Sink or swim? Copepod population maintenance in the Columbia River estuarine turbidity-maxima region

Maintenance of estuarine zooplankton populations in large river-dominated estuaries with short residence times has been an intriguing subject of investigation. During three different hydrological seasons, autumn 1990, summer 1991, and spring 1992, we intensively sampled zooplankton populations in the estuarine turbidity maxima (ETM) region of the Columbia River estuary of Oregon and Washington, USA. One of the principal objectives was to investigate retention mechanisms of the predominant zooplankton species, the harpacticoid copepod Coullana canadensis and the epibenthic calanoid copepod Eurytemora affinis. In the ETM, C. canadensis densities mirrored those of turbidity gradients and were almost always greater at the river bed, while E. affinis densities were greater higher in the water column during the flood and lower in the water column during the ebb. Cross-correlation and time-series analyses determined that C. canadensis densities were highly positively correlated with turbidity and that most of the variability was explained by the lunisolar diurnal (K1) and principal lunar (M2) tidal components occurring once every 23.93  h and once every 12.42 h, respectively. This indicates that C. canadensis populations are most probably maintained in the estuary through the same near-bottom circulation features that trap and concentrate particles in the ETM. In contrast, densities of the more motile species E. affinis were highly correlated with negative velocities, or ebb tide, and most of the variability in population densities could be explained by the principal lunar tidal component; therefore, we hypothesize that this species is probably vertically migrating on a tidal cycle into different flow layers to avoid population losses out of the estuary.