Implications of Drift Behavior for Daytime Densities


Sites close to the Modoc Point Road boundary and the Williamson River channel showed a strong diel cycle in simulated density. At some sites, different assumed drift behaviors had a marked effect on that diel cycle. Site A (fig. 9) is within about a one-half day of travel time from the Modoc Point Road boundary and within a few hours travel time from the Williamson River channel. If larval drift was assumed to occur only at night throughout the length of the Williamson River channel, then simulated larval densities at that site reflected the same day–night cycle, such that maximum densities were simulated at night. This held true if the nighttime-only drift behavior was assumed to be limited to that part of the channel upstream of the Delta. If instead nighttime-only larval drift occurred upstream but not downstream of the Modoc Point Road boundary, then the maximum simulated densities at site A occurred during the day. Site B is located several hours travel time farther from the Williamson River channel, and simulated concentrations there tended to be a maximum during daylight hours, regardless of the assumed drift behavior in the Williamson River channel (fig. 10).


Farther from the channel, in Goose Bay, the extent of larval drift in the channel was manifested as a varying strength of the diel cycle in the simulated density at site D (fig. 11). When nighttime-only drift behavior was assumed to persist throughout the channel, a large diel cycle was simulated at this site, but when nighttime-only drift behavior was assumed to occur only upstream of the Delta or upstream of the Modoc Point Road boundary, the diel cycle at this site was diminished. At sites located in Tulana (site 25980) and at the mouth of the Williamson River (site 25981), the difference in assumptions about nighttime-only drift behavior is not discernible in the simulated densities (not shown).


First posted April 2, 2012