Data from: Watershed versus within-lake drivers of nitrogen: phosphorus dynamics in shallow lakes
Data files
Jun 28, 2017 version files 77.44 KB
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15N seston data.csv
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chla macro all lakes three years .csv
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denitrification data.csv
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effects of changing fish mass on lakes.csv
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effects of changing TP on NP.csv
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N and P data lakes that shifted states.csv
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README_for_15N seston data.docx
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seston isotope data two years.csv
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stable lakes data 3 years.csv
Abstract
Research on lake eutrophication often identifies variables affecting amounts of phosphorus (P) and nitrogen (N) in lakes, but understanding factors influencing N:P ratios is important given its influence on species composition and toxin production by cyanobacteria. We sampled 80 shallow lakes in Minnesota (USA) for three years to assess effects of watershed size, proportion of watershed as both row crop and natural area, fish biomass, and lake alternative state (turbid versus clear) on total N: total P (TN:TP), ammonium, total dissolved phosphorus (TDP), and seston stoichiometry. We also examined N:P stoichiometry in 20 additional lakes that shifted states during the study. Lastly, we assessed importance of denitrification by measuring denitrification rates in sediment cores from a subset of 34 lakes, and by measuring seston δ15N in four additional experimental lakes before and after they were experimentally manipulated from turbid to clear states. Results showed alternative state had the largest influence on overall N:P stoichiometry in these systems, as it had the strongest relationship with TN:TP, seston C:N:P, ammonium, and TDP. Turbid lakes had higher N at given levels of P than clear lakes, with TN and ammonium two-fold and 1.4-fold higher in turbid lakes, respectively. In lakes that shifted states, TN was three-fold higher in turbid lakes, while TP was only two-fold higher, supporting the notion N is more responsive to state shifts than is P. Seston δ15N increased after lakes shifted to clear states, suggesting higher denitrification rates may be important for reducing N levels in clear states, and potential denitrification rates in sediment cores were among the highest recorded in the literature. Overall, our results indicate lake state was a primary driver of N:P dynamics in shallow lakes, and lakes in clear states had much lower N at a given level of P relative to turbid lakes, likely due to higher denitrification rates. Shallow lakes are often managed for the clear-water state due to increased value as wildlife habitat. However, our results indicate lake state also influences N biogeochemistry, such that managing shallow lakes for the clear-water state may also mitigate excess N levels at a landscape scale.