Title: Potassium enrichment stimulates the growth and reproduction of a clone of Daphnia
dentifera
David J. Civitello, Department of Biology, Indiana University, djcivite@indiana.edu
Current address: Department of Integrative Biology, University of South Florida,
civitello@usf.edu
Jessica L. Hite, Department of Biology, Indiana University, jlhite@indiana.edu
Spencer R. Hall, Department of Biology, Indiana University, sprhall@indiana.edu
Running title: Potassium stimulates Daphnia
Oecologia, Physiological Ecology – Original Research
Word count: Abstract – 250, Main text – 3660, Supplementary Information – 987 words
Main text - Figures: 4, Tables: 0, References 40
Supplement – Figures: 2, Tables: 0, References: 4
Keywords: Potassium, Daphnia, algae, growth rate, nutrient limitation, condition
Corresponding author: David J. Civitello, 4202 E Fowler Ave., Tampa, FL 33647, phone: 813974-4694, fax: 813-974-3263, email: civitello@usf.edu
Declaration of authorship: DJC and SRH designed the experiments. DJC and JLH performed the
experiments. DJC analyzed the data and wrote the first draft. All authors edited the manuscript.
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Appendix A: Additional methods and results relating to nutrient stimulation of Daphnia growth
and algal production in lake water.
Here we provide additional methods and results for the nutrient enrichment experiments
conducted in 12 hard water lakes in southwestern Indiana, USA in 2010. First, we present
estimates of the magnitude of nutrient stimulation for Daphnia (∆gi) and algae (∆ri) in each lake.
We also provide correlations among nutrient stimulation of algal growth (∆ri) and environmental
factors.
Nutrient stimulation of Daphnia growth
We estimated the magnitude of the nutrient enrichment treatments on Daphnia growth by
calculating growth differentials, ∆gi (Downing et al. 1999). Growth differentials quantify the
difference between the mean mass-specific growth rate in a nutrient-enriched treatment, g i (i =
P or K), and that of the unmanipulated control for each lake, g c ( gi  gi  gC ; Downing et al.
1999). We also averaged the ∆gi values across all of the lakes to estimate the overall effect of
nutrient enrichment in these lakes ( g i ). We estimated standard errors of these quantities by
bootstrapping (Dixon 1993).
Both nutrients significantly influenced Daphnia growth in five lakes (see Main Text). For
potassium, ∆gK values ranged from -0.065 – 0.097 d-1 (Fig. A1A). Phosphorus exerted similar
effects on Daphnia growth. For P, ∆gP values varied from -0.031 – 0.112 d-1 (Fig. A1B).
Nutrient stimulation of algal production
We estimated the effect size of the nutrient enrichment treatments of algal production by
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calculating growth differentials, ∆rj, based on algal production rates (Downing et al. 1999). In
general, we estimated these quantities as above for Daphnia. However, due to lower sample size,
we used the normal approximation for the standard error of the difference between two estimated
means (Sokal and Rohlf 1995) for ∆rj from each lake rather than bootstrapping.
Potassium and phosphorus exhibited different effects on algal growth. In general, K
stimulation of algal growth (∆rK) was small (range: -0.022 – 0.114, Fig. A1C). No ∆rK value
significantly exceeded 0 (K enrichment had no effect). In contrast, P stimulation of algal
production could be large. For P, ∆rP varied from -0.053 – 0.362, Fig. A1D). Phosphorus
enrichment significantly stimulated algal production in three lakes (see Main Text).
Environmental correlates of nutrient stimulation of algae
Phosphorus enrichment stimulated algal production in some lakes while potassium
generally had no effect. We explored three factors that could explain this variation across lakes –
initial density of edible algae (measured as chlorophyll a concentration), the ratio of carbon to
phosphorus in seston (C:P, by moles, determined for nine lakes), and background potassium
concentration (mg K+/L). Specifically, we used univariate linear regressions to determine
correlations between nutrient stimulation effects on algae (∆ri) and these environmental factors
(see Main Text for additional methodological details).
Phosphorus stimulation of algal production (∆rP) was not related to chlorophyll a (N = 11
lakes, R = 0.08, P = 0.82, Fig. A2A), seston C:P (N = 9, R = 0.43, P = 0.25, Fig. A2B), or
background potassium concentration (N = 11, R = 0.28, P = 0.41, Fig. A2C). In contrast,
stimulation of algal production by potassium (∆rK) was associated with chlorophyll a
concentration, a measure of algal density (N = 11, R = -0.66, P = 0.028, Fig. A2D). There was
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also a marginally significant correlation between ∆rK and seston C:P (N = 9, R = -0.66, P =
0.052, Fig. A2E). However, it was not related to the background concentration of potassium (N =
11, R = 0.40, P = 0.23, Fig. A2F).
Although phosphorus stimulated algal production in some lakes (Fig. 2C), this effect was
not significantly correlated with seston C:P ratio. In contrast, potassium exerted small,
nonsignificant effects on algal production (Fig. 2C). Still, the magnitude of these effects was
negatively correlated with chlorophyll a and seston C:P: K exerted greater stimulation of algal
production in lakes with less algal biomass and lower C:P ratios. Future work could address the
relevance and robustness of these relationships.
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LITERATURE CITED
Dixon P. M. 1993. The bootstrap and the jackknife: describing the precision of ecological
indices. Pages 267-288 in SM Scheiner and J Gurevitch, editors. Design and analysis of
ecological experiments. Oxford University Press.
Downing J. A., C. W. Osenberg, and O. Sarnelle. 1999. Meta-analysis of marine nutrientenrichment experiments: variation in the magnitude of nutrient limitation. Ecology,
80:1157-1167.
Sokal R. R. and F. J. Rohlf 1995. Biometry, 3rd edition. WH Freeman and Co, New York.
Sterner R. W. and J. J. Elser 2002. Ecological stoichiometry: the biology of elements from
molecules to the biosphere, Princeton Univ Pr.
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Figure Captions
Fig. A1. Nutrient stimulation of production in the bioassays, calculated as growth differentials.
Growth differentials for Daphnia dentifera (∆gi) and algae (∆ri) represent the difference in
individual (Daphnia) or community-wide (algae) growth rates between nutrient-enriched
treatments (either K or P) and unmanipulated controls. Enrichment with (A) potassium and (B)
phosphorus significantly increased Daphnia growth in five of 12 lakes. However, (C) K
enrichment did not alter algal production in any lake. In contrast, (D) P enrichment stimulated
algal production in three lakes. Asterisks indicate significant effects of nutrient enrichment as
reported in the main text. Dashed lines indicate no effect of enrichment (i.e., a growth
differential of zero). Lakes in the left column (panels A and C) are ordered by stimulation of K
of Daphnia. Similarly, lakes in the right column (panels B and D) are ordered by increasing P
stimulation of Daphnia. There are no algal growth data from University Lake
Fig. A2. Correlations between nutrient stimulation of algal production by phosphorus (∆rP) and
potassium (∆rK) and three environmental factors. P stimulation of algae was not related to (A)
chlorophyll a, (B) C:P of seston, or (C) background potassium concentration. In contrast, K
stimulation of algae decreased with increasing (D) chlorophyll a and (E) seston C:P. However, K
stimulation of algae was not related to (F) background K concentration. For C:P data, n = 9
lakes. For all other comparisons n = 12 lakes
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Fig. A1
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Fig. A2
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