, burnt and ploughed in initial land clearing. It is presently of very good quality and is subject to a Land for Wildlife agreement with CALM. Land clearing in the Gibson area generally followed the establishment of the Esperance Downs Research Station in 1950 and by 1954 20 000 acres of land had been developed (Rintoul 1973). Clearing in the immediate vicinity of the lake would have been in the decade mid 1950s to mid 1960s (Penny Blumann, personal communication). Previous investigations of the lake and surrounds include Penny Blumann’s Hooded Plover counts for Birds Australia, waterbird assessments by Clarke & Lane (2003) and Buchanan (2003), Rosemary Jasper’s land for wildlife assessment in 2004. A visit by Coral Turley of Esperance Wildflower Society in 1995 listed 51 species of plant and a visit by Katie Syme in 2002 listed 17 species of fungi and an additional 46 flowering plant species. The purpose of the present investigation is to describe in more detail the wetland characteristics and values of the lake as well as the nature conservation values of the adjacent bushland. The impetus for this study is the recognition of the importance of the Lake Warden catchment in a regional and state context and the requirement to describe its components and functional processes in greater detail. 2.0 ASSESSMENT OF BENJE-BENJENUP LAKE AND ADJACENT BUSHLAND The present assessment was undertaken between 25-27 April 2006; the survey was undertaken largely on foot recording birds present, other vertebrates if encountered (Appendix 1), readily identifiable plant species including weeds and noting the characteristics of plant structure. Common names for plants are given if listed in Bennett (1991). Electrical conductivity (as a surrogate for salinity) was measured in the lake, at brackish water seepage sites and at the inflow creek. These and other water quality data in Table 1 were measured in situ with a WTW P4 Multiline instrument. Measurements were made at the water surface. Total nitrogen and total phosphorus were analysed by Centre for Excellence in Natural Resource Management in Albany. 1 All wetland investigations are substantially influenced by previous rainfall and Benje-Benjenup is no exception. Rainfall for Esperance airport (Gibson) for 2005 was above average with 593 mm (cf.long term mean 568 mm). There were 194 mm for January – April 2006 (cf. sum of mean monthly rainfall for this period of 123 mm). There were 24.5 mm rain overnight at ‘Olimarena’on 25 April 2006. 3.0 RESULTS 3.1 Description of Benje-Benjenup Lake, catchment and surrounding vegetation According to the classification in Australian Nature Conservation Agency (1996) the lake is a ‘permanent inland saline lake’. My estimate of the area of its catchment is approximately 4 500 ha, an elliptical area which extends north west to a small plateau between Esperance locations 626 and 1446. Approximately 90-95% of the Benje-Benjenup sub-catchment has been cleared. Although the water level varies considerably the lake has held water permanently since at least 1954 (Nils Blumann, personal communication). At low water levels the alga Dunalliella is present which imparts a bright pink colour to the lake. An extensive line of tree stumps within the lake bed of unknown age but apparently quite old (Plate 1) as well as a crescent shaped limestone ridge on the south side (Plate 2) probably indicate a previous shoreline inside the present one. It is unlikely that any change (expansion) to the shoreline is due to hydrological change caused by land clearing due to the small amount of time elapsed since clearing and the apparent age of the stumps. The present shoreline is confined by raised sand dunes, particularly well developed on the north west and south east sides. In places there are exposures of granitic and ironstone rocks along the shoreline. Any connection between groundwater and lake water has not been investigated. It would appear to be unlikely due to both the permanence of the lake water and its hypersalinity. Rather, the lake appears to function as an independent, perched evaporation basin. If