Background information and definitions
Large debris (e.g. building materials, tyres) and accumulations of small debris (e.g. plant litter, wrack, plastics) can damage marsh vegetation. Debris can shade and crush vegetation. Movement of debris with water flows or tides can increase the area impacted beyond the footprint of the debris itself. Debris could also provide a refuge for animals that might damage vegetation, or release toxic chemicals (Uhrin & Schellinger 2011). Debris might be deposited directly onto marshes by humans, or carried there by wind and water (e.g. storms and tsunamis; Valiela et al. 1998).
Related actions: Put up signs to discourage littering.
Uhrin A.V. & Schellinger J. (2011) Marine debris impacts to a tidal fringing-marsh in North Carolina. Marine Pollution Bulletin, 62, 2605–2610.
Valiela I., Peckol P., D’Avanzo C., Kremer J., Hersh D., Foreman K., Lajtha L., Seely B., Geyer W.R., Isaji R. & Crawford R. (1998) Ecological effects of major storms on coastal watersheds and coastal waters: Hurricane Bob on Cape Cod. Journal of Coastal Research, 14, 218–238.
Supporting evidence from individual studies
A replicated, before-and-after, site comparison study in 2008–2009 in a coastal salt marsh in North Carolina, USA (Uhrin & Schellinger 2011) found that following removal of crab pots and tyres from the marsh, smooth cordgrass Spartina alterniflora height and density typically recovered to undisturbed levels within a year. Immediately before intervention, impacted patches contained fewer cordgrass stems (120–133 stems/m2) than undisturbed marsh (300–370 stems/m2). There was a similar trend (not statistically significant) for the maximum height of cordgrass (impacted: 34–62 cm; undisturbed: 72–80 cm). Patches cleared of crab pots consistently had similar cordgrass height to undisturbed marsh from 22 weeks after clearance (cleared: 25–81 cm; undisturbed: 30–87 cm) and statistically similar cordgrass density to undisturbed marsh from 42 weeks (cleared: 167–229 stems/m2; undisturbed: 302–414 stems/m2). Patches cleared of tyres consistently had similar cordgrass height to undisturbed marsh from 45 weeks (cleared: 50–85 cm; undisturbed: 53–87 cm). However, cordgrass densities remained significantly lower in cleared than undisturbed marsh over the whole study year (33 of 35 comparisons; cleared: 84–273 stems/m2; undisturbed: 287–538 stems/m2). Methods: In August or September 2008, seven wire crab pots and seven vehicle tyres were removed from a salt marsh. Vegetation was surveyed under the debris before removal, then regularly after removal until September 2009. In each survey, live cordgrass stems were counted and measured in 1–2 quadrats covering the footprint of each pot or tyre (including stems growing through the centre of tyres) and seven quadrats in undisturbed patches of the marsh.Study and other actions tested
A replicated, paired, site comparison study in 2013–2015 in a coastal salt marsh in New York State, USA (Ehl et al. 2017) found that patches cleared of stranded wooden debris developed similar vegetation cover to undisturbed marsh within two growing seasons. Immediately after removing debris, the cleared patches contained less vegetation (density: <5 shoots/m2; cover: 1–2%) than adjacent undisturbed marsh (density: 290 shoots/m2; cover: 96%). After one growing season, overall vegetation abundance had increased in the cleared patches (density: 98–114 shoots/m2; cover: 28–33%) but was still lower than in undisturbed marsh (density: 292 shoots/m2; cover: 96%). The same was generally true for the abundance of the two dominant species, smooth cordgrass Spartina alterniflora and saltgrass Distichlis spicata, although saltgrass cover was statistically similar in cleared patches and undisturbed marsh (see original paper for data). After two growing seasons, vegetation cover was statistically similar in the cleared patches and undisturbed marsh (density not recorded). This was true for overall cover (cleared: 69–74%; undisturbed: 79%), smooth cordgrass (cleared: 42–45%; undisturbed: 51%) and saltgrass (cleared: 23–25%; undisturbed: 21%). Methods: Five chunks of wooden storm debris (1–4 m2) were cleared from a salt marsh. Initial clearance took place in October 2013, although part of each patch was re-covered over winter then permanently cleared in March 2014. Between October 2013 (after initial clearance) and August 2015, vegetation was surveyed within each cleared patch and its adjacent undisturbed marsh.Study and other actions tested