Co-limitation by N and P Characterizes Phytoplankton Communities Across Nutrient Availability and Land Use
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Co-limitation by N and P Characterizes Phytoplankton Communities Across Nutrient Availability and Land Use A. R. Bratt,1,3* J. C. Finlay,1 J. R. Welter,2 B. A. Vculek,2 and R. E.Van Allen1 1
Department of Ecology, Evolution and Behavior, University of Minnesota, St. Paul, Minnesota 55108, USA; 2Department of Biology, St. Catherine University, St. Paul, Minnesota 55105, USA; 3Present Address: Department of Environmental Studies, Davidson College, Davidson, North Carolina, USA
ABSTRACT Historically, freshwater lakes have been widely assumed to be singly limited by phosphorus (P) because the dominant paradigm assumes that nitrogen fixation (N2 fixation) will compensate for any nitrogen (N) deficits. However, a growing body of evidence demonstrates that primary producer response to nutrient manipulation most frequently indicates co-limitation by N and P. Differences in N and P supply ratio have been shown to influence the identity and severity of nutrient limitation, but whether N and P concentration and the ratio of N to P concentrations can explain the frequency of co-limitation in aquatic primary producer assemblages remains unclear, especially in ecosystems subject to human perturbation that strongly increase nutrient availability. We determined how resource availability influences nutrient limitation by N and P of phytoplankton primary production across 12 lakes in Minnesota that vary in watershed land use and lake nutrient levels. We measured epilimnetic lake metabolism and indicators of N2
Received 21 August 2018; accepted 25 October 2019 Electronic supplementary material: The online version of this article (https://doi.org/10.1007/s10021-019-00459-6) contains supplementary material, which is available to authorized users. Author’s Contributions: ARB, JCF and JRW conceived and designed the study. ARB, BAV and REV performed the research. ARB, BAV and REV analyzed the data. ARB wrote the paper. *Corresponding author; e-mail: [email protected]
fixation to evaluate their influence on nutrient limitation status of planktonic algal assemblages. Despite large differences in land use (agricultural, urban, and suburban) and water column N and P availability, planktonic algal response to nutrient manipulation was consistently characterized by colimitation by N and P across years and months. Neither P availability (as concentrations of total and inorganic forms) nor N2-flux rate predicted responses to nutrient additions. N availability significantly influenced responses of phytoplankton to nutrient additions across years, but this effect was small. The ratio of total N to total P significantly influenced the response to single additions of N and P (these effects were negative and positive, respectively) in summer 2013. Importantly, higher lake primary production and heterocyte count (number of nitrogen fixing cells) were also associated with a stronger, positive response to N + P addition. Overall, these data suggest that planktonic algal assemblages are predominantly characterized by co-limitation by N and P despit
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