Lake Manarup background

• Research of whether sediments are a major source of nutrients in Lake Manarup, Lake Powell, Torbay Inlet, and Marbellup Brook - Geoscience team April 05.
• Field trip by local researchers September 2004 collated a snapshot of data provided in this INFO SHEET.
• Lake Manarup is now on the monthly water test sites in the Torbay catchment done by Geoff Bastyan for the Department of Environment.

Lake Manarup is important to landholders, visitors, birds and other wildlife. The waterbody is also known locally as Manarup Lagoon or Red Hill Lake. The lake is shallow and currently acts as a compensation basin for the Torbay Inlet drainage system. Seven Mile Swamp (Ewart Swamp) and paddocks south of Lake Powell drain via North Creek Drain, via a siphon under the Marbelup high level drain, into Lake Manarup. Lake Manarup is now separated from Torbay Inlet by floodgates, which are closed most of the time. A flap valve on the outlet from Lake Manarup prevents salt water entering it from Torbay Inlet. The valve is sometimes held open to allow water from the Inlet to enter the Lake, rather than opening the Inlet sandbar at inopportune times related to management of the Torbay Inlet drainage system (Hodgekin and Clark 1990). Lake Manarup is almost totally surrounded by fringing vegetation in good condition and has no public access.

ARE SEDIMENTS A MAJOR SOURCE OF NUTRIENTS? We need to know more about the conditions which cause the release or storing of nutrients in sediments in the Torbay catchment waterbodies. Nutrients cause algal blooms which are a major concern to everyone The Geoscience team from Canberra are trying to find out: If sediments act as a 'sink' or store of nutrients in Lake Powell, Lake Manarup, Torbay Inlet and Marbellup Brook. If sediments are an important source of nutrients compared with other catchment sources. The total amount and type of nutrients in sediments. The rate at which sediments are released into surrounding water. What processes control the storage and release of nutrients. The Geoscience team are spending two weeks in the Albany area doing studies using benthic chambers in Torbay as well as Oyster Harbour and Princess Royal Harbour. They are keen to share their information with the community at the open day at Lake Manarup Saturday 9 April 2005.

RESULTS: LAKE MANARUP FIELD TRIP 04

Netting in the fringing vegetation of the lake showed a narrow range of macroinvertebrates (small water creatures) including numerous amphipods, ostracods, isopods, estuarine shrimp and one oligochaete. This is typical for estuarine environments. One species of hardihead was caught in the fish net trawling.

Local potato farm, Chris Westcott on the right, and PhD fish researcher at the Centre for Excellence in Natural Resource Management Albany, Paul Close, bringing in the catch!

Water quality

Water samples were taken from the lake and analysed at the Albany Nutrient Analysis Laboratory. These samples were analysed for total nitrogen, total phosphorus, soluble phosphorus, nitrate and ammonium. Phosphorus was not detectable. Total nitrogen was detected, at 560 & 700 ug/ l or 0.56 & 0.7 ppm most likely the product of nitrogen fixing algae and is now at a level which raised concerns. Lake Manarup is fresh with a pH of 8.95. Results for nitrate & ammonium are still to come.

A series of water samples were taken at Lake Manarup, in the Marbelup high level drain, at the North Creek drain at the Elleker-Grasmere Road and North Creek drain on Chris Westcott's property.

Table 1. Lake Manarup

Sample Site: Lake access near bridge	Depth (metres)
Water Quality Parameter	Surface
Dissolved Oxygen	10.8
Temperature	20.0
Salinity (%)	0.63%
pH	8.95
Conductivity (mS/cm)	11.2
Turbidity (NTU)	11

Vegetation assessment

The dense native fringing vegetation surrounding Lake Manarup is in excellent condition. The vegetation consists of several species of sedge and rushes as:

Juncus pallidus (Pale Rush). This rush fringes Lake Man- arup, growing in permanently and seasonally wet positions. Tolerates fresh to brackish water.

Juncus kraussii (Shore Rush, also know as Sea Rush). This rush fringes Lake Manarup, growing in a range of elevations: shallow water, permanently wet and seasonally wet profiles. Tolerates brackish to saline conditions.

Gahnia trifida (Coast Saw-sedge) (possible id) Found grow- ing at seasonally wet elevation on lake bank. This sedge has a wide range of tolerance from fresh to saline.

Baumea juncea (Bare Twig Sedge). This was noted grow- ing above the waterline, at a seasonally wet elevation. Tolerates fresh to brackish water.

Surface drainage into North Creek drain

Of particular interest and note is the direction of surface drainage from the areas under potato cultivation south of Lake Powell. Surface water on areas of potato cultivation south of Lake Powell run into several parallel drains flowing in a southwards direction away from Lake Powell, into North Creek Drain. It would appear that there is no surface water flow overland or via drains directly into Lake Powell. This is important when considering potential nutrient sources to Lake Powell, as the existence of these drains and direction of flow suggests this area, traditionally under cultivation for potatoes, is not a nutrient source for Lake Powell.

Sorting the bugs - Julie Pech Department of Environment, Paul Close CENRM, Naomi Arrowsmith Department of Environment and Dr Peter Davies CENRM counting macro-invertebrates.

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