Quantifying the response of optical backscatter devices and transmissometers to variations in suspended particulate matter

Optical instruments have been used effectively in studies of sediment dynam ics for several decades. Without accurate instrument calibrations, calculat ed concentrations of suspended particulate matter (SPM) may be unreliable, with implications for interpretations of sedimentary processes and sediment fluxes. This review aims to quantify the effect of variations in SPM chara cteristics on the response of optical instruments (optical backscatter sens ors OBS and transmissometers) and to note the implications for users of the se instruments. A number of factors have a significant impact on instrument response, for example; a change in grain size from medium sands to fine si lts may lead to a x 100 increase in instrument response; flocculation of fi ne particles may decrease instrument response by x 2; and the presence of p lankton in suspension may lead to poor instrument calibrations of SPM conce ntration. Calibrations carried out in environments either with multi-modal bottom sediments, where flocculation of fine-grained sediments is likely, o r where the hydrodynamics or grain type are highly variable must also inclu de a determination of the changing nature of the suspended load in space an d time. A more complete understanding of instrument response to SPM and of calibration requirements may enable optical devices to be used to a greater potential as long-term measures of SPM concentration, and also enable impr ovements in calculations of net sediment fluxes. (C) 1999 Elsevier Science Ltd. All rights reserved.