Phytoplankton dynamics in relation to connectivity, flow dynamics and resource availability—the case of a large, lowland river, the Hungarian Tisza

Large lowland rivers with sufficient hydrological storage capacity are capable of supporting primary production, but the dynamics of the advecting phytoplankton is poorly understood. Our study aimed at exploring how longitudinal versus lateral connectivity, flow dynamics versus resource availability and continuous versus discontinuous environmental gradients shaped the species composition of phytoplankton. Samples were taken from February to October 2000 along the Hungarian Tisza River (HTR) and in its main tributaries. Longitudinal and seasonal patterns were related to resources (light and nutrients) availability and flow dynamics derived from a 1D hydrodynamic model. The HTR was autotrophic during the study period, but tributary input considerably exceeded net autochtonous production. The Szamos River was the major source of both phytoplankton and nutrients in the HTR. Chryso- and euglenophytes were flushed into the main river from floodplain oxbows during high discharge. Imported algae experienced discontinuity in environmental gradients when entering the main river. The merged impact areas of two dams (IAD) that separate the two large meandering patches of the HTR disrupted the longitudinal profiles of both physico-chemical variables and attributes of algal assemblages (biomass, species composition, richness, similarity between adjacent sampling sites). Hydraulic storage along the IAD selectively favoured the recruitment of cryptophytes that, however, could not compensate for the enhanced sedimentation of diatoms in terms of biomass. Although the meandering patches presented several small-scale differences in major environmental gradients, both patches supported the growth of planktonic diatoms. Changes in algal biomass were decoupled from nutrient availability. We conclude that various measures must be applied in various lowland rivers within the same catchment to control their trophic status as a component of the ‘good ecological status’ defined in the Water Framework Directive.