Calibration and Use of the Transparency Tube as an Estimate of Turbidity: An Analysis of Factors Controlling Light Attenuation in Estuarine Waters

Makenna Leigh Thompson, University of South Carolina

Abstract

The transparency tube is low-cost, portable, and easy to use to measure the transparency or clarity of the water. Filling the tube with water creates a small-scale water column where water will be slowly drained out of the bottom until the Secchi disk at the bottom of the tube is visible from the top opening, and the depth of the remaining water in the tube is recorded in centimeters. Unlike traditional Secchi disk measurements, the advantage of this instrument is that sampling depth in highly turbid waters is possible with a centimeter scale resolution. Transparency tube measurement, through careful calibrations, can then indirectly measure turbidity in nephelometric turbidity units (NTU). The current equations used to estimate turbidity (in NTU) from transparency tube measurements (in cm) are based on freshwater stream measurements. This study aims to create new conversion equations applicable to estimates of turbidity in estuarine waters in South Carolina by comparing paired transparency tube and turbidimeter measurements while taking into consideration the particulate and dissolved matter that may reduce water clarity. This study sampled 22 estuarine sites along the coast of South Carolina in various weather conditions and tidal stages throughout one year. Results suggest that there is a not a significant rate of change between equations, but rather a significant baseline of measurements implying there are inherent differences between the two environments that affect the transparency-turbidity relationship. This finding has significant implications for future water quality monitoring and management.