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domingo, 4 de junio de 2017

Leaf litter decomposition is a good tool to assess the effects of acidification on stream health


New research on functional stream ecology by Young AIL researchers

  • Leaves are a key source of energy for small forest streams
  • Small bugs (invertebrates) and microbes (bacteria and fungi) feed on these leaves, being key to transfer leaf energy to bigger animals such as fishes or birds
  • The rate at which bugs and microbes decompose leaf litter (litter decomposition) is tested as a measure of river health in response to human-caused stream acidification
  • Litter decomposition was strongly reduced in acidified streams
  • The magnitude of the reduction depends on river geology and the presence of bugs
  • Litter decomposition can be a useful tool to assess stream health

Text in Spanish and Portuguese

The burning of fossil fuels (in the industry or cars), the use of fertilizers and cattle production release large amounts of sulfur dioxide, nitrogen oxides and ammonia into the atmosphere. There, these compounds originate acid substances that can return to the earth’s surface as acid rain or dry acidic deposition (atmospheric acid deposition). Atmospheric acid deposition affects large areas worldwide, and is a transboundary problem as acid-forming substances released into the atmosphere can be transported over long distances and affect areas far away from where they originated. Atmospheric acid deposition poses more problems in acid-sensitive regions, which are regions with low buffering capacity, as those located at high elevation, where rainfall is high and soil thickness is low, and with weathering-resistant bedrock (for example, siliceous rocks can’t counteract acidity). In these regions, acid deposition leads to decreases in the concentrations of base cations (e.g., calcium, magnesium, which are essential elements for life), increases in the concentration of sulfur and nitrogen, and liberation of aluminum in the soil. This all translates into acidic water bodies, which may cause severe damage to freshwater organisms and to the benefits we get from them (such as clean water or angling).
The assessment of stream health is generally based on the presence of sensitive aquatic organisms. However, these indicators don’t necessarily inform about the capacity of streams to function and provide benefits. Therefore, it is crucial to understand how stream functions respond to acidification to create useful indicators of stream health.
Leaf litter decomposition is a fundamental ecosystem function in small forest streams. The rate at which litter decomposes depends on litter characteristics. Soft litter with high nutrient concentration decomposes faster than more recalcitrant litter, since microbial colonization is faster and microbial activities are higher. Leaf litter decomposition is also faster in the presence of bugs. Stream acidification generally slows down litter decomposition rates, as a result from inhibited microbial activity and disappearance of acid-sensitive shredders (bugs that feed on leaves). Leaf litter decomposition is thus particularly interesting as an assessment tool of acidification effects on stream functions since it is a key stream process, which has been widely studied and whose response to acidification can be predicted a priori.
Recently, a study carried out by Verónica Ferreira from the University of Coimbra (Portugal) and François Guérold from the University of Lorraine (France), and published in Ecological Indicators, assessed the potential for leaf litter decomposition to be used as an assessment tool to detect acidification effects on stream functions by means of a field experiment. The authors individually enclosed three leaf types differing in their suitability for bugs and microbes (alder Alnus glutinosa, sycamore Acer pseudoplatanus, and beech Fagus sylvatica) in fine mesh (where bugs cannot enter) and coarse mesh bags (both bugs and microbes can enter). The litter bags were incubated along an acidification gradient in two areas differing in geology in the Vosges Mountains, north-eastern France (Figure 1). This allowed the authors to assess if the response of litter decomposition to acidification depends on leaf type, mesh size, geology and acidification level. Also, to assess if the effects of acidification are consistent across studies, authors performed a meta-analysis, which combined the results of 17 empirical studies comparing reference and acidified streams.

Figure 1. Circumneutral stream flowing through granite bedrock in autumn (A) and an acidic stream flowing through sandstone bedrock in spring (B). Photos: François Guérold.
The authors of this study showed that acidification reduced litter decomposition in their field study by an average of 64% (Figure 2A). This reduction was quite similar to the overall result observed in the meta-analysis (63%, Figure 2B). The decline in decomposition rate was proportional to the acidification degree.
This study found that the effect of acidification was similarly strong for the three types of leaves, which suggests that decomposers are inhibited to a similar degree in leaves with distinct characteristics. On the other hand, authors found that decomposition resulting from bugs was more severely reduced by acidification than microbial-driven decomposition, although effects were strong for both mesh sizes. This probably occurred because most leaf-decomposing bugs are acid-sensitive. Finally, authors also showed that acidification caused a greater reduction in litter decomposition on streams flowing through sandstone rocks compared to those draining granite bedrocks, although the reduction of leaf decomposition by acidification was strong for both geological areas.
The effects of acidification on litter decomposition were overall strong and robust to leaf type, mesh size and geology. This supports the proposal of using litter decomposition as an indicator of stream health.

Figure 2. Inhibition (%, ±95confidence limit) of leaf litter decomposition in acidified streams, overall and as a function of leaf type, mesh size and geology in the field study (A) and in the published studies (B); significant inhibition is indicated by black symbols. Within each category, levels with the same letter do not significantly differ.
Ferreira V. & Guérold F. 2017. Leaf litter decomposition as a bioassessment tool of acidification effects in streams: evidence from a field study and meta-analysis. Ecological Indicators 79: 382–390, doi: 10.1016/j.ecolind.2017.04.044

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