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The effect of tidal transport on the zooplankton population of a local inlet Thomas, Andrew Charles


A series of cruises was made over 14 months to determine the effect of physical transport on the zooplankton of Indian Arm, whose deep water is separated from the marine influence of the Strait of Georgia by a series of shallow sills. The dominant transport process during the study period was tidal exchange. The topographic features of the inlet, coupled with density stratification found over the year, restricted the exchange of water and plankton to a surface phenomenon. Four copepod species were sampled using horizontally towed Clarke-Bumpus nets and vertically towed SCOR nets and one meter conical nets. These were analysed to determine the relationship between different life history patterns and tidal exchange, and the effect of this relationship on the populations of these species inside Indian Arm. Corycaeus anglicus is a surface water to mid depth organism found in highest numbers in Vancouver Harbour. The population density reaches a peak in the fall and remains high throughout the winter with transport concomittant with this peak. Euchaeta japonica exhibits ontogenetic depth preferences, nauplii and Stage I copepodites are found mainly in deep water below 200 meters, Stages II-IV are found mainly in shallow water, and Stages V and VI (adults) are found scattered over most of the water column. This species reproduces throughout the year in both Indian Arm and the Strait of Georgia. Euchaeta japonica was transported mainly as the Stage III copepodite and primarily during the winter months despite the fact that the species is found in surface waters in large numbers at other times of the year, thus producing an isolated population in Indian Arm during the summer. Metridia pacifica is a strong diel migrator found extensively in the upper 50 meters at night, and from 250 meters to 50 meters during the day. Transport of the species across the sill occurs at all times of the year but is significantly greater at night, regardless of the phase of tide. Eucalanus bungi is an ontogenetic migrator, overwintering in deep water (greater than 150 meters), and coming to the surface in the spring to spawn; juvenile stages are found in surface waters during the summer. The data show that this species is transported only during the summer while it is in surface water. Consequently nauplii and younger copepodites are the dispersal stages. The data suggest that the species does not reproduce in Indian Arm and that transport of the species during the summer months can account for the entire overwintering population found in the inlet. An analysis of the correlation of changes in the zooplankton community with physical parameters varying over the tidal cycle was made. Species known to migrate dielly show significant differences in numbers between day and night samples. Relatively few species show differences which can be correlated to the direction of tidal movement. The most significant changes seen in the zooplankton community occur in association with changes in hydrographic properties. Moreover, these changes are manifested not at the species level, but at the community level as changes in such parameters as diversity and dominance. The effect of transport on the zooplankton community in Indian Arm varies from species to species. Qualitatively, the effect depends upon the organisms' life cycle including a surface dwelling stage, and/or its ability to survive the surface water transport conditions. Quantitatively, the effect depends on the amount of time spent in surface water, which varies with the depth distribution of the organism, determined by its behavioral characteristics. The overall effect of tidal exchange will be to drive towards equilibrium the population of zooplankton found in Indian Arm and the Strait of Georgia. It is the biology of the individual species which determines the extent of interaction with this transport process and hence the amount of exchange which takes place.

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