Evaluation of laser in situ scattering instrument for measuring concentration of phytoplankton, purple sulfur bacteria, and suspended inorganic sediments in lakes

A laser in situ scattering and transmissometry (Lisst-100) probe has been u sed for estimating the particle-size distribution of phytopankton, purple p hotosynthetic sulphur bacteria (Chromatiaceae), and suspended inorganic sed iments in different lakes. Results from Lisst-100 have been compared to lab oratory measurements, such as those obtained by using a Galai laser size an alyzer (GL), an optical microscope (OM), and a flow cytometer (FC). Althoug h all of these instruments were shown to provide reliable values of the par ticle number concentration for the given populations, the Lisst-100 was the fastest and most reliable instrument because it did not require manipulati on of the samples-which is not the case of GL, OM and FC instruments -and a voided the tedious procedure of microscopic counts. The total particle volu me concentration results obtained with Lisst-100 differed from those obtain ed with GL for populations with large and porous aggregates, such as phytop lankton cells. The difference was attributed to the breakage of fragile alg al aggregates resulting from the measuring procedure used by GL. Although f or suspended sediment particles both instruments gave the same results for the total particle volume concentration, the particle-size distribution obt ained with GL was found always shifted to smaller diameters than with Lisst -100, probably because inorganic sediment particles present compact aggrega tes. When these aggregates break, they split into a high number of small pa rticles that contribute the same to the total volume concentration as the p revious aggregates. Finally, results of the total particle volume concentra tion with Lisst-100 were in accordance with those obtained with GL for the Chromatiaceae population, because cells remained in a dispersed phase. A go od correlation was found between the total particle volume concentration of Chromatiaceae measured with Lisst-100 and the concentration of bacteriochl orophyll a (BChl a), which is the parameter habitually used to estimate the concentration of Chromatiaceae. Therefore, Lisst-100 was found to be a rel iable instrument to estimate the Chromatiaceae concentration in aquatic eco systems.