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


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Davies, PE ORCID: 0000-0003-2651-4061, Cook, LSJ and Sloane, T 2006 , Orielton Lagoon: Changes in the benthic macroinvertebrate community between 1999 and 2005.

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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)
Authors/Creators:Davies, PE and Cook, LSJ and Sloane, T
Keywords: Orielton Lagoon, estuarine, freshwater, Tasmania, biota, monitoring, macroinvertebrate
Publisher: Freshwater Systems
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