Physical and Biological Processes Determining the Distribution of Zooplankton in a Tidal Estuary

The zooplankton of a small estuary, Great Pond, has been described. It has been shown that in the summer there were endemic populations of several zooplankton of which the copepod Acartia was the most numerous and widespread. 2. The rates at which Acartia must have been replaced to maintain its numbers in the surface mixed layer of Great Pond in face of dispersion by the tidal exchanges have been calculated. These rates are compared with the rates at which Acartia increased or decreased when isolated in bottles immersed in the pond. 3. This comparison shows that in the most landward parts of the pond the population in the mixed layer could not reproduce fast enough to maintain itself and was dependent on recruitment from the deeper layer. In the central part of the pond reproduction alone was sufficient to offset the seaward transport by the tidal exchanges. In the most seaward parts of the pond, mortality was so great that the small population was maintained only by transport from the landward parts of the pond. 4. Thus the comparison reveals the contribution of reproduction, growth and mortality and of tidal transport to the maintenance of the observed distribution of Acartia. Seasonal changes in these physical and biological processes account for the seasonal changes in the distribution of zooplankton in Great Pond.