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.M. Lawrie et al. J. Exp. Mar. Biol. Ecol. 242 1999 95 –106
column in each 5 cm depth class was recorded. For the initial experiment, six recordings were made at 10 min intervals for one set of individuals to assess the consistency of
their distribution pattern over time 60 min. Four further experimental runs were carried out with each gradient, recording the distribution of individuals twice 20 min intervals
for each run.
3. Results
3.1. Field studies At all states of the tide, Neomysis were distributed unevenly with depth Fig. 1. For
the majority of sampling occasions, individuals were aggregated into a band located just behind the tide edge in a water depth of
, 15–20 cm. The exception was during high spring tides when individuals occurred over a much broader depth range. Furthermore,
juveniles tended to gather in the shallower water in front of the adults At low tide, Neomysis were concentrated in the main channel, either in the shallow water at the
channel edge, at the sediment–water interface, or aggregated around physical structures, such as stones and or clumps of seaweed. Individuals were located in the lee of these
21
structures Fig. 2 where flow was typically 0–3 cm s .
1 ]
Flow rates measured at depth and maximum depth rapidly increased from shallow
3
to deep water during low water slack and ebb tide periods Fig. 3a–m. Flows of less
21
than 20 cm s were recorded only at the tide edge in relatively shallow water
, 15 cm
21
depth. Flows close to the sediment–water interface were always less than 10 cm s ,
and often close to zero. During rising and high-tide periods, flow rates in the water column were markedly lower and at high tide were often too small to register Fig. 3.
3.2. Laboratory studies In the experimental flume, the depth distributions of individuals were remarkably
consistent for each run Fig. 4a. At zero flow, 48 of the individuals were swimming in the water column between 2 and 20 cm from the bottom, with the remainder in the
, 2 cm section. As current velocity increased, the proportion of individuals in the bottom
section , 2 cm water depth also increased, with a consequent decline in the proportion
21
swimming in higher sections. At a flow of 7 cm s , more than 75 of individuals were
21
in the bottom section and at 10 cm s this had increased to more than 90. In other
words, as flow increased, more individuals moved towards the flume bed and the
21
boundary layer F 5 536, P , 0.0001. At flows greater than 10 cm s , individuals
4,174
were unable to maintain their position on the bottom of the tank and were washed backwards out of the experimental area.
It is possible that the observed changes in depth distribution with flow were due to entrainment of individuals into the low velocity boundary layer created by the floor of
the flume. To examine this hypothesis, another boundary layer was created by floating a solid wooden plate fixed between the collimators at the water surface. At low current
21
velocities 4 cm s more Neomysis swam in the upper section of the water column
S .M
. Lawrie
et al
. J
. Exp
. Mar
. Biol
. Ecol
. 242
1999 95
– 106
99
2
Fig. 1. Numbers of Neomysis recorded from 45 cm diameter corers 1590 cm at different water depths 0–5, 5–10, 10–15 cm etc. during different tidal states.
100
S .M
. Lawrie
et al
. J
. Exp
. Mar
. Biol
. Ecol
. 242
1999 95
– 106
Fig. 2. Numbers of Neomysis and flow measurements at standard locations around stones and weed clumps in low tide river channel. Direction of flow is from top to bottom of figure.
S .M. Lawrie et al. J. Exp. Mar. Biol. Ecol. 242 1999 95 –106
101
1 ]
Fig. 3. Variation in flow open circles at depth and closed circles at bed with water depth along transects
3
during different tidal states.
102 S
.M. Lawrie et al. J. Exp. Mar. Biol. Ecol. 242 1999 95 –106
Fig. 4. Depth distribution of Neomysis in flume a open topped, b with roof fitted to provide an additional boundary layer see text.
than had been recorded previously. However, as flow was increased, the general trends in depth distributions were as before Fig. 4b; F
5 296, P , 0.0001. At zero flow,
4,38 21
47 of individuals were in the water column . 2 cm depth. At 7 cm s , less than
21
25 were in the water column and at 10 cm s , 95 of individuals were within the
bottom 2 cm section. Also, very few individuals never more than 2–3 used the boundary layer created by the sides of the flume. Reducing flow rates decrementally
produced the reverse response to that described above. In the tilted tank zero flow, most individuals were found towards the deeper end of
the tank, purely reflecting the greater volume of water present Fig. 5. If volume is taken into account, the density of individuals was approximately uniform with depth,
except for the shallowest section Fig. 6. This was consistent across all experiments with the tilted tank and for both gradients.
S .M. Lawrie et al. J. Exp. Mar. Biol. Ecol. 242 1999 95 –106
103
Fig. 5. Numbers of Neomysis in water of different depths in static tanks with different bed gradients.
4. Discussion