this is the case, then for any given water level (volume) the salinity can be expected to gradually increase with time. Table 1. Water quality characteristics of Benje-Benjenup Lake on 27 April 2006 Depth near centre 1.1m Electrical conductivity of lake water (range) 180.7-182.0 mS/cm Electrical conductivity of inflow creek (flowing) 10.98 mS/cm Electrical conductivity of brackish seepage (range) 3.46-3.73 mS/cm Dissolved oxygen concentration of lake water 8.84 mg/l Ph of lake water 7.94 Turbidity of lake water 15 NTU Total nitrogen concentration of lake water 9.51 mg/l Total phosphorus concentration of lake water 0.57 mg/l Temperature of lake water 150C Note: 1the electrical conductivity (EC) of sea water is 52 mS/cm 2 the conversion from EC to mg/l TSS is x550 (George et al.1996) therefore the TSS equivalent of 180 mS/cm is 99 000 mg/l 3TSS is total soluble salts The vegetation surrounding the lake shows pronounced zonation and is determined in part by the height of the sand dunes. A typical transect from lake shore to paddock would commence with Samphire (Halosarcia sp.) with occasional Frankenia sp. on sandier soils. Low Saltwater Paperbark (Melaleuca cuticularis) to 3m are present with the former species as groundcover. In places there is regeneration of M. cuticularis 1.0-1.3 m (Plate 3). 2 As the soil gets deeper, Red-eyed Wattle (Acacia cyclops) and Rigid Wattle(A. cochlearis) to 3m are present as dense thicket. On lower dune slopes, Red Swamp Banksia (Banksia occidentalis) and Swamp Peppermint (Taxandria (formerly Agonis) linearifolia) to 4m are present with occasional Christmas tree (Nuytsia floribunda) to 5m. Showy Banksia (Banksia speciosa) to 4m are present on upper slopes and the dune crest. Understorey species associated with B. speciosa are Coastal Jugflower (Adenanthos cuneatus),Graceful Grass Tree Xanthorrhoea gracilis, Chittick (Lambertia inermis),Australian Bluebell Sollya heterophylla and Lasiopetelum rosarifolium. On more gravelly soil Lambertia inermis is present as a dominant species. Variations on this generalised description include: Vigorous regeneration of Banksia speciosa from a wildfire which burnt the west side of the lake in November 1993 (Plate 4). This fire burnt in extreme fire behaviour conditions of very low humidity, high wind speed and temperature (Nils Blumann, personal communication). Where the dunes are subdued there is less Banksia speciosa, it is partially replaced by Acacia cyclops and A. cochlearis, Melaleuca cuticularis as well as Taxandria linearifolia. On the south east side of the lake there is a platform with sandy soil between the littoral vegetation and the dune; different species present here include: Melaleuca brevifolia, Corky Honey-Myrtle (M. suberosa), Ashy Hakea (Hakea cinerea), Wedge-leaved Dryandra (Dryandra cuneata) with understorey of Scented Chaff Rush (Lepidobolus chaetocephalus) and Frankenia sp. The only weeds observed in the littoral vegetation were Fleabane (Conyza spp.) and Stinkwort (Dittrichia graveolens) and these were very occasional and restricted to the south side of the lake. 3.2 Description of the adjacent bushland The vegetation structure of the adjoining vegetation has been influenced by ‘logging’ (see above) during the 1960s as part of initial land clearing. The effect of this has been to reduce the vegetation to a structural uniformity particularly with respect to height and to lesser extent density. In spite of this the vegetation appears ‘natural’ and is in very good condition not showing any sign of degradation due to grazing, repeated fire or weed invasion. The vegetation is what is colloquially described as’sandplain- shrubland on deep, gutless sand’ (Plate 5). Dominant plants in the upper storey include Nuytsia floribunda, Banksia speciosa and Lambertia inermis to 5m. Medium height shrubs are Adenanthos cuneatus to 2m with lower shrubs Teasel Banksia (Banksia pulchella), Creeping Banksia (B. repens), Little Beaufortia (Beaufortia micrantha), Agonis spathulata, Jacksonia spinosa, Melaleuca striata, Curry Flower (Lysinema ciliatum) with the Grass tree Xanthorrhoea gracilis.