Influence of an ecosystem engineer, the emergent macrophyte Sparganium erectum, on seed trapping in lowland rivers and consequences for landform colonisation

Abstract

Summary 1. Plant physical ecosystem engineers can influence vegetation population and community dynamics by modifying, maintaining or creating habitats. They may also have the potential to act upon biotic processes, such as seed dispersal. 2. Examples exist of reduction in seed dispersal distances in vegetated compared to unvegetated terrestrial environments, and concentration of seed deposits associated with plant patches. Such effects in aquatic environments have been little studied, but the engineering effect of plant patches on patterns of flow velocity and sediment deposition in streams suggests that they may play a similar role. 3. In this study, we assess the potential of an emergent aquatic species, Sparganium erectum, to play a role in physically modifying river habitats and trapping seeds by examining patterns of seed deposition and substrate type in 47 river reaches across England and southern Scotland, U.K. 4. Areas of the river channel within or adjacent to S. erectum patches harboured more plant seeds and more species than unvegetated areas and had finer, sandier substrates with higher organic matter, total nitrogen and total phosphorus content. Most seed species were competitive, indicating that they were well suited to colonise the competitive environment of an S. erectum patch, and could potentially further stabilise accumulated sediments and contribute to landform development. 5. We demonstrate that S. erectum patches influence both the physical environment and the retention of seeds, in consistent patterns across the channel bed, for a range of lowland rivers that vary in stream power and geology and which can be expected to vary in levels of supply of fine sediment and seeds. 6. Our findings support the hypothesis that the fundamental influence of a riverine ecosystem‐engineering species on slowing fluid flow links the habitat creation process of sediment sorting and retention to seed trapping. We suggest the process is applicable to a wide range of aquatic and riparian vegetation. We also suggest that the mono‐specific and competitive growth, which is typical of these engineering species, will strongly influence the recruitment of trapped seeds.

Influence of emergent vegetation on nitrate cycling in sediments of a groundwater-fed river

“…This difference in DOC may have been due to root exudates and decomposition of dead plant material contributed by the emergent vegetation. Moreover, emergent and submerged macrophytes in river beds have also been found more effective than bare sediment patches in trapping fine sediments from fluvial sources ) and exogenous particulate organic matter (Sand-Jensen 1998;Trimmer et al 2009;O'Hare et al 2012). The trapped fine sediments with particulate organic matter could have been an additional source of the relatively higher DOC delivery into the underlying pore water through surface water down welling during high flow events (Gu et al 2008;Byrne et al 2013).…”

“…The trapped fine sediments with particulate organic matter could have been an additional source of the relatively higher DOC delivery into the underlying pore water through surface water down welling during high flow events (Gu et al 2008;Byrne et al 2013). S. erectum patches across 47 river reaches in the UK contained 1.8 times higher organic matter than bare river bed sediments (O'Hare et al 2012), which indicates higher potential for dissolved oxygen consumption through microbial respiration (Sobczak and Findlay 2002;Trimmer et al 2009) and release of DOC to serve as an electron donor during NO 3 -reduction (Forshay and Dodson 2011).…”

“…The sediments in the experimental reach of the river were gravelly alluvium consisting of pebbles, gravels and sands (20-50 cm deep) overlying unconsolidated Penrith sandstone . Emergent vegetation in the riverbed grows in both the main channel and along the channel margins (at the interface of riparian and fluvial riverine eco-tones) and is dominated by Bur reed (S. erectum L) (O'Hare et al 2012). The river meanders through a narrow floodplain and the catchment is predominantly agricultural with temporary grassland and some arable land use.…”

“…Among vegetation types growing in riverbeds, emergent macrophytes dominated by Bur Reed (Sparganium erectum) is the most common species in UK rivers (O'Hare et al 2010(O'Hare et al , 2012. O'Hare et al (2010) reported the presence of this species in 47 rivers in a region spanning from southern Scotland to southern England.…”

“…O'Hare et al (2010) reported the presence of this species in 47 rivers in a region spanning from southern Scotland to southern England. While the impact of this species on river flow modulation, Manning's roughness co-efficient and trapping of fluvial sediments has been evaluated (Clarke 2002;Gurnell et al 2006;O'Hare et al 2012), to our knowledge, its influence on sediment nutrient attenuation, particularly NO 3 -, has not been assessed.…”