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.M. Lawrie et al. J. Exp. Mar. Biol. Ecol. 242 1999 95 –106
function, have been discussed at length and seem to vary from species to species Berrill, 1968; Clutter, 1969; Mauchline, 1971; Dadswell, 1975; O’Brien, 1988; Ohtsuka
et al., 1995. In the case of Neomysis integer , which is restricted to estuarine habitats,
one explanation of aggregations may be that individuals move to areas of the estuary where they are less likely to be displaced Parker and West, 1979; Hough and Naylor,
1992; Roast et al., 1998. Within the Ythan estuary, Aberdeenshire, Scotland, aggregations appear most obvious
at the tide edge during the rising, falling and high-tide periods, but during the low-tide period are most conspicuous in the lee of rocks and macro-algal clumps, or as a
mono-layer in shallow water at the sediment–water interface. These patterns of distribution suggest that aggregations may be more common in low-flow areas, but the
relation between flow and abundance has not been quantified in the field to date at the relevant small scale cf. Roast et al., 1998. In addition, it is not clear to what extent the
distribution of aggregations is an inevitable consequence of passive entrainment of individuals into such low-flow areas, or whether Neomysis actively seeks such
conditions. In the present paper, we address these questions by recording the abundance of Neomysis at different states of the tide in relation to water depth and flow, and by
observing the response of Neomysis to flow under controlled laboratory conditions. Roast et al. 1998, have recently described the activities of Neomysis under a range of
conditions within a laboratory annular flume and they relate these to the physical environment of the Looe estuary, Cornwall, UK. In the present study, we were able to
make contemporary estimates of Neomysis densities and the flow environment in the field, and also to relate these to behavioural observations in a much larger flume
environment. Our work thus both compliments and extends that of Roast et al. 1998.
2. Materials and methods
2.1. Field studies All field observations were made within an extensive intertidal mudflat of the upper
Ythan estuary OS grid reference 005283 which supported high densities of Neomysis. Neomysis are present throughout the year at this site, which experiences a salinity range
of 15‰ low tide to 30‰ high tide. The present surveys were made in August and September 1996, when densities are high. The distribution of individuals during the
flood, high-water slack, ebb and low-water slack tidal periods was determined by sampling simultaneously along two replicate transects perpendicular to the tide edge,
running from high to low water about 20 m apart. Drop-corers 45 cm in diameter and 80 cm tall were located at 0.5 m intervals along each of the transects so that the top of
the corer remained well above the water surface i.e. the entire water column was sampled. The contents of each corer were fished to completion with a small hand net 1
mm mesh, and the Neomysis retained preserved in 70 alcohol and transferred to the laboratory where they were counted and sized. This procedure was repeated for each of
the four tidal states outlined above for both spring and neap periods. The data for the two transects were pooled.
S .M. Lawrie et al. J. Exp. Mar. Biol. Ecol. 242 1999 95 –106
97
Mean directional water flows were measured over the intertidal flat over a 13 h period from high tide to high tide, using a Novar ‘‘streamflo’’ portable flow-meter with a
1 ]
low-flow probe impeller. Maximum directional water current was measured at depth
3
H 3, where H is the depth from sediment to water surface, and at maximum water depth 1 cm from the bed at 0.5 m intervals along a transect perpendicular to the tide
edge for a minimum distance of 5 m from the tide edge, congruent with the Neomysis sampling.
During the low-tide period, Neomysis abundance, water depth and flow were recorded at standard positions around rocks and macro-algal clumps situated in the lower part of
the shore. With respect to the prevailing river flow, these were at the leading edge of the object, along its sides, in its lee and at two locations at least one metre away from either
side of the structure see Fig. 2. Neomysis were sampled simultaneously at these positions using standardised vertical hauls through the water column with a 1 mm mesh
hand-net. Water depth and flow rate were then measured at each position, as above. It was not possible to repeat this at other tidal states because of the increased water depth.
2.2. Laboratory studies In the laboratory studies, only mature individuals were used c. 12 mm–16 mm
length, rostrum to telson. These were freshly caught from the field site for each experiment and held in aerated, sea-water 33‰, 14
8C, in low light levels for no longer than 24 h prior to any experiment.
The distributions of Neomysis in relation to flow were recorded within a linear flume 8 m Armfield flume, 30 cm wide and 40 cm high. Approximately 100 individuals were
introduced to an experimental area of the flume between two collimators 2 m apart under static flow conditions. After allowing 10 min for the animals to distribute themselves,
the number of individuals within each of six depth sections in the water column was recorded 0–2 cm, 2–5 cm, 5–10 cm, 10–15 cm, 15–20 cm and
. 20 cm from the bottom Subsequently, the free-stream flow velocity was increased to the following rates:
21 21
21 21
21
1.5 cm s , 4 cm s
, 7 cm s , 8.5 cm s
, 10 cm s . This was repeated seven times.
Boundary layer development within the flume was dependent upon free-stream velocity and distance along the flume test section. At the maximum free-stream velocity 10 cm
21
s , the boundary layer was approximately 5 cm in depth 99 of free-stream
velocity. After each increase in flow velocity, individuals were allowed 2 min to distribute themselves and the number in each depth section recorded. For the initial
experimental run, five recordings of distribution were made with zero flow. Flow velocity is reported as the mean flow rate of the free-stream velocity measured for the
experimental test section based on the flow at a standard position in the flume tank above the boundary layer midstream, 10 cm above the bed and 5 cm behind the leading
collimator.
The depth distribution of Neomysis under static conditions was recorded in a tank 120 cm by 20 cm by 50 cm, tilted to produce two depth gradients: from 0 to 30 cm
depth or from 0 to 40 cm depth. Approximately 100 freshly caught Neomysis were placed centrally within the tank for each experiment. After allowing 10 min for the
animals to distribute themselves, the number of individuals within the entire water
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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
