Morphological stressors

Morphological stressors are caused by alterations to the channel, bed, riparian area and / or shoreline of a water body. Numerous morphological stressors often act together, primarily causing a change in ecosystem habitat.

Examples of pressures causing morphological stressors include:

• the construction of dams, locks and weirs, reservoirs, flood walls, barriers and dikes,
• the construction of hydropower projects,
• the underground burying of water bodies (often into urban sewer networks),
• urban and/or agricultural development on flood plains and riparian zones,
• the digging of trenches and canals for agricultural and forestry land drainage,
• dredging for gravel, sand and minerals, often for construction or in the creation of navigable waterways,
• land reclamation and dredging for coastal construction and beach development,
• the spread of invasive species such as zebra mussels whose presence can alter the physical characteristics of a water body’s bed and channel.

Effects on ecological status of aquatic ecosystems

Morphological stressors result from physical alterations of a water body, which can significantly alter the ecological status of aquatic ecosystems, and the ecosystem services they can provide.

Loss of natural habitat - alterations to water body channels, river beds and riparian zones are likely to alter the availability and (most commonly) the diversity of habitat for a range of aquatic and riparian biodiversity. Loss of habitats such as submerged boulders and woody debris, reed beds, riparian vegetation may result in reductions in ecosystem status. Such changes can be detected using bioindicators such as macroinvertebrate (aquatic insects), macrophyte (aquatic plants) or fish richness as well as riparian vegetation and ground beetles. 

Fine sediment dynamics - excessive deposition of fine sediments within a river channel as a result of morphological change (e.g. channel over-widening or stagnation) can lead to the burial and subsequent loss of suitable gravel beds for fish spawning and macroinvertebrate reproduction, and cause increased water turbidity. These changes may be detected in alterations to species richness, fish spawning success and resulting recreation service provision to anglers.

Reductions in connectivity - barriers such as locks, weirs, dams and hydropower projects fragment ecological flows of water, species, nutrients and sediments throughout a river basin. Such stressor impacts may be detected through changes in ecological communities of fish and macroinvertebrates upstream of barriers, or by changes in water quality downstream. Similarly, lateral connectivity between a water body and its riparian zone and flood plain may be fragmented through morphological alterations such as flood walls walls (Baattrud-Pedersen et al. 2015). The effects of such stressors may be detected by alterations to species populations which inhabit such liminal ecosystems, such as dragonflies and amphibians.

Common combinations as multiple stressors

Multiple stressors combinations involving morphological stressors are most commonly found in transitional waters, with nutrient, toxin (or chemical) and hydrological stressors the most frequent pairings. In rivers and lakes, morphological stressors are frequently observed alongside nutrient and hydrological stressors, and are a key determinant of the amount of time nutrients and sediments are present in an ecosystem. The interplay of nutrients and morphology stressors often boost eutrophication by increasing light (through the removal of vegetation shading) and nutrient availability (through the reduction of natural pollutant filtration and buffering in agricultural settings). 

The common pairing with nutrient stressors is partly influenced by the resulting hydrological stressors often caused by morphological stress (e.g. changes in water flow caused by dam construction), as nutrient and hydrology stressors frequently interact (Nõges et al 2015). Baattrud-Pedersen et al. (2016) recommend a trait-based approach for macrophytes to disentangle the impact of such combinations.

Examples of ecological impact

The European FP7 REFORM project focused on the ecological impact of hydrological and morphological stressors to identify appropriate restoration measures. Belletti et al (2015) have reviewed assessment hydromorphological methods for rivers. The impact of the most common pressures have been summarised in conceptual schemes on morphological alteration, river fragmentation and hydrological regime alteration (Garcia de Jalon et al. 2013), while Wolter et al. (2013) synthesised the impact on macroinvertebrates, macrophytes and fish. Specific research focused on the impact of fine sediments on invertebrates (e.g. Murphy et al. 2017).

A study carried out on 77 streams in south-east Sweden found that variability in invertebrate species and trait composition could be explained by variations in agricultural and hydromorphological pressures (Johnson et al 2017). However, whilst changes in species composition were significantly related to agricultural impacts, the unique variance accounted for by hydromorphological variables was not significant for either species or traits. In short, it was difficult to disentangle the unique effects of agricultural and hydromorphological pressures from their multiple pressure ‘cocktail’ (Freshwater Blog 2017a).

A good ecological status in rivers is most often associated with the presence of natural areas in floodplains. Floodplains can be important buffers of pollution, and give ‘room for the river’ to follow natural flow dynamics. Urbanisation and nutrient pollution are important predictors of ecological degradation (Grizetti et al 2017). 

Disturbance of riparian habitats was a strong predictor of shifts in invertebrate species and trait composition in Bruno et al’s 2016 study of Mediterranean basin ecosystems. This finding has been reported by a number of studies, as the removal of riparian zones often significantly alters stream food webs, shading and water temperature, negatively impacting invertebrate populations. Agricultural intensification was the most influential stressor for riparian functionality, followed by natural droughts in the Bruno et al study. Hydrological regulation weakly affected functional indicators. 

There is increasing evidence that river hydromorphology often has a strong impact on the health and diversity of aquatic ecosystems (EEA 2012). However, projects which restore river hydromorphology often have limited effects on freshwater biodiversity. To investigate this phenomenon, scientists undertook experiments on rivers in ten regions across Northern Europe, as part of the EU REFORM project (Hering et al 2015). Overall, the study suggests that the ecological success of river restoration for aquatic organisms doesn’t depend necessarily on the length of restored river, but rather the quality and diversity of habitats on the river’s bed. However, for floodplain organisms, relatively small-scale restoration projects may yield significant positive effects (Freshwater Blog 2015).

