The effects of haloclines on the vertical distribution and migration of zooplankton

While the influence of horizontal salinity gradients on the distribution and abundance of planktonic organisms in estuaries is relatively well known, the effects of vertical salinity gradients (haloclines) are less well understood. Because biological, chemical, and physical conditions can vary between different salinity strata, an understanding of the behavioral response of zooplankton to haloclines is crucial to understanding the population biology and ecology of these organisms. We studied four San Francisco Bay copepods, Acartia (Acartiura) spp., Acartia (Acanthacartia) spp., Oithona davisae, and Tortanus dextrilobatus, and one species of larval fish (Clupea pallasi), in an attempt to understand how and why zooplankton respond to haloclines. Controlled laboratory experiments involved placing several individuals of each species in two 2-m-high tanks, one containing a halocline (magnitude varied between 1.4 and 10.0 psu) and the other without a halocline, and recording the location of each organism once every hour for 2-4 days using an automated video microscopy system. Results indicated that most zooplankton changed their vertical distribution and/or migration in response to haloclines. For the smaller taxa (Acartiura spp., Acanthacartia spp., and O. davisae), this behavior took the form of accumulating in or below the halocline, while the effects on the larger species (C. pallasi and T. dextrilobatus) were more subtle. C. pallasi yolk sac and 3- to 6-day-old larvae seemed to pause or remain in the halocline during their diel migration, while 14- to 17-day-old larvae appeared to avoid the halocline by remaining in deeper, more saline water. There were very few statistically significant effects of haloclines on the vertical distribution of T. dextrilobatus. Subsequent mortality experiments with Acartiura spp., Acanthacartia spp. and T. dextrilobatus indicated that the behavioral changes seen in the halocline studies were not associated with any salinity-induced mortality per se, although more subtle affects of physiological stress could not be ruled out. These results point to a high degree of flexibility in vertical migration behavior within a given species as well as large variation between species. Such behavioral flexibility is likely to be very important in allowing planktonic organisms generally, and estuarine organisms in particular, to maintain or alter position relative to currents, food, and predators.