Sampling Methodology

This procedure has been taken from the report by Davis et al. (1990) "Towards more Effective Control of Nuisance Chironomids (Midges) in Metropolitan Wetlands, Perth, Western Australia" and the "Wetland Bioassesment Manual (Macroinvertebrates)" by Davis et al. (1999).

Equipment required for midge collection:

• Corer (100mm) - made from stainless steel
  (click here for diagram of corer construction)
• Waders
• Plastic Bags
• Elastic bands
• Waterproof paper

  	Corer in use
  	Core sample

Core sample

The corer is used to measure down approximately 600mm from the top of the sediment to the water level. Using the corer, with 100mm diameter, take a sediment sample, remembering to cover the hole in the top of the corer when removing it from the lake bed.

This ensures that the entire sample is retained within the corer as it is removed from the lake.

Place the core sample into a plastic bag along with a water proof paper tag with the wetland name, location number and date written on it in pencil (pen tends to smudge).

Fasten the top of the bag securely with an elastic band Make sure to record your sampling sites on a map so that the same sites can be sampled each time.

This allows any problem areas to be identified within the wetland.

Separating the midges from the mud sample.

Equipment required for separating the midge larvae from the mud sample

NOTE: Gloves should be worn whenever handling calcium chloride as it can be very irritating to the skin.

Calculations

Statistics are an important tool which allow a better understanding of the data collected. For the purpose of demonstrating the recommended statistics, a hypothetical wetland has been devised - "Lake Banshee".

Lake Banshee

Number of Samples (n) = 10
Total Number of Midge Larvae (T) = 277

Firstly, calculate the average number of midge larvae per core sample (X)

X	=	T/n
Average	=	Total Number of Midge Larvae/The number of samples
	=	277/10
	=	27.7

It is also recommended that the amount of variation between sampling locations be calculated. This is the standard error (SE).

* This function can be found on most scientific calculators

The average number of midges per core sample is 27.7 ± 10.21. This figure indicates that the "true" average number of midges per core sample varies between 17.49 to 37.91.

Now the number of midges per square meter can be calculated. This is done as follows:

Firstly, calculate the number of times your corer fits into a square meter (C)

Therefore, at Lake Banshee there were 3518 ± 1301 midges per metre²

The threshold for possible pesticide treatment is estimated to be 2000 larval midges per square metre. Once numbers exceed this level problems start to develop. This information is based on complaints received from residents of suburbs adjoining wetlands where larval densities have been regularly monitored. Many councils have found that treatment may be left until larval midge numbers exceed 5000 midges per square metre with little or no resident complaints.

It should be noted that as midge larvae tend to inhabit only shallow water a shallow wetland will actually contain more larval midges than a deep lake with the same midges density per square metre.

Chironomid identification

In most situations simply determining the number of midges present in a wetland may be satisfactory. However, midge identification can lead to a better understanding of the processes occurring within the wetland. There are a number of keys that can be used for identification. "A Guide to Wetland Invertebrates of Southwestern Australia" (Davis & Christidis, 1997) published by the Western Australian Museum, provides a key and illustrations of the species commonly found in Perth wetlands.

Physico-Chemical Monitoring

The amount of physico-chemical monitoring undertaken as part of the monitoring program will be at least partially dependent on the budget allocated to the work. Ideally, a number of parameters should be monitored at each site sampled for midge larvae.

• pH
• Electrical Conductivity (EC)
• Dissolved Oxygen (DO)
• Phosphorus (P) - total phosphorus and filterable reactive P
• Nitrogen (N) - total nitrogen, nitrite/nitrate and ammonium
• Chlorophyll a

All equipment needed for physico-chemical monitoring can be obtained from most laboratory supply stores.

pH

pH should be measured in the field with a portable, hand held, pH meter. Samples cannot be stored to measure later as the pH will alter rapidly by biological activity and temperature.

Methodology

Ensure that the pH meter is calibrated and that the protective sheath (if it has one) is removed from the sensor. Switch on the unit and allow it to warm up for the period of time recommended by the manufacturer (usually about 15 minutes). Place the probe into the wetland, ensuring it is fully immersed and wait for the reading to stabilise. Record the reading and move on to the next site and repeat the process.

