• algal communities;
  • biofilm;
  • chlorophyll;
  • diatoms;
  • light;
  • Mediterranean streams;
  • nutrient enrichment;
  • riparian forest

Nutrient input in streams alters the density and species composition of attached algal communities in open systems. However, in forested streams, the light reaching the streambed (rather than the local nutrient levels) may limit the growth of these communities. A nutrient-enrichment experiment in a forested oligotrophic stream was performed to test the hypothesis that nutrient addition has only minor effects on the community composition of attached algae and cyanobacteria under light limitation. Moderate nutrient addition consisted of increasing basal phosphorus (P) concentrations 3-fold and basal nitrogen (N) concentrations 2-fold. Two upstream control reaches were compared to a downstream reach before and after nutrient addition. Nutrients were added continuously to the downstream reach for 1 year. Algal biofilms growing on ceramic tiles were sampled and identified for more than a year before nutrient addition to 12 months after. Diatoms were the most abundant taxonomic group in the three stream reaches. Nutrient enrichment caused significant variations in the composition of the diatom community. While some taxa showed significant decreases (e.g., Achnanthes minutissima, Gomphonema angustum), increases for other taxa (such as Rhoicosphenia abbreviata and Amphora ovalis) were detected in the enriched reach (for taxonomic authors, see Table 2). Epiphytic and adnate taxa of large size were enhanced, particularly during periods of favorable growth conditions (spring). Nutrients also caused a change in the algal chl a, which increased from 0.5–5.8 to 2.1–10.7 μg chl · cm−2. Our results indicate that in oligotrophic forested streams, long-term nutrient addition has significant effects on the algal biomass and community composition, which are detectable despite the low light availability caused by the tree canopy. Low light availability moderates but does not detain the long-term tendency toward a nutrient-tolerant community. Furthermore, the effects of nutrient addition on the algal community occur in spite of seasonal variations in light, water flow, and water chemical characteristics, which may confound the observations.

Table 2.   Percent abundances of the most frequent taxa in three reaches of the Fuirosos stream. U1 and U2 untreated; E, enriched both in the periods before (bef) and after (aft) the enrichment of the E reach. Acronyms identifying the taxa are indicated.
Achnanthes biasolettiana GrunowABIA1.
Achnanthes lanceolata (Bréb.) GrunowALAN7.
Achnanthes minutissima Kütz.AMIN56.
Achnanthes lanceolata v. frequentissima Lange-Bert.ALFR0.
Amphora inariensis KrammerAINA1.
Amphora ovalis (Kütz.) Kütz.AOVA0.
Amphora pediculus (Kütz.) GrunowAPED0.
Cocconeis pediculus Ehrenb.CPED0.
Cocconeis placentula Ehrenb.CPLA13.720.31.88.412.332.4
Cymbella silesiaca Bleisch in Rabenh.CSLE0.
Diploneis oblongella (Nägeli) Cleve-EulerDOBL0.
Fragilaria capucina var. gracilis (Øestrup) HustedtFCGP0.
Fragilaria capucina var. capitellata (Grunow) Lange-Bert.FCCP0.
Fragilaria ulna (Nitzsch) Lange-Bert.FULN0.
Gomphonema angustatum (Kütz.) Rabenh.GADI1.
Gomphonema angustum C. AgardhGANT0.
Gomphonema minutum (C. Agardh) C. AgardhGMIN0.
Gomphonema pumilum (Grunow) E. Reichardt et Lange-Bert.GPUM1.
Meridion circulare (Grev.) C. AgardhMCIR0.
Navicula antonii Lange-Bert.NANT0.
Navicula accomoda Hust.NARB0.
Navicula capitatoradiata H. Germ.NCPR0.
Navicula cryptocephala Kütz.NCRY0.
Nitzschia linearis (C. Agardh) W. Sm.NLIN0.
Nitzschia palea (Kütz.) W. Sm.NPAL0.
Reimeria sinuata (W. Greg.) Kociolek et StoermerRSIN3.
Rhoicosphenia abbreviata (C. Agardh) Lange-Bert.RABB8.