Further reading

MARS publications:

Bruno D., Gutiérrez-Cánovas C., Sánchez-Fernández D., Velasco J. & C. Nilsson (2016). Impacts of environmental filters on functional redundancy in riparian vegetation. Journal of Applied Ecology. DOI: 10.1111/1365-2664.12619 (Read abstract)

Grizzetti B., Pistocchi A., Liquete C., Udias A., Bouraoui F. & W. van de Bund (2017).  Human pressures and ecological status of European rivers, Scientific Reports 7, 205. DOI:10.1038/s41598-017-00324-3 (Download article, 3.5mb)

Nõges P., Argillier C., Borja Á., Garmendia J.M., Hanganu J., Kodeš V., Pletterbauer F., Sagouis A., & S. Birk (2016). Quantified biotic and abiotic responses to multiple stress in freshwater, marine and ground waters. Science of the Total Environment, 540, 43-52. DOI: 10.1016/j.scitotenv.2015.06.045 (Read abstract)

Reports and publications:

Baattrup-Pedersen A., Andrews C., Belletti B., Campana D. Carlson P.E., Chapman D.S., Chormaski J., Comiti F., Garcia de Jalon D., Gonzalez del Tanago M., Gray A., Ives S. C., Johnson R.K., Kiczko A., Kjeldsen T.R., Kraml J., Laize C.L.R., Lebiedzinski K., Mader H., Maroto, Judit; Martinez-Fernandez, Vanesa; Mayr, Peter; McKie, Brendan G.,Okrusko T., Rinaldi M., Sandin L., Staras M., Vanbergen A.J., Woodcock B.A. & M.T. O'Hare (2015). Guidance on how to identify impacts of hydromorphological degradation on riparian ecosystems. Deliverable 3.4 of REFORM (REstoring rivers FOR effective catchment Management), a collaborative project (large-scale integrating project) funded by the European Commission within the 7th Framework Grant Agreement 282656. European Commission, 187pp. (External link to REFORM deliverable)

Baattrup-Pedersen A., Göthe E., Riis T. & M.T. O'Hare, (2016). Functional trait composition of aquatic plants can serve to disentangle multiple interacting stressors in lowland streams. Science of The Total Environment, 543, 230-238. DOI: 10.1016/j.scitotenv.2015.11.027 (Read abstract)

Belletti B., Rinaldi M., Buijse A.D., Gurnell A.M. & E. Mosselman (2015). A review of assessment methods for river hydromorphology. Environmental Earth Sciences, 73:2079–2100. DOI: 10.1007/s12665-014-3558-1 (Read abstract)

EEA (2012). European waters - assessment of status and pressures (Download report, 28mb)

Garcia de Jalon D., Alonso C., González del Tango M., Martinez V., Gurnell A., Lorenz S., Wolter C., Rinaldi M., Belletti B., Mosselman E., Hendriks D. & G. Geerling (2013).  Review on pressure effects on hydromorphological variables and ecologically relevant processes - Deliverable D1.2 of REFORM (REstoring rivers FOR effective catchment Management), a collaborative project (large-scale integrating project) funded by the European Commission within the 7th Framework Grant Agreement 282656. European Commission (External link to REFORM deliverable)

Hering D., Aroviita J., Baattrup-Pedersen A., Brabec K., Buijse T., Ecke F., Friberg N., Gielczewski M., Januschke K., Köhler J., Kupilas B., Lorenz A.W., Muhar S., Paillex A., Poppe M., Schmidt T., Schmutz S., Vermaat J., Verdonschot P. F. M., Verdonschot R.C. M., Wolter C. & Kail J. (2015). Contrasting the roles of section length and instream habitat enhancement for river restoration success: a field study of 20 European restoration projects. J Appl Ecol, 52: 1518–1527. doi:10.1111/1365-2664.12531 (Read abstract)

Johnson R.K., Angeler D.G., Hallstan S., Sandin L. & B.G. McKie, (2017). Decomposing multiple pressure effects on invertebrate assemblages of boreal streams, Ecological Indicators, 77, 293-303 (Read abstract)

Murphy J.F., Jones J.I., Arnold A., Duerdoth C.P., Pretty J.L., Naden P.S., Sear D.A. & A.L. Collins (2017). Can macroinvertebrate biological traits indicate fine-grained sediment conditions in streams? River Research and Applications. 1–12. DOI: 10.1002/rra.3194 (Read abstract)

Wolter C., Lorenz S. Scheunig S., Lehmann N., Schomaker C., Nastase A., Garcia de Jalon D., Marzin A., Lorenz A., Kraková  M., Brabec K. & R. Noble (2013). Review on ecological response to hydromorphological degradation and restoration. Deliverable D1.3 of REFORM (REstoring rivers FOR effective catchment Management), a collaborative project (large-scale integrating project) funded by the European Commission within the 7th Framework Grant Agreement 282656. European Commission (External link to REFORM deliverable)

Selected Freshwater Blogs:

Freshwater Blog (2017a). Untangling multiple pressure impacts in Swedish boreal streams (External website)

Freshwater Blog (2017b). Multiple pressures and the ecological status of European rivers (External website)

Freshwater Blog (2016). Functional redundancy and how river ecosystems respond to stress  (External website)

Freshwater Blog (2015). Habitat quality is more important than habitat length in river restoration projects (External website) 

Other websites:

REFORM (2015). REstoring rivers FOR effective catchment Management  (External website)