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

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Abstract:

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 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.

Pendant deux années complètes, nous avons maintenu modérément élevées les concentrations d'azote et de phosphore d'un ruisseau de tête de bassin fortement ombragé; nous avons comparé les effets sur le périphyton de ce ruisseau aux conditions observées dans un ruisseau témoin. Les deux ruisseaux ont aussi été échantillonnés pendant un an avant le traitement expérimental. Certaines valeurs de biomasse du périphyton (masse sèche sans les cendres et chlorophylle a) augmentent après l'enrichissement. L'augmentation de chlorophylle a est vraisemblablement due à un contenu plus élevé de chlorophylle par cellule, car le biovolume total des algues ne change pas avec l'enrichissement. Ces différences sont plus marquées durant les mois de forte lumière (novembre à mai) pendant lesquels les taux de croissance cellulaire (une valeur de remplacement de la productivité) sont aussi maximaux dans les conditions d'enrichissement. Les peuplements d'algues sont dominés par les diatomées et sont restés semblables dans le ruisseau expérimental et le ruisseau témoin durant la durée de l'enrichissement. Bien que les nutriments stimulent les taux de croissance des algues, les effets à long terme de l'addition de nutriments sur le périphyton restent de faible amplitude par comparaison à d'autres valeurs trouvées dans la littérature; il y a donc potentiellement une suppression des effets par la disponibilité de la lumière et la consommation des invertébrés. Ces facteurs et d'autres peuvent aussi être importants pour limiter le pool d'espèces d'algues et ainsi réduire la réponse taxonomique à l'enrichissement. Nos résultats indiquent que, dans les ruisseaux en tête de bassin où la couverture des arbres est intacte, un enrichissement chronique mais modéré en nutriments peut avoir peu d'effets décelables sur la composition des algues benthiques ou sur la biomasse du périphyton.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: September 1, 2005

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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