Measurements and consequences of retention in a side embayment in a tidal river

Embayments and tidal flow can affect the population structure of organisms, such as the zebra mussel (Dreissena polymorpha), that have a planktonic larval phase. To examine the potential retention of larvae in embayments in tidal rivers and predict the effect of embayments on transport in the river, a field experiment was conducted and a quasi one-dimensional model was developed. Dye was injected into North Tivoli Bay, which is adjacent to the tidal portion of the Hudson River, at the start of a flood tide, and measurements of concentration, stage, and velocity over 1.5 tidal cycles were used in a mass balance to estimate the fraction of dye mass in the bay. After the first ebb, 27% of the dye remained in the bay, and during the following flood the dye that returned increased the mass in the bay to 42% of the injection mass. The field measurements were used to calibrate a quasi one-dimensional model consisting of a series of well-mixed cells with a well-mixed bay adjacent to one of the river cells. The model predicts that a small fraction of larvae would be retained in the bay by the time larvae were competent to settle, but self-recruitment may be possible due to large numbers of larvae. If enough larvae are initially produced, recruitment in the river near the spawning site is possible because the small net southward flow typical of the spawning season provides little downstream transport of the peak of the dye cloud.