Research ArticleECOLOGY

Hippos (Hippopotamus amphibius): The animal silicon pump

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Science Advances  01 May 2019:
Vol. 5, no. 5, eaav0395
DOI: 10.1126/sciadv.aav0395
  • Fig. 1 Conservative Si mass balance of the Mara River.

    Upstream influx and downstream efflux is calculated by multiplying dissolved Si (DSi) and biogenic Si (BSi) concentrations at sites 1 and 10 (table S1) with a 10-day averaged discharge at both locations, respectively. The lateral influx by hippo egestion is calculated by multiplying the average biogenic Si concentration in feces (table S1) with the conservative estimate of feces deposition in the river [11.1 metric tons of DM day−1 (14)]. The average biogenic Si concentration in grass (table S1) is multiplied with the conservative estimate of grass eaten [42.0 metric tons of DM day−1 (14)] to calculate the uptake of biogenic Si by grazing. The difference between uptake and egestion in the water is the egestion on land. The difference between Si flux upstream and downstream is the Si that is added to the water from the MMNR. Biogenic Si dissolution and clay mineral formation are calculated using a Si isotope mass balance model (see the Supplementary Materials). Concentrations are reported as mass of Si. t, metric tons.

  • Fig. 2 Dumbbell-shaped phytoliths (indicated with white arrows).

    These phytoliths were found in grass (A), hippo feces (B), and suspended matter and sediment (C). Phytoliths account for 98 to 99% of total biogenic Si particle counts in suspended matter and sediment samples (see also fig. S1 for more and different pictures).

  • Fig. 3 Changes in silicon concentrations after river water has passed through a hippo pool.

    Duration, 24 hours; length of reach, 250 m; up to 80 hippos were present. (A) Dissolved Si concentrations. (B) Biogenic Si concentrations. All concentrations are reported as mass of Si.

  • Fig. 4 Dissolution of reactive Si in samples of different river compartments after exposure to rain water during 24 hours (1440 min).

    Results are expressed as percentage (%) of the total reactive Si (reported as mass of Si) in the samples [biogenic Si in grass and feces and alkaline-extractable Si (AlkExSi) determined with the continuous analysis method (42) for other samples] and are plotted as mean values (symbols) with SDs (error bars; n = 5). Series are fresh feces (order of hours old), dried feces (order of days old), old feces (order of weeks old), grass, sediment, and savannah soil.

  • Fig. 5 δ30Si values (in ‰ relative to the NBS28 quartz sand standard) of different ecosystem compartments.

    Each compartment of the terrestrial-aquatic Si supply chain had a relatively narrow range of δ30Si values. Mean values (±1σ) in grass (−0.42 ± 0.89‰), hippo feces (−0.52 ± 0.71‰), and suspended matter (−0.04 ± 0.35‰) are in close agreement with each other, although suspended matter is somewhat higher. δ30Si values in sediment (0.93 ± 0.43‰), river water dissolved Si (1.30 ± 0.24‰), savannah soil (1.83 ± 0.08), and porewater dissolved Si (1.84 ± 0.33‰) are higher, with savannah soil and porewater clearly distinct from river water and sediment.

  • Fig. 6 Map of the field sites in the MMNR (Kenya).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/5/eaav0395/DC1

    Supplementary Materials and Methods

    Fig. S1. Biogenic Si particles observed in suspended matter and sediment samples.

    Fig. S2. Results from mass balance calculations.

    Fig. S3. Results from the rain water batch dissolution experiment.

    Table S1. Abiotic data and results of chemical Si analyses.

    Table S2. Summary of Si isotope analyses presented in this paper.

    References (4460)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Materials and Methods
    • Fig. S1. Biogenic Si particles observed in suspended matter and sediment samples.
    • Fig. S2. Results from mass balance calculations.
    • Fig. S3. Results from the rain water batch dissolution experiment.
    • Table S1. Abiotic data and results of chemical Si analyses.
    • Table S2. Summary of Si isotope analyses presented in this paper.
    • References (4460)

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