The impacts of river barriers on aquatic plants

What we found

There is a common misconception that macrophyte (aquatic plant) communities are sedentary. However, macrophytes frequently disperse both upstream and downstream in rivers, and this dispersal is vital for recolonization of vacant habitat patches, and to allow genetic exchange between populations. Downstream dispersal is mainly through a process called hydrochory: the dispersal of seeds and vegetative parts in river flows. The dominant mode of upstream dispersal is zoochory: where seeds and other reproductive parts are transported by birds and fish as they move around catchments.

The presence of barriers in our rivers can disrupt macrophyte dispersal. Large dams create reservoirs where the still water prevents hydrochory: seeds and other vegetative parts being carried by river currents drop to the bottom of the reservoir, starving areas downstream of new recruits.

Even relatively small barriers can disrupt the movement of fish, especially the weaker swimming members of fish communities that tend to transport macrophytes upstream.

How are plants dispersed? Illustration adapted from the original paper by Jones et al. (2020).

Why it is important

River barriers can alter macrophyte community composition and distributions. Heavily fragmented rivers tend to support less diverse macrophyte communities, and the modified habitats are more prone the establishment of invasive species. Apart from their contribution to river biodiversity, macrophytes play important functional roles in river ecosystems by providing habitat and food, altering flow and sediment dynamics, and regulating water chemistry. Hence, by disrupting macrophyte dispersal, barriers can have knock on effects on river ecosystem functioning.

How we did it

We evaluated the published studies on macrophyte dispersal dynamics and assessed the evidence for barrier impacts on dispersal and community composition. This involved extensive searches for literature in Google Scholar and Web of Knowledge databases. There were many important existing studies looking at aquatic plant dispersal, and the effects of dams and reservoirs on macrophyte communities. However, these existed as disjointed studies, and by consolidating and critically assessing the evidence in a single paper, this review provides a more accessible and comprehensive evaluation of the problem.

What it means

The review has shown that river barriers can have damaging effects on aquatic plant communities, with potentially important consequences for river ecosystem function.

Our research highlighted the importance of looking beyond impacts on fish to fully understand the impact artificial barriers have on our rivers.

How do barriers stop plants dispersal? Illustration adapted from the original paper by Jones et al. (2020).

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FishBites is a science communication series by the FishBEE research group. The FishBEE team integrates the Centre for Sustainable Aquatic Research (CSAR), based at Swansea University, UK.

This post is based on the research paper authored by Dr Pete Jones and co-authors, entitled Impacts of artificial barriers on the connectivity and dispersal of vascular macrophytes in rivers-A critical review. Available here

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Jones, Peter & Consuegra, Sofia & Börger, Luca & Jones, Joshua & Garcia de Leaniz, Carlos. (2020). Jones et al (2020) Impacts of artificial barriers on the connectivity and dispersal of vascular macrophytes in rivers-A critical review. Freshwater Biology. 10.1111/fwb.13493.