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formed patches less than 1m2 and thus had non-significant correlograms at this scale. At a larger scale, the 8m survey also revealed patches of different sizes, e.g., less than 8m2 (P. elegans, E. cf flava and G. duebeni) and 8-24m2 (C. capitata, S. martinensis and L. conchilega). The two bivalve species M. balthica and C. edule had significant spatial structures at this scale but the sizes of their patches could not be estimated because of their location at the edge of the survey area. At the largest scale investigated, 8 of the 11 species with aggregated distributions exhibited significant spatial patterns. Many of these formed patches between 40-60m2, P. elegans, E. cf flava, C. edule and M. balthica, for example. The sediments were also spatially structured within the study area. Again, mapping, together with spatial autocorrelation analysis revealed that at the smallest scale, patches of 1.5-2m2 of increased % organic carbon content, % silt/clay and sorting coefficient were present while patches >3.5m 2 of increased median particle size (Md 4)) were present. These variables formed larger patches in the 8m survey but their sizes could not be ascertained with any confidence because they were located at the edge of the survey area. The 40m survey indicated that these sediment variables formed patches approximately 80-100m 2 across the centre of the survey area. High-density patches formed by tube-dwelling polychaetes on intertidal sandflats have been well documented (e.g., Sanders et al., 1962; Daro and Polk, 1973; Dupont, 1975; Featherstone and Risk, 1977; Noji and Noji, 1991; Morgan, 1997). Patches of spionid polychaetes vary greatly in size from 0.04-9m2 (Marenzelleria viridis; Zettler and Bick, 1996) to 150,000m2 (Clymenella torquata; Sanders et al., 1962). However, comparisons of the spatial patterns identified in this study with those of tube-builders found at other intertidal sandflats are difficult since most studies have been carried out in either very different habitats and/or focused on defining smaller-scale patterns. The few studies explicitly investigating spatial distributions at this scale have found that soft-bottom macroinvertebrates generally exhibit significant spatial structuring on intertidal sandflats, forming patches of various sizes. For example, Thrush et al. (1989) investigated the spatial arrangements of polychaetes and bivalves in the intertidal sandflats of Manukau Harbour, New Zealand. Of the 36 populations studied, 30 were found to have significant spatial patterns, as assessed by variance to 53
mean ratio and spatial autocorrelation analysis, forming patches of increased densities between 5 and 30m radius. McArdle and Blackwell (1989) found the bivalve Chione stutchburyi formed patches of increased density of 5-15m in a sandy lagoon in Ohiwa Harbour, New Zealand. Morrisey et al. (1992), using a nested sampling design, showed that the abundances of the infauna in the soft sediments of Botany Bay, Australia, were patchy at scales from 1 metre to several kilometres. None of the species in the study by Thrush et al. (1989) exhibited identical spatial patterns at all of the 6 sandflats which suggests that patterns and patch sizes are partly affected by their environment and that different studies are likely to give different spatial patterns for the same species. In addition, Hewitt et al. (1997) showed that spatial patterns varied temporally within the same site. Heterogeneity results from a complex interaction of biotic and abiotic processes (Livingston, 1987; Caswell and Cohen, 1991). At the scale investigated in this study, metres to tens of metres, spatial heterogeneity of macrofauna may result from many processes including disturbance, larval settlement, competition and sediment heterogeneity. In most studies, the causes of the patterns are only hypothesized or inferred from the data available. For example, Thrush et al. (1989) suggested that feeding pits generated by rays may have been responsible for the generation of small- scale heterogeneity within larger homogeneous density patches at two of their sites, while McArdle and Blackwell (1989) proposed that the pattern of C. stutchburyi at their site may have resulted from an environmental gradient. Morgan (1997) suggested that a large, gregarious settlement of larvae must have been one of the prerequisites for the formation of very dense patches of P. elegans in the intertidal sandflats of the Baie de Somme, France. Determining the process(es) responsible for an observed pattern at this scale (metres to hundreds of metres) is very difficult (McArdle et al., 1997). Although controlled, manipulative experiments are possible at smaller scales (centimetres to a metre) their conclusions cannot be scaled-up to larger scales since different processes are likely to be responsible for giving rise to patterns. For example, Hewitt et al. (1996) showed that it was possible to predict the spatial patterns (
- Page 17 and 18: The scales of observation, or the s
- Page 19 and 20: systematic sampling design to inves
- Page 21 and 22: aised sediment within an otherwise
- Page 23 and 24: Fauchald and Jumars (1979) describe
- Page 25: Dalmeny House and sewage discharged
- Page 28 and 29: E 7— E 6-ac. MHWS MHWN t co 4 —
- Page 30 and 31: variance (TTLQV) techniques (see Lu
- Page 32 and 33: analysis using Moran's and Geary's
- Page 34 and 35: Holme and McIntyre (1984). Percenta
- Page 36 and 37: Pattern Analysis - Grid Surveys Sur
- Page 38 and 39: 57 64 1=1 0 0 0 0 0 0 0 O 0 0 0 0 0
- Page 40 and 41: Maps produced by kriging and other
- Page 42 and 43: 200 180 1160 140 100 1-3 80 g 60 c.
