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Untangling multiple pressure impacts in Swedish boreal streams

July 4, 2017
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A Swedish boreal stream. Image: mrdonb | Flickr Creative Commons

Freshwater ecosystems across the world are affected by multiple pressures acting in tandem, which can cause complex and unpredictable results for their health and diversity. As a result, the topic is a key focus for many aquatic scientists globally.

Human land use is an important driver of multiple pressures. Two common land-use pressures stem from agricultural activities and hydromorphological alterations. Agricultural activities can cause increased nutrient concentrations as a result of fertiliser run-off, cause sediment build-ups as a result of soil erosion, and alter ecosystem hydrology and connectivity through water abstraction.

Hydromorphological alterations are those which change the hydrology (i.e. water flows) and morphology (i.e. the shape, course and banks) of a river or lake. Hydromorphological alterations are commonly caused by the construction of flood defences, hydropower plants and drainage channels, and can significantly change the habitat quality and quantity available to aquatic organisms.

The multiple pressure impacts resulting from agricultural activity and hydromorphological alterations are the subject of a new study carried out on 77 streams in south-east Sweden. Writing in Ecological Indicators, scientists led by Richard K Johnson from the Swedish University of Agricultural Sciences in Uppsala quantified multiple pressure impacts on the insect life in these boreal streams.

Using species and trait composition techniques on ecological monitoring data sampled between 2008 and 2013, the researchers report that agricultural and hydromorphological pressures had significant joint effects on invertebrate assemblages in the studied streams. They found that variability in invertebrate species and trait composition could be explained by variations in agricultural and hydromorphological pressures.

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’.

As a result, the research team used multivariate regression, a data analysis method which allowed them to rank individual pressure impacts. They found that whilst agricultural pressures were the best predictors of both species and trait composition for the streams’ invertebrates, the negative impacts of hydromorphological pressures on their populations could be far more pronounced.

Disturbance of riparian habitats was a strong predictor of shifts in invertebrate species and trait composition. This finding has been reported by a number of other past studies (e.g. Gregory et al 1991 – pdf), as the removal of riparian zones often significantly alters stream food webs, shading and water temperature, which impact invertebrate populations.

Community responses were less pronounced at sites affected by both loss of riparian integrity and elevated nutrients, suggesting that negative hydromorphological pressure impacts were mitigated by moderate increases in nutrient concentrations from agricultural run-off.

In quantifying and ranking the impacts of both individual and combined multiple pressures, the study provides a valuable case study for water managers and policy makers seeking to design ecological restoration schemes. More broadly, it is another ‘building block’ in the global effort to gain knowledge on multiple pressure interactions and impacts.

Johnson RK et al (2017) Decomposing multiple pressure effects on invertebrate assemblages of boreal streams, Ecological Indicators, 77, 293-303

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