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Orielton Lagoon: Changes in the benthic macroinvertebrate community between 1999 and 2005

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Davies, PE and Cook, LSJ and Sloane, T (2006) Orielton Lagoon: Changes in the benthic macroinvertebrate community between 1999 and 2005. Project Report. Freshwater Systems, Hobart.

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Abstract

This report describes the results of a survey for benthic macroinvertebrates (BM) conducted in Orielton Lagoon in May 2005. This survey was designed as a repeat of a survey conducted in June 1999 (Davies and Cook 2004). These surveys were commissioned by Sorell Council and designed by Freshwater Systems to describe changes in BM assemblages that may be attributed to remediation works at the Lagoon. The works included modifications to the Sorell causeway and the associated drainage of Orielton Lagoon, and diversion of suburban wastewater from the Lagoon for land disposal/irrigation. These actions were aimed at increasing both the water and sediment quality of the Lagoon, with the specific aim of reducing nutrient loads, eutrophication and the incidence of algal blooms and associated odour problems. Orielton Lagoon is a shallow (average 1.3 m deep) lagoon about 265 ha in area, separated from Pittwater by a causeway originally constructed in 1868, and modified in 1906 and 1953. The causeway constricted tidal exchange and this, along with a large influx of nutrients from wastewater disposal from Midway Point, led to high internal nutrient loadings and high rates of algal production (Brett 1988, Kinhill 1993). Blue green algal blooms were a frequent occurrence in the mid to late 1990's, causing scums, odours and public health and aesthetic concerns. Culvert infrastructure at the causeway was modified in 1998 to enhance tidal exchange with Pittwater, flush water-column nutrients from the lagoon and stabilise salinities. This BM survey was conducted in two parts and was originally conceived as a BACI (before-after control impact) paired design with sites in Orielton Lagoon being compared with sites in a control embayment before and after the cessation of WWTP discharge to the lagoon. Natural differences in hydrodynamics and geomorphology between Orielton Lagoon and other nearby semi-estuarine or embayment sites limit their use as strict statistical controls, and they should be referred to as reference systems. Two reference embayments were selected, to improve inferential ability from the survey and to ensure representative sampling of marine embayments in the local area that were as similar as possible to Orielton Lagoon in its original hydrodynamic state, and that were experiencing low to moderate nutrient loads. The two embayments chosen were upper Pittwater, downstream of Samphire Island, and the Carlton River estuary. Sampling was conducted on soft silt-mud sediments of broadly similar size composition to those observed in Orielton lagoon. Results of both the pre- and post-mitigation sampling are described here, and compared with changes observed at the reference sites.

Item Type: Report (Project Report)
Keywords: Orielton Lagoon, estuarine, freshwater, Tasmania, biota, monitoring, macroinvertebrate
Publisher: Freshwater Systems
Date Deposited: 28 Nov 2006
Last Modified: 18 Nov 2014 03:12
URI: http://eprints.utas.edu.au/id/eprint/435
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