Title: Potassium enrichment stimulates the growth and reproduction of a clone of Daphnia dentifera

David J. Civitello, Department of Biology, Indiana University,

Current address: Department of Integrative Biology, University of South Florida,

Jessica L. Hite, Department of Biology, IndianaUniversity,

Spencer R. Hall, Department of Biology, IndianaUniversity,

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: 813-974-4694, fax: 813-974-3263, email:

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.

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, (i = P or K), and that of the unmanipulated control for each lake, (; 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 (). 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 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 ∆rjfrom 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 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.

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 nutrient-

enrichment 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.

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

Fig.A1


Fig.A2

Civitello et al.1March 2014