Size matters: Effects of propagule size on dispersal in rivers

Biological particles (e.g., bacteria, eggs, fruit/seeds, and larvae) of a
wide range of sizes (i.e., 10-6 – 10-1 m) are transported over various
distances (i.e., 100 - 104 m) downstream in rivers.  We examined
the effects of propagule size on downstream dispersal by releasing
biodegradable microbeads (density ~ 1200 kg m-3) of three size classes
(~150, 250, 350 µm) at the Speed River, Guelph, ON.  Hitting
distance estimates and longitudinal dispersion coefficients declined with
particle size and were significantly different between 150 and 350 µm
microbeads. The magnitude of these differences was relatively small (~ 5
m) because of the slow velocity (9.5 ± 0.01 cm s-1) and low turbulence
(shear velocity = 1.9 ± 0.13 cm s-1) in the river.  We examined
the dispersion of larval and juvenile unionid mussels (size range = 56 –
415 µm, 247.54 ± 60.38 [mean ± SD] µm, N = 174) across a broader range of
flow conditions by applying laminar and turbulent flow models in three
river reaches of increasing velocity and turbulence.  Model
results indicated that the dispersal of smaller larvae increased
disproportionately with increasing water column turbulence.
 Given that the peak in the size frequency distribution of larvae
and juveniles corresponded to Rouse numbers (ratio of gravitational
settling to water column turbulence) P < 1, we suggest a trade-off
in propagule size in the taxon, whereby the increased dispersal of smaller
juveniles may come at the cost of reduced settlement success in turbulent
environments.