ALGAL RESPONSE TO NUTRIENT ENRICHMENT IN FORESTED OLIGOTROPHIC STREAM 1

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. 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.  U1‐bef U1‐aft U2‐bef U2‐aft E‐bef E‐aftAchnanthes biasolettiana Grunow ABIA 1.1 1.2 0.4 0.1 5.4 0.7Achnanthes lanceolata (Bréb.) Grunow ALAN 7.2 1.3 5.7 7.1 7.3 2.2Achnanthes minutissima Kütz. AMIN 56.2 55.0 81.2 71.4 52.2 34.5Achnanthes lanceolata v. frequentissima Lange‐Bert. ALFR 0.0 0.1 0.1 0.9 1.0 0.0Amphora inariensis Krammer AINA 1.9 2.0 0.3 0.1 1.0 1.4Amphora ovalis (Kütz.) Kütz. AOVA 0.0 0.0 0.0 0.0 0.0 1.3Amphora pediculus (Kütz.) Grunow APED 0.9 2.2 0.1 0.6 3.3 1.3Cocconeis pediculus Ehrenb. CPED 0.1 0.2 0.0 0.1 0.2 1.7Cocconeis placentula Ehrenb. CPLA 13.7 20.3 1.8 8.4 12.3 32.4Cymbella silesiaca Bleisch in Rabenh. CSLE 0.0 0.2 0.0 0.1 0.0 0.1Diploneis oblongella (Nägeli) Cleve‐Euler DOBL 0.6 0.0 0.9 0.2 0.0 0.0Fragilaria capucina var. gracilis (Øestrup) Hustedt FCGP 0.3 1.0 0.1 0.0 0.1 3.5Fragilaria capucina var. capitellata (Grunow) Lange‐Bert. FCCP 0.0 0.2 0.0 0.1 0.4 0.6Fragilaria ulna (Nitzsch) Lange‐Bert. FULN 0.2 1.1 0.1 0.1 0.0 1.4Gomphonema angustatum (Kütz.) Rabenh. GADI 1.6 0.6 1.6 1.8 1.0 0.8Gomphonema angustum C. Agardh GANT 0.2 0.1 0.6 1.2 1.4 0.1Gomphonema minutum (C. Agardh) C. Agardh GMIN 0.2 0.0 0.3 0.1 0.3 0.5Gomphonema pumilum (Grunow) E. Reichardt et Lange‐Bert. GPUM 1.7 0.0 2.0 1.4 1.1 0.0Meridion circulare (Grev.) C. Agardh MCIR 0.0 0.1 1.5 1.7 0.4 0.2Navicula antonii Lange‐Bert. NANT 0.8 0.1 0.1 0.2 0.8 0.2Navicula accomoda Hust. NARB 0.0 0.0 0.0 0.0 0.0 0.0Navicula capitatoradiata H. Germ. NCPR 0.3 0.0 0.1 0.1 0.0 0.3Navicula cryptocephala Kütz. NCRY 0.5 0.1 0.1 0.3 0.5 0.2Nitzschia linearis (C. Agardh) W. Sm. NLIN 0.2 0.0 0.0 0.2 0.0 0.1Nitzschia palea (Kütz.) W. Sm. NPAL 0.0 0.0 0.3 0.2 0.5 0.2Reimeria sinuata (W. Greg.) Kociolek et Stoermer RSIN 3.4 2.0 0.6 1.2 4.9 2.8Rhoicosphenia abbreviata (C. Agardh) Lange‐Bert. RABB 8.1 5.0 0.2 0.4 3.6 9.9