Direct evidence of tidally oriented behavior of the copepod Eurytemora affinis in the Seine estuary

The calanoid copepod Eurytemora affinis dominates the zooplankton communities in most northern hemisphere estuaries. A recurrent behavior noticed in several estuaries suggests that this species maintains its horizontal position through vertical migration. In order to investigate this behavioral strategy in detail, we sampled E. affinis nauplii, copepodids and adults at high frequency at a fixed point in the low salinity zone of the Seine estuary. We took water samples in sub-surface and near bottom during a 12- and 50-h tide cycle, in May 2002 and May 2004, respectively. Since nauplii have weak swimming capabilities compared to copepodid and adult stages, we proposed here to consider them as passive particles. The variation of the relative abundances of nauplii to older stages regarding the vertical position was therefore viewed as direct evidence of the behavior of later stages. We showed that copepodids and adults tried to stay close to the bottom during the ebb, were re-suspended in the water column 1 h after the low tide during the maximum current velocity, and then vertically migrated back to the bottom layer when current velocity decreased. We also showed that copepodids behaved as adults, with only a lower velocity, taking more time to go near the bottom.