A comparison of sampling methods for riverine zooplankton
Sluss, Tamara D.; Jack, Jeffrey D.; Thorp, James H.
Research of zooplankton in large rivers is more recent and less common than lentic and marine studies and over ten different sampling devices have been used to sample riverine zooplankton around the world to date. The objectives of this study were to (i) demonstrate the need for standardizing collection methods for riverine zooplankton; (ii) report the results of field tests on the relative effectiveness of three sampling devices (alpha bottle, Schindler trap, and manual bilge pump) in collecting copepods, cladocerans, and rotifers from a large river during low and high flow periods at two depths in the Ohio River; and (iii) discuss advantages and disadvantages of sampling devices and provide sampling suggestions based on this field study and experience sampling riverine zooplankton. Our results indicate that there is great variability in the densities of zooplankton captured by different sampling devices and variability between replicate measurements of the same sampling device. The alpha bottle and the pump collected significantly different densities of every taxa (excepting Bosmina) with significant differences due to sampling device. All other taxa showed no significant differences between devices indicating that the Schindler may be the least effective of the three devices. The Schindler captured less total zooplankton than the other devices and those differences were driven by the capture of fewer Bosmina and Keratella by the Schindler device. Additionally, during the three sampling events, there were only significant differences between samples taken from different depths for cyclopoid copepods indicating a relatively well-mixed sampling region both vertically and laterally, from shore to channel. Although this study was conducted in only one large river and across two sampling seasons, the differences due to sampling device makes it clear that conclusions drawn from cross-study comparisons when different sampling devices were used are precarious. It may be proper for river researchers to standardize collection methods while also becoming aware of the consequences of using different methods in various riverine habitats and during disparate seasonal and hydrologic conditions.