Table 1.
Details moored Teledyne RDI 75-kHz, 20°-beam angle Longranger ADCPs.
Figure 1.
Overview of relative echo intensity data (dI), amount of plankton, and vertical current data (w), movement of plankton, from an upward looking ADCP in the sub-tropical Canary Basin (at 22.5°N, 27°W, mooring LOC163 (Table 1), ranging vertically between z = −1476 and −886 m.
Below −1150 m noise is increased, due to lower amounts of plankton once they have migrated upwards after dusk. Time in 2007 is yearday +365. (a) Entire 18 months of depth versus local solar time series of plankton, or echo intensity, relative to its time mean. (b) Monthly averaged summer time data from white rectangle in a. Symbols ‘s’ indicate local sunrise and sunset, ‘t’ nautical twilight and ‘a’ astronomical twilight. (c) Month of July raw data (rectangle in a.) of which b. is the average. (d) as c., but for w. (e) as b., but for w.
Figure 2.
DVM-variations in day length inferred from raw data of Figure 1 at z = −1400 m, grouped in 24 hour (daily) periods.
(a) Relative echo intensity. (b) Vertical current; note the much clearer upward migration of the plankton, confined in a shorter period, compared with the less clear downward migrations. In both panels, the black lines denote times of local sunrise and sunset.
Figure 3.
Seasonal DVM-variations in day length experienced by the plankton, as calculated in half-overlapping steps from monthly data as in Figure 1b (o) and Figure 1e (*) for z = −1021 m.
Thick solid lines indicate the times of true local day length determined from the sun. (a) Total day length; dashed line is the solid line multiplied by a factor of 77%. This factor is the average day length determined from plankton DVM, being slightly larger for dI-determined day length (o) and slightly smaller for w-determined day length (*). (b) Near sunrise and sunset.
Figure 4.
(a) As Figure 3a, but for z = −587 m at the equator (0°N, 37°W, mooring LOC144 (Table 1). (b) Composite mean relative echo intensity from upward looking ADCP at (0°N, 37°W) between −973 and −483 m and downward looking ADCP at (1°N, 38°W, mooring LOC164 (Table 1) between −1075 and −1565 m. (c) The associated sunrise and sunset profiles with depth with respect to the minimum morning and maximum afternoon values in dI, or plankton, variation (first derivative of, black lines). The filled blue fields indicate errors. For reference, similar profiles have been computed from minimum morning and maximum afternoon w (red dashed lines). Times of local sunrise and sunset are followed (to within errors) down to about −650 m; for z<−650 m the times between minima and maxima for both d(dI) and w decrease roughly linearly with increasing depth; at large depths (z<−1400 m) errors become large, especially for w for which values become unrealistic.
Figure 5.
Latitudinal dependence of DVM-amplitude of seasonal variation in day length determined from (derivative of) daily relative dI-values (o, derivative of) and from daily w-values (*) around 1000 m (except 1400 m for mooring LOC113 at 33°N) using harmonic analysis on a one-year periodicity (single frequency of 1/365 cpd) for all moorings in Table 1.
(a) Day length amplitudes. The red graph indicates the suns variation in day length according to equation (2). The dashed red line indicates the same graph shifted by 0.6 hours (see text). Note that at 1000 m the average acoustics data residual is 77% (9.3 hours) of the sun's day length. (b) Normalized residual variance, following harmonic analysis. High values are expected near the equator, where a yearlong harmonic is expected to be small. For those data, vertical averages over 200 m were computed and an expected reduced “error” is indicated by purple symbols. Similarly, averages over 4 bins are computed for data from 15°N, which showed unexpected high normalized residual variance.
Figure 6.
Two typical snapshots of raw one-day period plankton movements (w-data) around −1000 m at mooring LOC164 (22.5°N, 27°W) for winter and summer.
In winter, the more diffuse downward migration peaks later than expected as compared with the summer-data. The discrepancy between the absolute time difference between plankton movement (w-peaks) in summer and winter (dashed lines in lower panel) is larger in the morning than in the evening. This explains the larger amplitude of DVM-variations with day length observed in Figure 5 for peak-w (*) by about 40 minutes on average.