Research ArticleECOLOGY

Ocean currents promote rare species diversity in protists

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Science Advances  15 Jul 2020:
Vol. 6, no. 29, eaaz9037
DOI: 10.1126/sciadv.aaz9037
  • Fig. 1 Genealogy in oceanic currents.

    (Left) The coalescence model predicts the protist species composition in a sample of oceanic water taken from an area of size L0 × L0. Different colors represent different species. Arrows represent the velocity field induced by ocean currents. (Right) Trajectories of the coalescence model with ocean currents. Individuals are represented by tracers that are transported backward in time and can coalesce with other tracers if they reach a close distance. Coalescence events are marked by open circles; trajectories of individuals that have coalesced are shown in the same color. Tracers are removed from the population at an immigration rate μ (marked by crosses). See also movie S1.

  • Fig. 2 Coalescence model predicts effect of chaotic advection by oceanic currents on SAD.

    The two panels show SADs (A) in the presence (orange lines) and (B) absence (green lines) of oceanic currents for the coalescence model. Here and below, SAD curves are rescaled so that P(1) = 1 to ease visualization. Model details and parameters are presented in Materials and Methods. Dashed lines are power laws to guide the eye (see also Fig. 3).

  • Fig. 3 Rare SADs present a steeper decay with abundance in oceans than in lakes.

    Continuous lines represent SADs of protist communities from (A) 157 oceanic samples (29) and (B) 206 freshwater samples (40). Total numbers of individuals in each sample are in the ranges of (A) (103, 105) and (B) (104, 106). In both panels, power laws (dashed lines) are shown to guide the eye.

  • Fig. 4 Power law exponents of SADs.

    We run our models for different population sizes and different values of flux parameters for ocean samples (see Materials and Methods). We select 157 oceanic samples and 206 freshwater samples as in Fig. 3. We fit the power law exponent α of the SADs to the model and to the data using maximum likelihood. (A) Continuous distributions of the exponent obtained by kernel density estimation. (B) Dependence of the exponent on four main parameters of the oceanic flow: forcing frequency ω, wave perturbation amplitude ϵ, mean wave amplitude B0, and phase speed c. In each subpanel, other parameters are kept constant (see Materials and Methods). Correlation tests of ω, ϵ, B0, and c with the exponent α yield Pearson coefficients rP = 0.72, 0.48, −0.04, and −0.58 and P values P = 3 × 10−9, 6 × 10−4, 0.77, 10−5, respectively.

  • Fig. 5 Chaotic advection by oceanic currents increases species number S in a water sample.

    (A) Ratio Socean/Slake as a function of the sample size N for the model and the data. We simulate the model at increasing sample sizes N in powers of 2 and obtain continuous curves by interpolation. Other parameters are presented in Materials and Methods. Averaged data are obtained by binning for both oceans and lakes. (B and C) Number of species S in samples of N individuals in (B) oceans and (C) lakes. A power law (Eq. 3) fits the data better than the Ewens sampling formula (Eq. 2) for both (B) oceanic (normalized log-likelihood −19.39 versus −449.52) and (C) lake samples (normalized log-likelihood −6.64 versus −117.21). Fitted exponents are z = 0.73 and z = 0.65 for oceans and lakes, respectively. The results of the coalescence model are shown with and without oceanic currents (orange and green lines, respectively). The Ewens sampling formula provides a better fit than the power law in both cases [normalized log-likelihood −420.95 versus −2131.43 in (B) and −434.66 versus −1902.62 in (C)].

Supplementary Materials

  • Supplementary Materials

    Ocean currents promote rare species diversity in protists

    Paula Villa Martín, Aleš Buček, Thomas Bourguignon, Simone Pigolotti

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    • Figs. S1 to S9
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