The retention of larval rainbow smelt Osmerus mordax in the well-mixed, turbid upper estuary of the St. Lawrence River is achieved by active tidal vertical migrations. Time-series analysis of their vertical distribution obtained during two 98 h sampling series conducted in June and July 1986 revealed that larvae were near the surface during flood tides and nearer the bottom during ebbs. The nature of vertical migration changes with larval size. In June, young larvae concentrated at the surface during flood. Their subsequent descent appears to be a passive sinking and dispersal throughout the water column beginning after the flooding tide attains maximum speed. In July, older larvae concentrated during both flood and ebb and the amplitude of their vertical migration increased. Thus, older larvae use currents more efficiently as reflected by their average position in the estuary, which was farther upstream In July than in June, and by the dfferential larval size distribution along the estuary, larger larvae being located farther upstream than smaller larvae in July. Passive vertical transport was not responsible for the observed vertical movements as larvae concentrated in surface or bottom layers when mixing of the water column was maximal. Series of hourly average densities calculated from discrete depth samples revealed cyclic variations associated with tidal advection. As smelt larvae are advected by water masses, their longitudinal distribution was influenced by both active vertical migrations and hdal circulation in the estuary

Field studies on a variety of organisms have suggested that environmental variability plays a major role in determining spatial patterns in distribution and species diversity of estuanne organisms due to the effect of abiotic fluctuations on the physiology of animals. However, there is no study examining the effect of environmental variability on zooplankton distribution and diversity in estuaries. As vertical m~gration is obligatory behavior for the retention of planktonic animals at intermediate positions in the St. Lawrence Estuary (Canada), vertical stratification of the water column is considered the major source of environmental variability for zooplankton in this system. To evaluate the importance of this source of variability as a factor controlling the distribution and diversity of zooplankton in the estuary, we examined the relative contribution of each of the environmental factors of salinity, temperature, turbidity and vertical stratification in explaining spatial patterns of summer zooplankton distribution and diversity. Multivariate analyses revealed the presence of a longitudinal succession of seasonally stable species assemblages (tidal freshwater, true-estuarine and euryhalinemarine assemblages) whose spatial distribution was mainly a function of salinity and vertical stratification. Turbidity and temperature played a minor role in explaining spatial distribution. For all sampling periods, the limit between the true-estuarine and euryhaline-marine assemblages, the lowest number of population centers and the lowest zooplankton abundance all corresponded to the most vertically stratified waters. In contrast, population centers of all species were concentrated in the most abiotically stable parts of the estuary. We conclude that environmental variability is a major factor determining zooplankton distribution and diversity in the estuary. The spatio-temporal stability of the species assemblages and the seasonal variabihty in the abundance of some species also suggest that trophic interactions may play an important role in the regulation of zooplankton populations in the estuary. KEY WORDS: Zooplankton assemblages. Salinity. Turbidity. Temperature. Spatio-temporal structure 0 Inter-Research 1994 Resale of fuLJ article not permitted

ABSTRACT. Phytoplankton biomass, primary production and bacterial abundance and production were measured across the freshwater-saltwater transition zone of the St. Lawrence River [Canada) during the seasonal period of maximum concentrations of fish larvae and macrozooplankton. The estuarine front was characterized by steep gradients in b~olog~cal as well as physical properties. Maximum turbidity and high phytoplankton biomass occurred in the well-mixed, low salinity (0.2 to 4 psu) region immediately upstream of the salt wedge. Peak zooplankton and ichthyoplankton biomass occurred within and slightly downstream of this frontal region The hypothesis that lower food chain processes were controlled exclusively by allochthonous carbon and bacterial heterotrophy was not supported. Photosynthetic rates per unit chlorophyll a [chl a) remained high across the freshwater-saltwater transition and the low light penetration was offset by a shallow mean depth of mixing. Bacterial concentrations and activity remained relatively constant across the transition, while chl a declined sharply downstream of the front, consistent with grazing losses. Photosynthesis contributed 34 to 66% of the total production (bacteria + phytoplankton). Freshwater phytoplankton advected from upstream contributed another 20 to 30%. These first-order estimates underscore the combined importance of photosynthesis plus bacterial processes within the downstream food web of large river ecosystems.

ABSTRACT. We documented the vertical distribution, species composition, size and feeding of bivalve veligers in vertically stratified and well-mixed waters of the Baie des Chaleurs, Quebec, Canada. Larvae aggregated at certain depths in both types of water column. When a pycnocline was present, the mean depth of veligers was generally below it. Veligers' vertical distribution was not related to that of water chlorophyll a except at night in stratified conditions. In stratified waters, gut chlorophyll a content of larvae was associated with water chlorophyll a concentration. In mixed waters, larval gut

We tested the hypothesis that the active vertical migration of the larvae of anadromous rainbow smelt (Osmerus n~ordax) observed in the turbid, well-mixed portion of the St. Lawrence Estuary is an adaptation to improve feedlng incidence by retaining larvae in productive, up-estuary waters. We documented the vertical distribution, diet and feeding incidence of larvae of 3 length classes over 4 tidal cycles at each of 3 stations located along the longitudinal axis of the upstream portion of the estuary. The percentage of smelt larvae with gut contents increased with larval length and with distance upstream for all 3 length classes. Vertical migrations of larvae were not related to feeding incidence in the vertical plane, except in the case of the smallest length class, indicating that active vertical migrations were not behavioral responses to vertical changes in prey availability. The predominance of calanoid copepods and freshwater cladocerans in the diets of larvae with the highest feeding frequencies and the predominance of mysids in the diets of larvae with the lowest feeding frequencies suggested that larvae were more successful feeding on copepods and cladocerans than on the larger mysids. Apparent tidal rhythmicity in diet and feeding success was largely due to the tidal displacement of water masses at fixed sampling stations. Feeding incidence at the 2 sampling stations situated within the maximum turbiditv zone was greatest in larvae advected from upstream at the end of ebbing tide. We conclude that active vertical migration of smelt larvae is an adaptation to maximize the longitudinal retention of high densities of larvae in a zone of high prey biomass.