The effect of settling velocity on the transport of mussel larvae in a cobble‐bed river: Water column and near‐bed turbulence

Abstract

Simple transport models predict that the distance small organisms such as larvae “drift” downstream in rivers is determined by their settling velocity, the release height, and the stream velocity. However, natural turbulent conditions in a river may also affect the downstream transport and dispersion (spread) of larvae. The main goal of this study was to examine how stream velocity and larval settling velocity (Mucket [ Actinonaias ligamentina ] and the Wavy‐rayed Lampmussel [ Lampsilis fasciola ] differ by 2.5 times) affect the transport of freshwater unionid mussel larvae in the Grand River, Ontario, Canada. Larvae were released and captured in a series of nets downstream. Larval spread in rivers appeared to be strongly affected by stream flow conditions. Larvae were spread more rapidly with increased stream velocity likely due to increased turbulence in the water. Overall there was a good agreement between measured downstream decrease in capture of larvae and predictions from a 3‐dimensional advection–diffusion model that considered the spread due to hydrodynamics. However, in contrast to the predictions of simple transport models, differences in settling velocity had no detectable effect on the transport of larvae. Future studies are necessary to further explore the role of settling velocity and other factors under different stream flow conditions, which may also be important for dispersal of other organisms and particles. Simple transport models predict that the distance organisms drift downstream in rivers is determined by their settling velocity ( w s ), the release height ( z r ), and the stream velocity ( U ). Unfortunately, empirical evidence is lacking on whether and how factors such as w s affect mussel larvae dispersion in rivers under natural turbulent conditions. The main goal of this study was to examine how U and w s 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 w s (0.9 ± 0.02 [mean ±  SE] and 2.2 ± 0.02 mm s − 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 w s . Further studies are needed to better understand the effect of differences in w s and z r under different hydrodynamic conditions, which may also be important for other dispersal phenomena.