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Figure 12.7 Chlorophyll a concentrations in a coastal marine environment, along a transect 8 naut. miles long in St. Margaret's Bay (Nova Scotia, Canada). (a) Data series exhibiting a linear trend, and (b) correlogram of the detrended series where lags (abscissa) are given as distances along the transect. After Platt et al. (1970).

the two sampled variables is obvious. Correlograms for the nonfiltered (Fig. 12.8b) and filtered (Fig. 12.8c) series clearly show the same periodic signal, of ca. 8 lags x (15 days lag-1) = 120 days. Nonfiltered data provide the same information as the filtered series, but not quite as clearly. According to the authors, these periodic variations could be an example of free oscillations, since they seem independent of any control by the environment which is stable all the year round. The same ecological application will be used again to illustrate the calculation of cross-correlation (next Subsection) and Schuster's periodogram (Section 12.4).

Trich. Chl a

Trich. Chl a

Figure 12.8 (a) Filtered time series of chlorophyll a and Trichodesmium in tropical surface waters off Barbados. Marks along the abscissa are spaced by 75 days. On the ordinate, units are 103 filaments Trichodesmium L-1 and |J,g chlorophyll a L-1. Correlograms of (b) the nonfiltered series and (c) the filtered series. After Steven & Glombitza (1972).

Figure 12.8 (a) Filtered time series of chlorophyll a and Trichodesmium in tropical surface waters off Barbados. Marks along the abscissa are spaced by 75 days. On the ordinate, units are 103 filaments Trichodesmium L-1 and |J,g chlorophyll a L-1. Correlograms of (b) the nonfiltered series and (c) the filtered series. After Steven & Glombitza (1972).

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