Calibration of the transparency tube for estimating turbidity in estuarine systems

Turbidity, quantified in turbidity units (nephelometric or formazin), is a
common and valid measure of water quality related to transparency. A
transparency tube (TT) is an economical tool developed to estimate water
clarity as an alternative to the Secchi disk, but it is also frequently
used to estimate turbidity. Although the relationship between TT measures
and turbidity is well characterized for freshwater river and lake systems,
this relationship has not been tested for estuarine waters. The objective
of the current study was to empirically determine the TT-turbidity
relationship for estuarine waters and compare these results with the
traditional freshwater system conversions. We obtained 107 measurements of
TT depth, turbidity, total suspended solids, CDOM absorbance, Secchi
depth, and chlorophyll a at 22 estuarine locations in South
Carolina over a one-year period. Linear regressions provide conversion
equations that can be applied to estuarine waters. The TT-turbidity
relationship for estuaries was compared with freshwater systems. Our
results suggest that the slope of the relationship differs between systems
(-1.11 vs. -1.41), resulting in different turbidity estimates for TT
measurements for estuarine vs. inland waters. We propose a combined
conversion table incorporating estuarine and freshwater (riverine and
lacustrine) systems. A TT-turbidity conversion for estuarine waters
significantly benefits current coastal water quality programs and citizen
science groups by producing more accurate turbidity estimates for
screening and routine monitoring efforts.