Cumulative effects of nutrients and pH on the plankton of two mountain lakes

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Abstract:

We conducted enclosure experiments to examine the cumulative effects of nutrient enrichment and acidification on the plankton of two mountain lakes with differing nutrient conditions. The low-nitrate lake responded to N, N plus acid, and N plus acid plus P additions, showing four- to seven-fold increases in chlorophyll a, increased photosynthetic rate, compositional shifts toward large chlorophytes, and decreased zooplankton biomass. The high-nitrate lake responded minimally to either N or P alone but responded strongly to combined additions of N plus acid plus P, showing eightfold increases in chlorophyll a, increased cell density and photosynthetic rates, and compositional shifts toward chlorophytes and the dinoflagellate Gymnodinium. In both study lakes, changes in chlorophyll a were linked to addition of limiting nutrients regardless of pH, whereas shifts in phytoplankton species composition were apparently affected by both nutrient conditions and acidity. The most striking changes in species composition and biomass occurred in combined N plus acid plus P treatments, indicating that continued nutrient enrichment may interact with acidification to produce marked changes in the plankton of mountain lakes.

Des expériences en enclos nous ont permis d'étudier les effets cumulatifs de l'enrichissement en nutriments et de l'acidification sur le plancton de deux lacs de montagne possédant des conditions nutritives différentes. Le lac aux faibles concentrations de nitrates réagit à l'addition de N, de N + acide et de N + acide + P, par des accroissements de 4-7 fois de la chlorophylle a, un taux de photosynthèse plus élevé, des changements de composition favorisant les chlorophytes de grande taille et une réduction de la biomasse du zooplancton. Le lac à haute teneur en nitrates réagit peu à l'addition de N ou de P seuls, mais il réagit fortement à des additions combinées de N + acide + P, affichant des augmentations de la chlorophylle a par un facteur de 8, un accroissement de la densité cellulaire et du taux de photosynthèse et des changements de composition favorisant les chlorophytes et le dinoflagellé Gymnodinium. Dans les deux lacs étudiés, les variations de la chlorophylle a sont reliées à l'addition de nutriments limitants, indépendamment du pH, alors que les changements dans la composition du phytoplancton sont affectés à la fois par les conditions nutritives et l'acidité. Les changements les plus spectaculaires de composition spécifique et de biomasse se produisent à l'addition conjointe de N + P + acide, ce qui indique que l'enrichissement prolongé en nutriments peut interagir avec l'acidification pour produire des changements importants dans le plancton des lacs de montagne.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: July 1, 2004

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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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