The 78 h salinity record ranges between 35.83 and 35.93 psu (Fig. 11.7b), which
is three times more than the variability recorded in the repeated yoyo CTD casts
(Fig. 11.5). The offset in absolute psu values between these two records results from
the almost 20 m deeper location of the lander. A similar pattern is displayed in the
temperature record varying between 10.55 and 10.28
C (Fig. 11.7c). Both, salinity
and temperature do not exhibit such a clear semi-diurnal pattern (M2) coeval to the
tidal signal recorded as pressure variability. Salinity maxima are exactly in parallel
to temperature minima, including extreme values and they occur always during the
short period between high and low tide and vice versa. However, T_tide frequency
analysis indicates a distinct M4 signal which originates from shallow water
overtides of principle lunar constituent exhibiting a signal noise ratio of 2.1e + 03
and 2.4e + 03 for salinity and temperature, respectively. While minima in salinity
and maxima in temperature plot preferentially during low tide and high tide,
warmer temperature and less saline waters prevail for longer time windows than
the more pronounced and frequently shorter pulses of cooler and more saline
waters.
The tidal current analysis was again performed using T_Tide (Pawlowicz et al.
2002). The ADCP data of 78 h deployment time (Fig. 11.7d, flow direction is given
by vector) show a current system that is dominated by currents from the WNW and
the ESE. Displayed are the currents at bin 13 (first bin 0-0.88 m, bin size 0.5 m),
6.88 m above the device. Within the 95% confidence level, the principal tidal
constituents that influence current dynamics at the deployment site are M2 (principle lunar semidiurnal), M3 (lunar terdiurnal), 2MK5, and M6 (principal lunar
shallow water overtides). Most prominent is the M2 (principal lunar semidiurnal
constituent) tide with a sound noise ratio of 260 and a direction of 150
, followed by
M3 (lunar terdiurnal constituent) with a sound noise ratio of 35 and a direction of
130
. Generally, current speeds do not exceed 0.1 m/s with mean values around
0.03 m/s (Fig. 11.8c). Overall, a complex circulation pattern emerges that is
controlled by various frequencies.
11.4 Discussion and Conclusion
Yoyo CTD casts are a tool to document the existence of e.g. a semi-diurnal tides
influencing benthic communities in deeper water settings. However the signal is
very rough and any other signals than the prominent M2 can not be resolved. The
distinct differentiation between the various tidal constituents is only possible with a
lander system equipped with high resolution data sampling devices. Moreover,
repeated CTD casts may disturb the signal of the water masses due to the weight
and the size of the instrument passing through the water and creating small scaled
turbulences causing mixing. This is demonstrated in the larger amplitude between
minima and maxima in both temperature and salinity recorded in the lander data in
contrast to the lower variability recorded by the yoyo-casts. Moreover, single casts
11 Water Mass Measurements Around Benthic Communities: A Comparative Study. . .
193
is three times more than the variability recorded in the repeated yoyo CTD casts
(Fig. 11.5). The offset in absolute psu values between these two records results from
the almost 20 m deeper location of the lander. A similar pattern is displayed in the
temperature record varying between 10.55 and 10.28
C (Fig. 11.7c). Both, salinity
and temperature do not exhibit such a clear semi-diurnal pattern (M2) coeval to the
tidal signal recorded as pressure variability. Salinity maxima are exactly in parallel
to temperature minima, including extreme values and they occur always during the
short period between high and low tide and vice versa. However, T_tide frequency
analysis indicates a distinct M4 signal which originates from shallow water
overtides of principle lunar constituent exhibiting a signal noise ratio of 2.1e + 03
and 2.4e + 03 for salinity and temperature, respectively. While minima in salinity
and maxima in temperature plot preferentially during low tide and high tide,
warmer temperature and less saline waters prevail for longer time windows than
the more pronounced and frequently shorter pulses of cooler and more saline
waters.
The tidal current analysis was again performed using T_Tide (Pawlowicz et al.
2002). The ADCP data of 78 h deployment time (Fig. 11.7d, flow direction is given
by vector) show a current system that is dominated by currents from the WNW and
the ESE. Displayed are the currents at bin 13 (first bin 0-0.88 m, bin size 0.5 m),
6.88 m above the device. Within the 95% confidence level, the principal tidal
constituents that influence current dynamics at the deployment site are M2 (principle lunar semidiurnal), M3 (lunar terdiurnal), 2MK5, and M6 (principal lunar
shallow water overtides). Most prominent is the M2 (principal lunar semidiurnal
constituent) tide with a sound noise ratio of 260 and a direction of 150
, followed by
M3 (lunar terdiurnal constituent) with a sound noise ratio of 35 and a direction of
130
. Generally, current speeds do not exceed 0.1 m/s with mean values around
0.03 m/s (Fig. 11.8c). Overall, a complex circulation pattern emerges that is
controlled by various frequencies.
11.4 Discussion and Conclusion
Yoyo CTD casts are a tool to document the existence of e.g. a semi-diurnal tides
influencing benthic communities in deeper water settings. However the signal is
very rough and any other signals than the prominent M2 can not be resolved. The
distinct differentiation between the various tidal constituents is only possible with a
lander system equipped with high resolution data sampling devices. Moreover,
repeated CTD casts may disturb the signal of the water masses due to the weight
and the size of the instrument passing through the water and creating small scaled
turbulences causing mixing. This is demonstrated in the larger amplitude between
minima and maxima in both temperature and salinity recorded in the lander data in
contrast to the lower variability recorded by the yoyo-casts. Moreover, single casts
11 Water Mass Measurements Around Benthic Communities: A Comparative Study. . .
193