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This study did not set out to explicitly determine the mechanisms by which many of the species increased in abundance with weed cover. Size-frequency measurements, however, suggested that for P. elegans, increased larval settlement, possibly facilitated by reduced water velocity, and/or increased juvenile survivorship, possibly due to fewer epibenthic predators, may have led to the increased densities obtained. This is supported by the results of Chapter 6 in which it was demonstrated that during periods of high larval availability, P. elegans was capable of rapidly numerically dominating after small-scale disturbances on Drum Sands. Given its reproductive strategy, P. elegans would probably have been able to show this response to the larger-scale disturbances imposed by macroalgal mat development. Furthermore, Hull (1987, 1988) postulated that larval settlement could have accounted for the observed increases in the numbers of several species during his weed-implantation experiment. The positive effect of V. subsimplex on P. elegans recruitment in this study is in contrast with the findings of the weed implantation experiment (Chapter 4) in which the main P. elegans larval recruitment during May 1997 did not successfully occur in the weed treatment plots. Perkins and Abbott (1972), Nicholls et al. (1981) and Soulsby et al. (1982) suggested that algal mats had a detrimental effect on particularly the juveniles of many species in their surveys. The positive effect of V subsimplex on larval recruitment during this survey was significant during periods of relatively low larval availability (end September/early October). Therefore, if V subsimplex had been present at the same time as E. prolifera, large P. elegans densities may have resulted under V subsimplex and the differences between the experiment and the survey may have been appreciably larger (see next section). It is likely that increased larval settlement and/or survival was probably the process(es) by which many of the other species also increased in abundance under V. subsimplex mats, although without size measurements and information on their reproductive strategies and population structure, this cannot be concluded with any real certainty. 127
The possible role of algal-mat establishment in P. elegans patch formation on Drum Sands. Once established, the V. subsimplex mats rapidly increased both the numbers of species present and the numbers of individuals of those species already present on Drum Sands, including large increases in the numbers of P. elegans. This effect was not short-lived and became even more pronounced during January, 20 weeks after the weed mats had established. Unfortunately, during February the weed mats, together with their accumulated sediments and associated faunal assemblages, disappeared, presumably due to erosion by winter storms. Therefore, although the V. subsimplex mats significantly increased the densities of P. elegans, they did not lead to P. elegans patch formation during the year this study was carried out. The possibility that algal mats may have created P. elegans patches during other years cannot be ruled out. It is possible that the increased water movement due to storms was unusually strong during the winter of 1997-1998. Indeed, sediment erosion was probably the reason why the numbers of P. elegans had decreased in the control plots between October and January (from 52.8 ± 8.1 to 18.2 ± 5.9). However, the majority of the pre-existing P. elegans patches appeared unaffected by erosion during this winter. It is perhaps more likely that the timing of weed establishment during 1997 was influential in the erosion of weed plots. During 1997, the weed mats did not appear until September which is much later than weed mats usually establish on sandflats in Britain and much later than it occurred on Drum Sands during 1996 (late May/early June). Since juvenile P. elegans numbers in particular increased in sediments covered by V. subsimplex, the late development of these mats may not have increased P. elegans densities as much as in other years since the large recruitment phase in May/June was missed. Since the numbers of P. elegans were possibly lower under weed mats compared to those of previous years, and the period over which sediment accumulation and stabilisation could occur was shorter, the sediments where weed mats developed may have been more prone to erosion during the high-energy wave movement during winter compared to previous years. Furthermore, the unusual presence of the weed during the winter period may also have contributed to sediment erosion. 128
- 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
- Page 118 and 119: artefacts associated with the metho
- Page 120 and 121: present in high numbers around sewa
- Page 122 and 123: lack, hydrogen sulphide-smelling se
- Page 124 and 125: CHAPTER 5 THE EFFECTS OF MACROALGAL
- Page 126 and 127: METHODS Survey design - During late
- Page 128 and 129: The sediments could not be sampled
- Page 130 and 131: RESULTS Species abundances - Table
- Page 132 and 133: 90 — 80 — "-e-' 70 — 60 — 4
- Page 134 and 135: 35 — *** 30 25 — 1.) = .-c‘l
- Page 136 and 137: Pygospio elegans size distributions
- Page 138 and 139: which is difficult to compare with
- Page 140 and 141: eason why some invertebrates showed
- Page 144 and 145: This reliance upon the early establ
- Page 146 and 147: CHAPTER 6 INITIAL COLONISATION OF D
- Page 148 and 149: esulting community at any stage of
- Page 150 and 151: ambient sediment had been removed.
