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Nitrogen regulation of algal biomass, productivity, and composition in shallow mountain lakes, Snowy Range, Wyoming, USA

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Abstract:

We investigated the effects of increased nitrate (NO3), alone and in combination with phosphorus (P), on phytoplankton, epilithon, and epipelon in shallow lakes of the Snowy Range, Wyoming, using two enclosure experiments during early and late summer. Phytoplankton responded strongly to N and N + P, but not to P, with increased cell density, chlorophyll a, and photosynthesis and shifts in composition from chrysophytes to cyanophytes, chlorophytes, and diatoms. Zooplankton density and biomass were unaltered despite the additional phytoplankton stock, probably as the result of poor food quality. In the late summer, algae on tiles responded to N and N + P additions with greater chlorophyll a and increases in cyanophyte and chlorophyte density. Algae on sediment dominated whole-enclosure algal biomass but were spatially variable and responded insignificantly to nutrients. Consequently, N controlled productivity and community composition of phytoplankton and algae on hard substrates but had less impact on ecosystem algal biomass because of the large pool of nutrient-sufficient sediment algae. Phytoplankton were more photosynthetically efficient than the benthos, however, such that primary productivity did shift more toward the water column.

Nous avons étudié les effets d'un accroissement du nitrate (NO3), seul ou en combinaison avec le phosphore (P), sur le phytoplancton, l'épilithon et l'épipélon de lacs peu profonds de la chaîne des monts Snowy au Wyoming à l'aide de deux expériences en enclos au début et à la fin de l'été. Le phytoplancton réagit fortement à N et N + P, mais non à P, par une augmentation de la densité des cellules, de la chlorophylle a et de la photosyntèse et par un changement de composition (les chrysophytes sont remplacés par les cyanophytes, les chlorophytes et les diatomées). La densité et la biomasse du zooplancton restent inchangées malgré le stock additionnel de phytoplancton, probablement à cause de la mauvaise qualité de la nourriture. En fin d'été, les algues poussant sur des tuiles réagissent à des additions de N et de N + P par une augmentation de la chlorophylle a et de la densité des cyanophytes et des chlorophytes. Les algues croissant sur les sédiments représentent la majorité de la biomasse des algues dans l'ensemble des enclos, mais elles varient d'un endroit à un autre et elles réagissent de façon non significative aux nutriments. En conséquence, N contrôle la productivité et la composition des communautés de phytoplancton et d'algues vivant sur des substrats durs, mais son impact sur la biomasse des algues de l'écosystème est réduit parce qu'il y a un important pool d'algues sur les sédiments qui ont suffisamment de nutriments. Le phytoplancton a, cependant, une photosynthèse plus efficace que celle du benthos, ce qui fait que la productivité primaire se déplace plus vers la colonne d'eau.[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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