Extensive fish production systems in continental
areas are often created by damming headwater streams.
However, these lentic systems favour autochthonous organic
matter production. As headwater stream functioning is essentially
based on allochthonous organic matter (OM) supply, the
presence of barrage fishponds on headwater streams might
change the main food source for benthic communities. The
goal of this study was thus to identify the effects of barrage
fishponds on the functioning of headwater streams. To this
end, we compared leaf litter breakdown (a key ecosystem
function in headwater streams), their associated invertebrate
communities and fungal biomass at sites upstream and downstream
of five barrage fishponds in two dominant land use
systems (three in forested catchments and two in agricultural
catchments).We observed significant structural and functional
differences between headwater stream ecosystems in agricultural
catchments and those in forested catchments. Leaf litter
decay was more rapid in forest streams, with a moderate, but
not significant, increase in breakdown rate downstream from
the barrage fishponds. In agricultural catchments, the trend
was opposite with a 2-fold lower leaf litter breakdown rate
at downstream sites compared to upstream sites. Breakdown
rates observed at all sites were closely correlated with fungal
biomass and shredder biomass. No effect of barrage fishponds
were observed in this study concerning invertebrate community
structure or functional feeding groups especially in agricultural
landscapes. In forest streams, we observed a decrease
in organic pollution (OP)-intolerant taxa at downstream sites
that was correlated with an increase in OP-tolerant taxa. These
results highlighted that the influence of barrage fishponds on
headwater stream functioning is complex and land use dependent.
It is therefore necessary to clearly understand the various
mechanisms (competition for food resources, complementarities
between autochthonous and allochthonousOM) that control
ecosystem functioning in different contexts in order to
optimize barrage fishpond management.