- Page 152 and 153: emoved since they were the only tax
- Page 154 and 155: All statistics were performed using
- Page 156 and 157: RESULTS Univariate analysis of spec
- Page 158 and 159: 3.5 3 5 2 11 5 1 0.5 0 40 35 Ca 30
- Page 160 and 161: of non-patch areas (Figure 6.3(vi))
- Page 162 and 163: the individuals colonising patch az
- Page 164 and 165: Multivariate analysis of community
- Page 166 and 167: Month Sample statistic (Global R) N
- Page 168 and 169: 2NP 3NP 4NP .•,, 6NP 5NP 6P 1NP i
- Page 170 and 171: Figure 6.8: Two-dimensional MDS ord
- Page 172 and 173: - - 5P ... 4P . 6P • .‘2NP 1NP
- Page 174 and 175: I 50. 1 60. 70. 80. 90. 100. BRAY-C
- Page 176 and 177: 'P2-AZ P3-AZ N2-AZ .- - - " .„ ..
- Page 178 and 179: o • o -o + 350 — 300 = 250 7 g
- Page 180 and 181: The importance of the ambient commu
- Page 182 and 183: In April, when P. elegans larval av
- Page 184 and 185: not only for errant polychaetes, bu
- Page 186 and 187: observed in this study. How crucial
- Page 188 and 189: Micro-scale spatial patterns of mac
- Page 190 and 191: METHODS Experimental design - A pre
This study did not set out to explicitly determine the mechanisms by which many of<br />
the species increased in abundance with weed cover. Size-frequency measurements,<br />
however, suggested that for P. elegans, increased larval settlement, possibly facilitated<br />
by reduced water velocity, and/or increased juvenile survivorship, possibly due to<br />
fewer epibenthic predators, may have led to the increased densities obtained. This is<br />
supported by the results of Chapter 6 in which it was demonstrated that during periods<br />
of high larval availability, P. elegans was capable of rapidly numerically dominating<br />
after small-scale disturbances on Drum Sands. Given its reproductive strategy, P.<br />
elegans would probably have been able to show this response to the larger-scale<br />
disturbances imposed by macroalgal mat development. Furthermore, Hull (1987,<br />
1988) postulated that larval settlement could have accounted for the observed<br />
increases in the numbers of several species during his weed-implantation experiment.<br />
The positive effect of V. subsimplex on P. elegans recruitment in this study is in<br />
contrast with the findings of the weed implantation experiment (Chapter 4) in which<br />
the main P. elegans larval recruitment during May 1997 did not successfully occur in<br />
the weed treatment plots. Perkins and Abbott (1972), Nicholls et al. (1981) and<br />
Soulsby et al. (1982) suggested that algal mats had a detrimental effect on particularly<br />
the juveniles of many species in their surveys. The positive effect of V subsimplex on<br />
larval recruitment during this survey was significant during periods of relatively low<br />
larval availability (end September/early October). Therefore, if V subsimplex had<br />
been present at the same time as E. prolifera, large P. elegans densities may have<br />
resulted under V subsimplex and the differences between the experiment and the<br />
survey may have been appreciably larger (see next section).<br />
It is likely that increased larval settlement and/or survival was probably the<br />
process(es) by which many of the other species also increased in abundance under V.<br />
subsimplex mats, although without size measurements and information on their<br />
reproductive strategies and population structure, this cannot be concluded with any<br />
real certainty.<br />
127
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