Table 1.
Biological response variables measured in the field surveys and corresponding response variables tested in laboratory and instream nutrient limitation assays.
Table 2.
Summary of seasonal nutrient characteristics of streams and stream waters tested.
Streams are listed in order of increasing average total phosphorus (P) concentrations.
Fig 1.
Matched views of a nutrient-diffusing substrate (NDS) rack at deployment and after 21-days of colonization in a nitrogen and phosphorus (N+P) co-limited stream, the Big Wood River.
Fig 2.
Green algae growth results from six streams with Nitrogen (N), Phosphorus (P), or N+P additions.
Averages ± 95th percentile confidence intervals (CI) of the mean (n = 3). Primary nutrient limitation is concluded when the CI of the single nutrient additions does not overlap the mean of the controls. Co-limitation of N+P is determined by significant (p<0.05) interaction term in 2-factor ANOVA test.
Fig 3.
Green algae growth in ambient waters (no additions).
The asymptotic relation with total P suggests saturation with a half-saturation value (50% of maximum) of about 65 μg/L TP and above about 120 μg/L TP, no further growth increases with P were noted. Error bars show standard deviation.
Table 3.
Percent phosphorus (P) and nitrogen (N) removed from the water column during the 11-day P and N growth series.
Fig 4.
Duckweed and periphyton responses to Phosphorus (P) enrichment (left) and Nitrogen (N) enrichment (right).
Error bars show standard deviations. EC50 (± SE): the concentration causing an increase in growth to 50% of maximum growth; saturation concentrations–further increases in P or N result in little further growth, calculated as 95% of the absolute asymptote.
Table 4.
Nutrient limitation conclusions from the nutrient-diffusing substrate (NDS), average (ranges) of ambient nutrient concentrations in the streams at the time of the tests, and ancillary chemistry and channel measurements.
Fig 5.
Chlorophyll (a) accrual on nutrient-diffusing substrates (NDS) with Nitrogen (N), Phosphorus (P), or N+P additions.
Averages ± 95th percentile confidence intervals (CI) of the mean (n = 6). Nutrient limitation is interpreted as with Fig 2.
Table 5.
Nutrient limitation conclusions from streams tested under similar conditions with both the green algae bottle test and nutrient-diffusing substrates.
Fig 6.
Nitrogen to phosphorus ratios corresponding with empirically determined nutrient limitation from the different stream water experiments.
(N-nitrogen limited; NP-nitrogen and phosphorus co-limited; P-phosphorus limited).
Fig 7.
Benthic chlorophyll a density over time in a nitrogen-limited stream.
A. Benthic chlorophyll a relative to both nitrogen (N) and phosphorus (P); B. Overlain with the three nutrient limitation experiments conducted on the stream that demonstrated consistent N limitation; C. A simplified comparison without P shows nearly perfect inverse correspondence between chlorophyll a and N.
Fig 8.
Benthic chlorophyll a on artificial substrates at nitrogen (N) limited sites versus field samples.
Curve fit to chlorophyll a from NDS controls (no nutrient additions), excluding P-limited sites, following 21-day accrual (top), and the same line fit overlaying 62 benthic chlorophyll a samples collected from natural substrates in associated field collections (bottom). Field data from [28, 39].
Fig 9.
Benthic chlorophyll a on artificial substrates at phosphorus (P) limited sites versus field samples.
Curve fit to chlorophyll a from NDS controls (no nutrient additions), excluding N-limited sites, following 21-day accruals overlain with 62 benthic chlorophyll a samples collected from natural substrates.