SINK OR SWIM - COPEPOD POPULATION MAINTENANCE IN THE COLUMBIA RIVER ESTUARINE TURBIDITY-MAXIMA REGION

Maintenance of estuarine zooplankton populations in large river-domina ted estuaries with short residence times has been an intriguing subjec t of investigation. During three different hydrological seasons, autum n 1990, summer 1991, and spring 1992, we intensively sampled zooplankt on populations in the estuarine turbidity maxima (ETM) region of the C olumbia River estuary of Oregon and Washington, USA. One of the princi pal objectives was to investigate retention mechanisms of the predomin ant 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 a lmost 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 analyse s determined that C. canadensis densities were highly positively corre lated with turbidity and that most of the variability was explained by the lunisolar diurnal (K-1) and principal lunar (M-2) tidal component s occurring once every 23.93 h and once every 12.42 h, respectively. T his indicates that C. canadensis populations are most probably maintai ned in the estuary through the same near-bottom circulation features t hat trap and concentrate particles in the ETM. In contrast, densities of the more motile species E. affinis were highly correlated with nega tive velocities, or ebb tide, and most of the variability in populatio n densities could be explained by the principal lunar tidal component; therefore, we hypothesize that this species is probably vertically mi grating on a tidal cycle into different flow layers to avoid populatio n losses out of the estuary.