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NO3 uptake in shallow, oligotrophic, mountain lakes: The influence of elevated NO3 concentrations

K. R. Nydick·B. M. LaFrancois·Jill Baron
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Abstract

Nutrient enrichment experiments were conducted in 1.2-m deep enclosures in 2 shallow, oligotrophic, mountain lakes. 15 N-NO 3 isotope tracer was used to compare the importance of phytoplankton and benthic compartments (epilithon, surface sediment [epipelon], and subsurface sediment) for NO 3 uptake under high and low NO 3 conditions. NO 3 uptake approached saturation in the high-N lake, but not in the low-N lake. The capacity of phytoplankton and benthic compartments to take up NO 3 differed among treatments and between lakes, and depended on water-column nutrient conditions and the history of NO 3 availability. Phytoplankton productivity responded strongly to addition of limiting nutrients, and NO 3 uptake was related to phytoplankton biomass and photosynthesis. However, more NO 3 usually was taken up by benthic compartments (57–92% combined) than by phytoplankton, even though the response of benthic algal biomass to nutrient additions was less pronounced than that of phytoplankton and benthic NO 3 uptake was unrelated to benthic algal biomass. In the low-N lake where NO 3 uptake was unsaturated, C content or % was related to NO 3 uptake in benthic substrates, suggesting that heterotrophic bacterial processes could be important in benthic NO 3 uptake. These results suggest that phytoplankton are most sensitive to nutrient additions, but benthic processes are important for NO 3 uptake in shallow, oligotrophic lakes.

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K. R. Nydick, B. M. LaFrancois, Jill Baron. 2004. NO3 uptake in shallow, oligotrophic, mountain lakes: The influence of elevated NO3 concentrations. https://doi.org/10.1899/0887-3593(2004)023%3C0397%3Anuisom%3E2.0.co%3B2

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