- Page 44 and 45: 2.5 1.5 0.5 0 3 T (i) % Silt/clay%
- Page 46 and 47: v : m pattern Id pattern Ip pattern
- Page 48 and 49: " v : m pattern Id pattern Ip patte
- Page 50 and 51: The results show that at the smalle
- Page 52 and 53: Nephtys hombergii's spatial distrib
- Page 54 and 55: (vii) G. duebeni (ix) % Organic con
- Page 56 and 57: 8m survey - spatial patterns Figure
- Page 58 and 59: (1) P. elegans (iii) L. conchilega
- Page 60 and 61: a) Ts 1.4 0.6 u 0.2 -0.2 1.4 'E5 0.
- Page 62 and 63: 200m 150m 100m 50m (ix) C. edule 56
- Page 64 and 65: 73 ‘a• el 1.4 (ix) G. duebeni 1
- Page 66 and 67: DISCUSSION The main aims of this st
- Page 70 and 71: stutchbutyi, at Wirroa island, New
- Page 72 and 73: exhibited by the tube-building poly
- Page 74 and 75: CHAPTER 3 THE POPULATION STRUCTURE
- Page 76 and 77: Asexual reproduction by fragmentati
- Page 78 and 79: METHODS Survey design - It has been
- Page 80 and 81: RESULTS The species abundances in e
- Page 82 and 83: corresponds to 44 setigers using Eq
- Page 84 and 85: 1 0000000 00 rg 0 00 d- - Xauanbau
- Page 86 and 87: Reproductive activity of Pygospio e
- Page 88 and 89: P. elegans larvae at Drum Sands hav
- Page 90 and 91: Pygospio elegans showed great seaso
- Page 92 and 93: Previous studies have produced simi
- Page 94 and 95: The sole reliance on a planktonic m
- Page 96 and 97: abundance are highly seasonal, were
- Page 98 and 99: CHAPTER 4 THE EFFECTS OF MACROALGAL
- Page 100 and 101: studies may have been completely di
- Page 102 and 103: METHODS Study site - The exact posi
- Page 104 and 105: 1 C N W 4----111" 1.5m 2 NW C Contr
- Page 106 and 107: sediment sampling, together with re
- Page 108 and 109: RESULTS Species abundances - The me
- Page 110 and 111: ; 15 35 — 30 — 25 — 10 — 5
- Page 112 and 113: statistical difference from net plo
- Page 114 and 115: Pygospio elegans size distribution
- Page 116 and 117: used, approximately equivalent to t
mean ratio and spatial autocorrelation analysis, forming patches of increased densities<br />
between 5 and 30m radius. McArdle and Blackwell (1989) found the bivalve Chione<br />
stutchburyi formed patches of increased density of 5-15m in a sandy lagoon in Ohiwa<br />
Harbour, New Zealand. Morrisey et al. (1992), using a nested sampling design,<br />
showed that the abundances of the infauna in the soft sediments of Botany Bay,<br />
Australia, were patchy at scales from 1 metre to several kilometres. None of the<br />
species in the study by Thrush et al. (1989) exhibited identical spatial patterns at all of<br />
the 6 sandflats which suggests that patterns and patch sizes are partly affected by their<br />
environment and that different studies are likely to give different spatial patterns for<br />
the same species. In addition, Hewitt et al. (1997) showed that spatial patterns varied<br />
temporally within the same site.<br />
Heterogeneity results from a complex interaction of biotic and abiotic processes<br />
(Livingston, 1987; Caswell and Cohen, 1991). At the scale investigated in this study,<br />
metres to tens of metres, spatial heterogeneity of macrofauna may result from many<br />
processes including disturbance, larval settlement, competition and sediment<br />
heterogeneity. In most studies, the causes of the patterns are only hypothesized or<br />
inferred from the data available. For example, Thrush et al. (1989) suggested that<br />
feeding pits generated by rays may have been responsible for the generation of small-<br />
scale heterogeneity within larger homogeneous density patches at two of their sites,<br />
while McArdle and Blackwell (1989) proposed that the pattern of C. stutchburyi at<br />
their site may have resulted from an environmental gradient. Morgan (1997)<br />
suggested that a large, gregarious settlement of larvae must have been one of the<br />
prerequisites for the formation of very dense patches of P. elegans in the intertidal<br />
sandflats of the Baie de Somme, France.<br />
Determining the process(es) responsible for an observed pattern at this scale (metres<br />
to hundreds of metres) is very difficult (McArdle et al., 1997). Although controlled,<br />
manipulative experiments are possible at smaller scales (centimetres to a metre) their<br />
conclusions cannot be scaled-up to larger scales since different processes are likely to<br />
be responsible for giving rise to patterns. For example, Hewitt et al. (1996) showed<br />
that it was possible to predict the spatial patterns (
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