The effect of settling velocity on the transport of mussel larvae in a cobble-bed river: Water column and near-bed turbulence
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Simple transport models predict that the distance organisms drift downstream in rivers is determined by their settling velocity (ws), the release height (zr), and the stream velocity (U). Unfortunately, empirical evidence is lacking on whether and how factors such as ws affect mussel larvae dispersion in rivers under natural turbulent conditions. The main goal of this study was to examine how U and ws affect the transport of freshwater unionid mussel larvae (glochidia) in a turbulent reach of the Grand River, Ontario, Canada. Glochidia of Actinonaias ligamentina and Lampsilis fasciola, which had a 2.5-fold difference in their ws (0.9 – 0.02 [mean – SE] and 2.2 – 0.02mms-1, respectively), were released and captured in a series of drift nets downstream. Larval dispersion in rivers appeared to be strongly affected by hydrodynamic conditions. The results indicated that glochidia are dispersed more rapidly with increased U. This is likely due to increased turbulence and lateral and vertical mixing, which were consistent with the predictions of a 3-dimensional advection–diffusion model. The decline of glochidia with distance was well described with an inverse power function, but only on days when the average U measured at 40% water depth was .40 cm s-1. In contrast to the predictions of simple transport models, the observed downstream transport did not differ significantly between glochidia with different ws. Further studies are needed to better understand the effect of differences in ws and zr under different hydrodynamic conditions, which may also be important for other dispersal phenomena.