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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