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Many species appear to selectively use tidal currents for transport into or out of estuaries, or to maintain residence within an estuary. Further, some species are preferentially active during nocturnal hours, possibly to reduce their vulnerability to predators. The interaction between predicted tidal current flow and the day-night cycle was investigated for 11 locations within the United States. Tidal current constants for each of these were obtained from the United States National Ocean Survey. Additionally, times of sunrise and sunset were computed for each location and day between the years 1978 and 2000. These sets of information were combined by integrating the predicted current velocity between successive times of sunrise or sunset or times of slack current for each day and location. The result is the average daily net transport possible at that time and location. Plots of these transport values reveal seasonal patterns when either transport in a preferred direction or retention is more readily accomplished. Such calculations do not incorporate other influences on tidal currents, such as variations in winds or freshwater discharge, but they do reveal the consistent seasonal and regional opportunities for transport or retention that a species would encounter over long time spans. |
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