Periphyton response to long-term nutrient enrichment in a shaded headwater stream

We maintained elevated but moderate concentrations of nitrogen and phosphorus continuously for 2 years in a heavily shaded headwater stream and compared effects on stream periphyton with a reference stream. Both streams were sampled for 1 year before treatment. Some measures of periphyton biomass (ash-free dry mass and chlorophyll a) responded positively to enrichment. Increased chlorophyll a was likely a result of higher chlorophyll per cell, as total algal biovolume did not change with enrichment. These differences were greatest during high-light months (November- May), when cellular growth rates (a proxy for production) were also highest with enrichment. Algal assemblages were dominated by diatoms and remained similar between the treatment and reference streams throughout the enrichment period. Although nutrients stimulated algal growth rates, the long-term effects of nutrient addition on periphyton biomass were small in magnitude compared with other published values and were potentially suppressed by light availability and invertebrate consumption. These and other factors may have also been important in limiting the algal species pool and thus a taxonomic response to enrichment. Our results indicate that in headwater streams with intact tree canopies, chronic nutrient enrichment at moderate concentrations may have little detectable effect on benthic algal composition or periphyton biomass. Although nutrients stimulated algal growth rates, the long-term effects of nutrient addition on periphyton biomass were small in magni- tude compared with other published values and were potentially suppressed by light availability and invertebrate consump- tion. These and other factors may have also been important in limiting the algal species pool and thus a taxonomic response to enrichment. Our results indicate that in headwater streams with intact tree canopies, chronic nutrient enrichment at moder- ate concentrations may have little detectable effect on benthic algal composition or periphyton biomass. Resume : Pendant deux annees completes, nous avons maintenu moderement elevees les concentrations d'azote et de phosphore d'un ruisseau de tete de bassin fortement ombrage; nous avons compare les effets sur le periphyton de ce ruisseau aux conditions observees dans un ruisseau temoin. Les deux ruisseaux ont aussi ete echantillonnes pendant un an avant le traitement experimental. Certaines valeurs de biomasse du periphyton (masse seche sans les cendres et chlorophylle a) augmentent apres l'enrichissement. L'augmentation de chlorophylle a est vraisemblablement due a un contenu plus eleve de chlorophylle par cellule, car le biovolume total des algues ne change pas avec l'enrichissement. Ces differences sont plus marquees durant les mois de forte lumiere (novembre a mai) pendant lesquels les taux de croissance cellulaire (une valeur de remplacement de la productivite) sont aussi maximaux dans les conditions d'enrichissement. Les peuplements d'algues sont domines par les diatomees et sont restes semblables dans le ruisseau experimental et le ruisseau temoin durant la duree de l'enrichissement. Bien que les nutriments stimulent les taux de croissance des algues, les effets a long terme de l'addition de nutriments sur le periphyton restent de faible amplitude par comparaison a d'autres valeurs trouvees dans la litterature; il y a donc potentiellement une suppression des effets par la disponibilite de la lumiere et la consommation des invertebres. Ces facteurs et d'autres peuvent aussi etre impor- tants pour limiter le pool d'especes d'algues et ainsi reduire la reponse taxonomique a l'enrichissement. Nos resultats indiquent que, dans les ruisseaux en tete de bassin ou la couverture des arbres est intacte, un enrichissement chronique mais modere en nutriments peut avoir peu d'effets decelables sur la composition des algues benthiques ou sur la bio- masse du periphyton. (Traduit par la Redaction) Greenwood and Rosemond 2045