We assessed the eutrophication effects on leaf litter decomposition and primary production, and on
periphytic algae, fungi and invertebrates. According to the subsidy-stress model, we expected that when
algae and decomposers were nutrient limited, their activity and diversity would increase at moderate
levels of nutrient enrichment, but decrease at high levels of nutrients, because eutrophication would lead
to the presence of other stressors and overwhelm the subsidy effect. Chestnut leaves (Castanea sativa
Mill) were enclosed in mesh bags and immersed in five streams of the Ave River basin (northwest
Portugal) to assess leaf decomposition and colonization by invertebrates and fungi. In parallel, polyethylene
slides were attached to the mesh bags to allow colonization by algae and to assess primary
production. Communities of periphytic algae and decomposers discriminated the streams according to
the trophic state. Primary production decomposition and biodiversity were lower in streams at both ends
of the trophic gradient