Biomass responses of total algal response to fertilisation with nitrogen in mesocosms conducted in August and September.

<p>Time series include; a) total phytoplankton biomass (mg wet mass L⁻¹), b) Chl <i>a</i> (<i>µ</i>g L⁻¹) and c) the ratio of Chl <i>a</i> : total phytoplankton biomass. Symbols represent mean and standard error (± SE, <i>n</i> = 3) for each nitrogen treatment, including amendments with NH₄⁺ (shaded triangle, coarse dashed line), NO₃⁻ (shaded square, medium dashed line) and urea (shaded circle, fine dashed line), as well as unamended (control) mesocosms (solid circle, solid line).</p

Principal component analysis of experimental phytoplankton assemblages at the a) division, b) genus, and c) species level of taxonomic resolution.

<p>Genera and species were selected if their cumulative biomass over the course of each experiment was more than 1% of the total for any of the 12 enclosures. Algal densities were log₁₀(x +1)-transformed as needed, and categorical nitrogen treatments (e.g.,+or – urea) were included as passive variables. All samples were included in each PCA; however, to simplify presentation, sample ordination points are not presented and only select taxa are identified. Coloured arrows indicate cyanobacteria (blue), chlorophytes (green), cryptophytes (red), diatoms (yellow), dinoflagellates (brown), and chrysophytes (purple). Proportion of total variation explained by first (x) and second (y) principle axes are presented.</p

Least-squares regression analysis of the linear relationship between microscopic and chromatographic estimates of phytoplankton abundance.

<p>Phytoplankton biomass was measured by microscopy, while concentrations of taxonomically-diagnostic biomarker pigments were analysed by spectrophotometry (chlorophyll <i>a</i>) and high performance liquid chromatography (all other pigments). Data were log₁₀(x+1) transformed prior to analysis (<i>df</i> = 58). Algal biomass was summed according to distribution of indicator pigments prior to statistical analysis.</p

Biomass responses of important phytoplankton species to fertilisation with nitrogen in mesocosms conducted in Augusts and September.

<p>Biomass presented as mg wet mass L⁻¹. Symbols represent mean and standard errors (± SE, <i>n</i> = 3) for each of the nitrogen treatments, including addition of NH₄⁺ (shaded triangle, coarse dashed line), NO₃⁻ (shaded square, medium dashed line) and urea (shaded circle, fine dashed line), as well as unamended (control) mesocosms (solid circle, solid line).</p

Repeated-measures analysis of variance for the response of selected phytoplankton taxa to added nitrogen.

<p>Probability (<i>p</i>) values were calculated for treatment and treatment-time interaction effects. Tukey’s HSD <i>post hoc</i> results represent mean treatment values ordered from largest to smallest and significant differences (>) at α = 0.05, for urea (U), nitrate (NO), ammonium (NH), and the control (C). If a treatment falls on both sides of a “>” this indicates no significant difference from the treatments on either side. All phytoplankton biomass (mg L⁻¹) data were log₁₀(x+1??transformed prior to analysis to meet assumptions of normality. Probabilities were not corrected for number of comparisons.</p

Biomass responses of major phytoplankton groups to fertilisation with nitrogen in mesocosms conducted in August and September.

<p>Algal groups (mg wet mass L⁻¹) include; a) cyanobacteria, b) chlorophytes, c) diatoms, d) chrysophytes, e) cryptophytes and f ) dinoflagellates. Symbols represent mean and standard errors (± SE, <i>n</i> = 3) for each of the nitrogen treatments, included amendments with NH₄⁺ (shaded triangle, coarse dashed line), NO₃⁻ (shaded square, medium dashed line) and urea (shaded circle, fine dashed line), as well as unamended (control) mesocosms (solid circle, solid line).</p

Repeated-measures analysis of variance of total phytoplankton biomass, chlorophyll <i>a</i>, chlorophyll <i>a</i> : biomass ratio, and biomass of major algal groups.

<p>Probability (<i>p</i>) values were calculated for treatment and treatment-time interaction effects. Tukey’s HSD <i>post hoc</i> results represent mean treatment values ordered from largest to smallest and significant differences (>) at α = 0.05, for urea (U), nitrate (NO), ammonium (NH), and the control (C). If a treatment falls on both sides of a “>” this indicates no significant difference from the treatments on either side. All phytoplankton biomass (mg L⁻¹) data, but not chlorophyll <i>a</i> concentrations (µg L⁻¹), were log₁₀(x+1) transformed prior to analysis to meet assumptions of normality. Probabilities were not corrected for number of comparisons. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0053277#s2" target="_blank">Methods</a> for additional information.</p

Map of Wascana Lake, Saskatchewan, Canada.

<p>Map shows a) the continental location, b) the gross drainage area (1400 km²) and c) depth contour map with the location of the mesocosm experiment (shaded box).</p

Seasonal limnological trends in Wascana Lake, Saskatchewan May–August 2009.

<p>(a) total dissolved (TDP) and soluble reactive phosphorus (SRP) concentrations, (b) total dissolved nitrogen (TDN) concentration and phytoplankton biomass (as Chl <i>a</i>), and (c) final concentrations of Chl <i>a</i> (fertilized treatment minus control) after 72-h bottle bioassay incubations of Wascana Lake water receiving growth-saturating concentrations of NH₄ (N), PO₄^3- (P), or both N and P (N+P). Analysis of variance with Tukey’s post hoc tests identified statistically significant (asterisk) phytoplankton biomass response (<i>p</i> < 0.05) relative to control bottles. Vertical dashed grey lines show the start dates of the monthly mesocosm experiments.</p

Model fits describing N effect on mesocosm plankton dynamics.

<p>Least squares regression analysis (<i>n</i> = 15) of phytoplankton and bacterial abundance and production (y) as functions of urea load (x). Models were selected using Akaike information criterion corrected for small sample sizes (AIC_c), and ranked based on AIC_c score, with best-fitting models in bold. Models with no explanatory power (i.e. r² = 0) are omitted. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0188652#pone.0188652.g002" target="_blank">Fig 2</a> for graphical representation of best-fit models.</p