reaching the bottom waters only every hour for a short time may miss extreme data
values.
The comprehensive data recorded by the lander (Figs. 11.7 and 11.8) reveal
different tidal constituents among which the M4 is the most prominent one
governing the salinity and temperature variability (Fig. 11.7b, c). So far, this tidal
signal has not been described from other bottom water masses around benthic CWC
communities. Although both physical parameters vary almost with the same amplitude, the water masses seem to be dominated by the variability in salinity
(Fig. 11.7b), since it exactly parallels the variations in sigma-theta (σ Θ ) (Fig. 11.8a).
The tides migrate from the open Atlantic in the west towards the east in to the
Bay of Biscay. A high tide signal at Ria de Camari~ nas (43
07.578
0 North
009
10.934
0 West) occurs 20 min later at Gijo ´n (43
34.002
0 North 005
41.00
West) and 32 min later at Bayonne (43
31.812 North 001
31.950
0 West). This
general W-E pattern is slightly deflected on the shelf break and the various
escarpments along the Cantabrian Margin towards an ENE direction. This is clearly
seen in the current pattern (Figs. 11.7d and 11.8d) with ENE directions during rising
tide and WNW directions during falling tide. Although bottom water currents are
fairly low in contrast to data presented by Pingree and Le Cann (1989), there are
elevated horizontal velocities around high tides and reduced to almost current-less
conditions during low tide.
Variations in horizontal current velocities longer than 48 h seen in Fig. 11.8c
may be related to a lunar cycle. Salinity and temperature variations are smaller
during elevated horizontal velocities, which may be the result of enhanced mixing
of water masses. The pronounced parallel signal of saltier and cooler water indicates a clear advection of the deeper, cooler and more saline water, which is
ascribed to the upper MOW. The presence of this water mass is also responsible
for the steep gradient in salinity between 600 and 1000 mbsl displayed in Fig. 11.4,
which does not show up shallower nor deeper. This gradient, which is also a density
gradient, is a necessary prerequisite to concentrate nutrients, since their momentum
to sink into deeper water depth is reduced in these denser water masses. (Dullo et al.
2008; Fl€ ogel et al. 2014; Hebbeln et al. 2014). During times of reduced horizontal
velocities the signal of advected MOW is more pronounced due to reduced mixing.
Internal waves may result in similar pronounced peaks (Gill 1982; Jeans and
Sherwin 2001), however, they would have been recorded in a distinct increase in
vertical current velocity, which is not seen in the data.
Living frameworks of CWCs have not been observed, although physical parameters would argue for their occurrence (Dullo et al. 2008). However, we cannot
exclude their possible existence entirely, since we had limited ground truthing
information of the whole escarpment. The frequent findings of fishing lines and
broken skeletons of dead CWCs may indicate their destruction by industrial fishing
activities.
In conclusion, only high-resolution lander data highlight the dynamical environmental control of the bottom water mass around benthic communities which
have not been known to occur in such settings. Although, repeated CTD casts may
provide information about the amplitude in temperature and salinity variability our
11 Water Mass Measurements Around Benthic Communities: A Comparative Study. . .
195
values.
The comprehensive data recorded by the lander (Figs. 11.7 and 11.8) reveal
different tidal constituents among which the M4 is the most prominent one
governing the salinity and temperature variability (Fig. 11.7b, c). So far, this tidal
signal has not been described from other bottom water masses around benthic CWC
communities. Although both physical parameters vary almost with the same amplitude, the water masses seem to be dominated by the variability in salinity
(Fig. 11.7b), since it exactly parallels the variations in sigma-theta (σ Θ ) (Fig. 11.8a).
The tides migrate from the open Atlantic in the west towards the east in to the
Bay of Biscay. A high tide signal at Ria de Camari~ nas (43
07.578
0 North
009
10.934
0 West) occurs 20 min later at Gijo ´n (43
34.002
0 North 005
41.00
West) and 32 min later at Bayonne (43
31.812 North 001
31.950
0 West). This
general W-E pattern is slightly deflected on the shelf break and the various
escarpments along the Cantabrian Margin towards an ENE direction. This is clearly
seen in the current pattern (Figs. 11.7d and 11.8d) with ENE directions during rising
tide and WNW directions during falling tide. Although bottom water currents are
fairly low in contrast to data presented by Pingree and Le Cann (1989), there are
elevated horizontal velocities around high tides and reduced to almost current-less
conditions during low tide.
Variations in horizontal current velocities longer than 48 h seen in Fig. 11.8c
may be related to a lunar cycle. Salinity and temperature variations are smaller
during elevated horizontal velocities, which may be the result of enhanced mixing
of water masses. The pronounced parallel signal of saltier and cooler water indicates a clear advection of the deeper, cooler and more saline water, which is
ascribed to the upper MOW. The presence of this water mass is also responsible
for the steep gradient in salinity between 600 and 1000 mbsl displayed in Fig. 11.4,
which does not show up shallower nor deeper. This gradient, which is also a density
gradient, is a necessary prerequisite to concentrate nutrients, since their momentum
to sink into deeper water depth is reduced in these denser water masses. (Dullo et al.
2008; Fl€ ogel et al. 2014; Hebbeln et al. 2014). During times of reduced horizontal
velocities the signal of advected MOW is more pronounced due to reduced mixing.
Internal waves may result in similar pronounced peaks (Gill 1982; Jeans and
Sherwin 2001), however, they would have been recorded in a distinct increase in
vertical current velocity, which is not seen in the data.
Living frameworks of CWCs have not been observed, although physical parameters would argue for their occurrence (Dullo et al. 2008). However, we cannot
exclude their possible existence entirely, since we had limited ground truthing
information of the whole escarpment. The frequent findings of fishing lines and
broken skeletons of dead CWCs may indicate their destruction by industrial fishing
activities.
In conclusion, only high-resolution lander data highlight the dynamical environmental control of the bottom water mass around benthic communities which
have not been known to occur in such settings. Although, repeated CTD casts may
provide information about the amplitude in temperature and salinity variability our
11 Water Mass Measurements Around Benthic Communities: A Comparative Study. . .
195