Measurement of Turbidity

Transcription

1 Measurement of Turbidity What is turbidity: Turbidity is a measure of water clarity and how much the material suspended in water decreases the passage from light through the water. Suspended materials include soil particles (clay, silt, sand), algae, plankton and microbes. All these particles are typical in the size range of mm (clay) to 1.0 mm (sand). Higher turbidity increases water temperatures because suspended particles absorb more heat. This reduces the concentration of dissolved oxygen because warm water can hold less O than cold. Higher turbidity also reduces the amount of light penetrating the water, which reduces photosynthesis and the production of Oxigen. Suspended materials can block fish gills, reducing resistance to disease, lowering growth rates and affecting egg and larval development. As the particles settle as sediment they can blanket stream bottom, smoother fish eggs and benthic macro-invertebrates. Source of turbidity include soil erosion, waste discharge, urban runoff, eroding stream banks, large numbers of bottom feeders (e.g. carp, which stir up bottom sediments) or excessive algal growth. Measurement and units: Units are nephelometric turbidity units (NTU) or formazin turbidity units (FTU) depending on the calibration standards used: (Source: University of Wisconsin, Water Action Volunteers program (2003)) Turbidity is generally measured using a turbidity meter (based on optical measurement compared to calibration standards: intensity of light (light source in the TM) scattered at an angle of 90 ). Another approach is to measure the transparency (measurement of light absorption and scattering) instead of turbidity. Water transparency can be measured using a Secchi disk or a transparency tube: 48

2 Measurement of turbidity in the ocean For measuring the turbidity from the boat, use the Secchi Disk (on board of PANGAEA). The measurements should be conducted at the same spots where the salinity and temperature measurements are taken. Measurement procedure (proposed by DAVIES-COLLEY and others): (reference: Davies-Colley, R.J. et al (1993): Colour and clarity of natural waters. Ellis Horwood) te: Correctly the size of the Secchi-Disk has to be adapted, because the size of the disk does affect its visibility. Using always the same disk size makes your own measurements comparable, but they may be not comparable to other measurements. 1. Lower the disk on the sunny side of the boat. There is still a discussion on whether the sunny or the shaded side is better. If there is too much glare, try it on a different place of the boat, but not in the deepest shadow (PANGAEA is big enough to find a decent spot) 2. Remove sunglasses 3. Allow sufficient time (preferably 2 min) when looking at the disk near its extinction point for the eyes to adapt completely to the prevailing luminance level. 4. Record the depth at which the disk disappears. Slowly raise the disk and record its depth of reappearance. The Secchi depth is the average of the depth of disappearance and reappearance. Repeat this procedure 3 x. 5. The readings should be made as near to mid-day as possible 6. The water depth should be at least 50% greater than the Secchi depth, so that the disk is viewed against the water background, not bottom-refected light. The Secchi-depth gives information about the clarity of the fresh water layer in the Fjords. As darker this layer is, as less sunlight penetrates into the underlying salt water layer. This is affecting the ecosystems in the Sounds Tasks: - Measure the Secchi Depth at the same position you conduct the salinity and temperature measurements. - Compare and discuss the results! - Discuss the influence of the clarity on the ecosystems of the coral reefs! 49

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