Shading may alter the colonization pattern and dominance between two invasive submerged aquatic plant species Champika Ellawala Kankanamge . Fleur E. Matheson . Tenna Riis Received: 7 January 2020 / Accepted: 8 April 2020 Ó Springer Nature B.V. 2020 Abstract Egeria densa and Elodea canadensis are two common invasive submerged macrophytes in streams and rivers worldwide. We conducted a six- week growth experiment with monocultures of E. densa and E. canadensis and mixed communities of the two species under five different shade levels (35%, 63%, 79%, 90% and 95%). Our aim was to test the effect of shade and competition on colonization of vegetative propagules of these species in flowing waters. We found that biomass accrual in both species was greatly reduced at high shade (C 79%) and that E. densa individuals accrued more biomass than E. canadensis individuals at all shade levels. At the least shade level (35%), interspecific competition with E. densa stimulated biomass accrual of E. canadensis individuals, compared to those within an E. canaden- sis monoculture. At higher shade (C 63%), this facilitation response disappeared, although E. canadensis plants continued to have longer stem length and a greater degree of branching than E. densa plants at B 79% shade. At the highest shade level, competition with E. canadensis reduced biomass accrual of E. densa. Hence, shade influenced the competitive interactions between these two species. At low shade, the competitiveness of E. canadensis was enhanced, but in the field E. densa appears to be the dominant species in streams, especially those with higher levels of shade and less frequent disturbance by flushing flow events. Keywords Submerged macrophytes Á Monoculture communities Á Mixed communities Á Stream shade Á Dominance Introduction Macrophyte beds affect stream ecosystem structure by modifying flow velocity, substrate conditions, stream depth and cross-sectional area (Sand-Jensen 2002; Champion and Tanner 2000). They also play a critical role in the functioning of stream ecosystems as they influence primary production (Alnoe et al. 2015), Handling Editor: Te´lesphore Sime-Ngando C. Ellawala Kankanamge (&) Á F. E. Matheson National Institute of Water and Atmospheric Research, Gate 10 Silverdale Road, Hamilton 3216, New Zealand e-mail: ellawala@cee.ruh.ac.lk C. Ellawala Kankanamge Department of Civil and Environmental Engineering, Faculty of Engineering, University of Ruhuna, Hapugala, Sri Lanka T. Riis Department of Bioscience, Aarhus University, Ole Worms Alle 1, 8000 Aarhus C, Denmark T. Riis WATEC, Aarhus University Center for Water Technology, Ny Munkegade 120, 8000 Aarhus C, Denmark 123 Aquat Ecol https://doi.org/10.1007/s10452-020-09770-0