Deutsche Hydrographische Zeitschrift - German Journal of Hydrography 
154 
b) Short-term moorings MK1 and MK2 
The time covered by MK1 and MK2 proved to 
be a phase of very slow deep currents. Rotor stalls 
range between 46 and 89%, i.e. velocities were 
smaller than 1-1.5 cm/s which is the threshold velo 
city of the rotor (see Table 1). A comparably high 
percentage of rotor stalls was also observed by 
Kontar and Sokov [1994] in the Clarion-Clipperton 
province of the northeastern tropical Pacific. Their 
-20 -10 0 10 20 
Fig.5: Progressive vector diagram of moorings MK1 (a) 
and MK2 (b). Time marks (x) are given every 5 
days, the numbers at the tracks give the di 
stance from bottom in metres. 
current measurements (55 days, sampling interval 
15 minutes) give comparable mean values and rotor 
stalls of 50% at 15 mab, and of 80% at 5 mab. 
At MK1, the mean velocity magnitudes are bet 
ween 0.3 and 2.1 cm/s, with maximum velocities 
ranging from 4.3 cm/s at 0.6 mab to 6.7 cm/s at 50 
and 503 mab. The PVD shows a mainly southward 
flow (Fig.5a). Time marks (x) are given every 5 
days. The spacing of the time marks indicates a re 
latively regular flow between 50 and 500 mab. At 13 
mab, the flow is extremely weak and there is a pro 
nounced directional shear during the last 2 weeks of 
the record. 
At MK2, mean magnitudes range from 0.9 to 
2.4 cm/s, with maxima of 7.2 cm/s at 4 mab, and 
11 cm/s at 503 mab. The PVD reveals a strong di 
rectional shear between 13 and 50 mab and bet 
ween 200 and 500 mab (Fig. 5b). The spacing of the 
time marks is less regular than for MK1 and indi 
cates the intermittent character of flow at this posi 
tion. 
Time series of all current meters (D1, MK1, 
MK2) are presented by Klein [1996]. 
The BBL and its sublayers 
The term ‘bottom or benthic boundary layer’ is 
not standardized. Sometimes it is used as a sy 
nonym for bottom mixed layer, e. g. Armi and Mil 
lard [1976], or it is used in the biological sense as 
'... the sediment community and assemblage of or 
ganisms in the overlaying water column associated 
with the bottom (within 100 m of the seabed)’ (Smith 
and Hinga [1983]). Here, the term BBL is used for 
that part of the water column which is in any way af 
fected by the the seabed. Theoretically, the BBL 
can be subdivided into several sublayers which are 
distinguished by their hydrographical characteristics 
and/or their dynamical structure. Not all sublayers 
can be resolved by these measurements, and not 
all layers are necessarily fully developed or even 
existent. Nevertheless, if there is information about 
bottom roughness and near-bottom currents, the 
properties of the sublayers described below can be 
estimated by theoretical approaches.