Zooplankton sample variability in a tidal estuary: An interpretative model1

Abstract

A series of 347 vertical zooplankton hauls was made at a 175‐h anchor station in the upper St. Lawrence estuary, Québec, and current meter profiles were recorded. The 95% confidence interval of a single observation in most of the copepod time series was about 25–400% of the mean. Temporal variability consisted for the most part of semidiurnal fluctuations superimposed on a long term trend. Other periodic components were also evident. Most components were related to the advection, at discrete frequencies, of longitudinal zooplankton gradients of different magnitude and sign. The relationship was generalized by a mathematical model which shows that a distinct component of sample variability can be found at all frequencies of the current tidal constituents, provided that a longitudinal gradient is present. The relative magnitude of the variability components, for a constant gradient slope in both time and space, is identical to that of the amplitude/frequency product of the current tidal constituents. Other components not explained by the model can be related to periodic vertical displacements of the pycnocline caused by the interaction of an internal tide with a sill in the sampling area.