Effect of nutrients on the sporulation and diversity of aquatic hyphomycetes on submerged substrata in a Mediterranean stream

Eutrophication in streams often results in enhanced decomposition rates and chemical conditions that greatly affect the suitability of the habitat for many species. We performed an enrichment experiment in a 50-m reach of a Mediterranean stream by the continuous addition of N and P (enriched reach) while another consecutive reach upstream served as the reference (unenriched reach). After 1.5 years of enrichment, conidia production and species composition of aquatic hyphomycetes were measured in communities colonizing four streambed substrata (Platanus acerifolia and Populus nigra leaves, epipsammic and epilithic biofilms). Samples collected from the two reaches were subjected to two nutrient treatments in the laboratory [low nutrient (−NP) and high nutrient (+NP) concentrations] to test for short-term (48 h) nutrient effects. Long-term nutrient additions did not affect potential sporulation rates while short-term nutrient enrichment produced insignificant increase in these rates on most substrata collected in the unenriched reach. Major differences in potential sporulation rates were observed between substrata, the highest rate being for leaves (especially Platanus acerifolia). However, enrichment produced significant changes in fungal community composition. Clavariopsis aquatica, Alatospora acuminata and Lemonniera sp. were dominant in the enriched reach while Heliscella stellata, Trisceloporus acuminatus and Clavatospora longibrachiata were characteristic of the unenriched reach. Our results suggest that fungal N demands can be fulfilled at relatively low levels of dissolved nitrate and further increases in the nutrients availability may not result in enhanced fungal activity.