Electrical Conductivity (EC)

Electrical conductivity should also ideally be measured in the field though it is possible to take a water sample to analyse later.

Methodology

Similar to that described for pH.

Temperature

Temperature must be recorded at the wetland.

Methodology

Most pH meters will also measure temperature so utilise the pH meter and record the temperature at the same time as the pH.

Dissolved Oxygen (DO)

Again, DO should ideally be measured in the field as it is temperature dependent and disturbing the water may add more oxygen to it.

Methodology

Requires the use of a DO meter.

Phosphorus (P)

There are two main forms of P, which are routinely measured as part of wetland monitoring - Total and orthophosphate.

Total P - Represents the total amount of P present within a water sample in both organic and inorganic forms.
Orthophosphate - Represents the amount of inorganic P readily available in the water column for uptake by plants.

Methodology

Ideally, nutrients should be measured at each site however this can become costly and it may be advisable to use a bulked sample. To make a bulked sample take the same quantity of water from each of the ten sites sampled for midge larvae and pour them all together into a clean bucket. Mix the water well and then, for total phosphorus, place a small amount of water into a clean bottle (it is possible to obtain suitable bottles from laboratories who test for nutrients) labelled with the wetland name, site number (if applicable) date and nutrient to be analysed (total P). Then place the sample into the freezer to store until analysis. For orthophosphate filter a small quantity of water through a 0.45mm filter and pour it into a clean bottle. Label the bottle with the same information as for total phosphorus and freeze the sample until analysis. These samples can then be taken to an analytical laboratory for testing.

Nitrogen (N)

Nitrogen occurs in a number of forms in wetlands, the following are the types that are routinely monitored:

Total nitrogen - Refers to the amount of nitrogen present in a sample in both the organic and inorganic forms.
Ammonium - this is the main form of nitrogen produced by the breakdown of organic material and urea. Ammonium is one of the forms of N most commonly utilised by aquatic plants. This form of N is readily oxidised to nitrite and then to nitrate.
Nitrite/Nitrate - Nitrate is one of the forms of N most commonly utilised by aquatic plants.

Methodology

The sampling methodology for N is the same as for P. For total N, use unfiltered water and freeze the sample for later analysis. For ammonium and nitrite/nitrate filtered water is utilised and the samples are frozen. Remember to label the bottle clearly with either total N, nitrite/nitrate or ammonium, the wetland name, sample date and site number.

If there are constraints on the amount of nutrients that can be analysed then it may be necessary to analyse fewer forms of nutrients. In this case the recommended nutrients to test are total phosphorus and total nitrogen. Over time this will still give a good indication as to the nutrient status of the waterbody being tested.

Chlorophyll a

The amount of chlorophyll a in a waterbody is a good measure of phytoplankton productivity, which in turn provides an indication of the potential food resources available to larval midges.

Methodology

As soon as possible after collection filter a known quantity of water through a 47 micron glass microfibre filter. Ideally a litre should be filtered however this can rarely be achieved, usually 500 or 250ml is filtered. Use tweezers to carefully fold the filter paper in half and then in half again. Take another filter paper and fold it around the sample filter paper. Place the filter paper in an envelope with the date, wetlands name, site number and amount of water filtered. Place the envelope into a plastic clip-seal bag and freeze the sample in the dark until analysis. Chlorophyll a can be measured from a bulked sample if required.

Before commencing nutrient monitoring it is advisable to contact the laboratory that will be used for analysis to ensure that their standards for sample collection will be followed.

References
Davis, J.A., Pinder, A.M., Trayler, K.M. and Harrington, S.A. (1990). Towards More Effective Control of Nuisance Chironomids (Midges) in Metropolitan Wetlands, Perth, Western Australia. Unpublished Report for the Midge Research Steering Committee, Australia.
Davis, Dr. J., Horwitz, Dr. P., Norris, Dr. R., Chessman, Dr. B., McGuire, M., Sommer, B. and Trayler, Dr. K. (1999). Wetlands Bioassessment Manual (Macroinvertebrates). National Wetlands Research and Development Program, Australia.
Davis, J. and Christidis, F. (1997). A Guide to Wetland Invertebrates of Southwestern Australia. Western Australian Museum, Australia.