Figure 12.17 Coherence spectra between pairs of variables sampled along a transect 8 nautical miles long in St. Margaret's Bay (Nova Scotia, Canada). Dashed lines: approximate 95% confidence limits. After Platt et al. (1970).
In order to identify the factors controlling the spatial heterogeneity of marine phytoplankton (patchiness), Denman & Platt (1975) analysed values of chlorophyll a and temperature, recorded continuously along a transect in the St. Lawrence Estuary. Two pumping systems were towed, at depths of 5 and 9 m, over a distance of 16.6 km (10 nautical miles). The sampling interval was 1 s, which corresponds to 3.2 m given the speed of the ship. After detrending, computations were carried out using the Fast Fourier Transform. Four coherence and phase spectra were calculated, as shown in Fig. 12.18.
For a given depth (Fig. 12.18a: 5 m; b: 9 m), the coherence between temperature and chlorophyll a was high at low frequencies and the phase was relatively constant. At higher frequencies, the coherence decreased rapidly and the phase varied randomly. The lower panels of Fig. 12.18 indicate the absence of covariation between series from different depths. The authors concluded that physical processes played a major role in the creation and control of phytoplankton heterogeneity at intermediate scales (i.e. from 50 m to several kilometres). Weak coherence between series from the two depths, which were separated by a vertical distance of only 4 m, suggested the presence of a strong vertical gradient in the physical structure. Such
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