Growth versus food relationships.

<p>The left hand graphs show growth rates against <i>ln</i> food concentration in each experiment, with the fitted regression equations giving growth rate as a function of <i>ln</i> food concentration in mg C L^-1; each point in the graphs is the mean of 4 replicates at each food concentration. The right hand graphs show threshold food concentrations (TFC, mean ± 95% CI), slopes of the regression lines of growth rate on food concentration (SL, mean ± 95% CI) and growth rates at 0.2 mg C L^-1 (GR, mean ± 95% CI) in high latitude (HL) and temperate <i>Daphnia</i> (T). The animals were fed <i>Nannochloropsis</i> or <i>Synechococcus</i> (HL clone only) at 18°C or 26°C. The different letters indicate statistically different values (p<0.05, ANOVA with Tukey HSD or confidence intervals comparisons).</p

Location of the collection site for the zooplankton used in the growth experiments.

<p>The high latitude clone of <i>D</i>. <i>pulex</i> (<i>Daphnia</i>-HL) originated from a thermokarst pond in the SAS Valley (subarctic Québec) and the temperate clone from Metis (Jardins de Métis) in southern Québec (<i>Daphnia</i>-T).</p

The FA content and composition of two picophytoplankton food types used in the growth experiments.

<p>The concentrations of saturated (SAFA), monounsaturated (MUFA) and polyunsaturated (PUFA) fatty acids (±SE, n = 5) in <i>Nannochloropsis</i> (N) and <i>Synechococcus</i> (S). The different letters indicate a significant difference in total fatty acid content at p<0.001.</p

Effects on <i>Daphnia</i> growth rates.

<p>The first three-way ANOVA tested for the effects of temperature (T), food type (F), food concentration (FC) and their interactions on growth rate of <i>Daphnia</i>-HL, and the second tested the effects of temperature (T), clone (Cn), food concentration (FC) and their interactions on growth rate of the two <i>D</i>. <i>pulex</i> clones, <i>Daphnia</i>-HL and <i>Daphnia</i>-T.</p><p>Effects on <i>Daphnia</i> growth rates.</p

The content and composition of FA in <i>Daphnia</i>-HL fed either <i>Nannochloropsis</i> (N) or <i>Synechococcus</i> (S) at 18°C or 26°C.

<p>Each values is a mean of 5 replicates ±SE. Different letters indicate significant differences in total fatty acid contents at p<0.01 (two-way ANOVA with Tukey HSD).</p

Changes in food web dynamics of low Arctic ponds with varying content of dissolved organic carbon

<p>An influx of terrestrial dissolved organic carbon (DOC) into freshwater habitats can regulate a range of ecosystem characteristics, from water clarity to productivity. To understand the extent to which DOC can regulate ecosystem functioning, we conducted a survey to determine the source of DOC in low Arctic ponds close to the Arctic Circle (Kangerlussuaq, Greenland), including its role in food web dynamics. We used a multiple element (carbon, nitrogen, and hydrogen) stable isotope approach to examine the proportional contribution of different sources to aquatic consumers in nine arctic ponds that spanned a broad gradient of DOC (6.6–60.1 mg^L-1). Our results show that benthic and pelagic primary production decreased along a gradient of increasing DOC content. Additionally, the changes in the organic matter pool with increasing DOC translated into changes in consumer resource use. We found significant differences in resource use between species. All consumers relied on benthic autotrophic material when DOC was low; but when DOC was high some consumers changed their diet. Collectively, our findings demonstrate how the concentration of DOC influences aquatic production and our study can be used as a baseline to predict how the aquatic food web may respond to regionally changing DOC concentrations.</p