13.4 Larval Settlement and External Fertilization
413
--- 50
~
S 0
.... .... 0
..0 40
.c
~
~
8 30
II.l
S
.;::
I::
ell
II.l
20
::E
10
0.0
0.2
0.4
0.6
0.8
1.0
l.2
1.4
1.6
1.8
2.0
Height above substratum (m)
Fig. 13.7: Mean larvae settlement time as a function of height above substratum
for various friction velocities
A mean time, T, of settlement can be found by averaging over the heights
from Zl = 0 to Zl = h:
- 1loh
h
T = -
T(Zl)dz1 = 0.75-.
h 0
K,U.
(13.34)
As should be expected, the average time, T, increases directly with water depth
and inversely with the friction velocity. For a surf zone, when the turbulent
diffusion coefficient, K z , is given by Eq. (13.33), the mean time T becomes:
-
h
T = 2.15-.
K,U.
(13.35)
In fact, the average velocity with which the larva or spore is transported to the
substratum by turbulent mixing alone is:
(13.36)
In the surf zone, the friction velocity K,U. varies from 0.7 m/s to 1 mis, which
is much higher velocity than the typical sinking velocity of planktonic larvae
(typically 0.001-0.01 m/s).