Lagrangian drift experiment on the Middle Daugava River (Latvia) during the filling phase of the spring floods

The 2nd real-time Lagrangian drift experiment within the river-floodplain system of the Middle Daugava (South-East Latvia) was conducted on April 8, 2010, by applying a drifting sampling platform (a maritime life-raft) and a Lagrangian sampling strategy. Repeated measurements of the water temperature, pH, oxidation-reduction potential, conductivity, turbidity, dissolved oxygen concentration and the chlorophyll-a concentration were performed at a depth of 0.5 m with a 30 min frequency. In addition, a subsurface sampling of the zooplankton was performed hourly by applying standard methods. Repeated measurements of the Secchi depth, the drift speed and the river depth were also performed in-situ. The drift was started at 08:00, 10 km upstream from Daugavpils and finished at 19:00, 62 km further downstream. During the drift, 23 series of in-situ measurements were performed and 11 zooplankton samples were collected. The Spearman's rank correlation and the Multivariate and Partial Mann-Kendall tests were applied to the selected variables to identify statistically significant downstream trends as well as their correlation with the drift speed and river depth. Significant downstream trends were observed for the drift speed, the Secchi depth, turbidity, temperature, pH, conductivity and the dissolved oxygen. Most trends were different from those recorded during the drainage phase of floods (Gruberts et al. 2012). A possible impact of the drift speed on the chlorophyll-a concentration was detected. Downstream trends of other physical, chemical and biological parameters were independent from in-channel hydrological conditions. At 28.5 km downstream from Krauja, an abrupt shift in the Secchi depth indicated lateral interaction between the main channel and the adjacent floodplain area, which resulted in enrichment of the water column with suspended material and rotifers like Synchaeta sp. and Filinia longiseta.