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artefacts associated with the method of weed attachment and it was assumed that the wire hoops did not confound the interpretation of the results. There seemed very little evidence from the results of the present experiment at Drum Sands to suggest that the plastic mesh had an effect upon either the physical or physico-chemical conditions of the sediments or upon the fauna. The effects of weed cover on species abundances are sometimes both dramatic and complex and can play an important role in structuring benthic assemblages. Hull (1988) suggested that the observed changes result from the interaction of many factors. These include a reduced current velocity enhancing larval settlement, shelter from predation (although some epibenthic predators may be attracted to weed mats), a reduction in oxygen exchange between the sediment surface with the overlying water, accumulation of silt, anoxia and the production of toxic H2S. Consequently, the effects of weed cover on the fauna (Bonsdorff, 1992) and the mechanisms by which these effects are brought about (Raffaelli et al., 1991) are poorly understood. Many of the effects observed during controlled algal-manipulation experiments are similar to those resulting from organic enrichment (Pearson and Rosenberg, 1978), notably the large increases in Capitella capitata and Malacoceros fuliginosus (Hull, 1987, 1988; Cha, in prep.). Hull (1987) suggested that areas such as the Ythan Estuary, Scotland, where the community comprises many opportunistic species, are likely to cope with the anoxic environment which algal mats create and, therefore, the observed effects of macroalgae are likely to be small. This is in contrast to Drum Sands where reduced sediments were well below the sediment surface and opportunistic species such as C. cap itata were present in very low numbers suggesting that the fauna there were likely to show a more dramatic response to the disturbance imposed by weed cover. A review of the literature revealed that C. capitata tends to be the only benthic infaunal species to show a consistent response to weed cover in controlled manipulation experiments and in general, polychaetes exhibit a mixed response to macroalgal cover (Woodin, 1977). Warren (1976) found that C. capitata colonised by larval settlement giving increased densities during July and October. The timing of C. capitata larval availability therefore coincided with the period of E. prolifera cover in weed treatment plots in this experiment and presumably enabled this species to 103
increase in numbers under the favourable conditions afforded by the weed mats on Drum Sands. Larval settlement of C. capitata was possibly facilitated in the weed plots by a reduction in water flow, and its success within these plots may have been due to its ability to tolerate the reducing conditions while being able to feed on the increased detritus (Price and Hylleberg, 1982). Pygospio elegans showed a significant decline in abundance within the weed plots during the present experiment after 6 weeks and 20 weeks of E. prolifera cover. The responses of P. elegans abundance to weed cover has been shown to vary between experiments. For example, Bonsdorff (1992) found that drifting algal mats in the shallow sandy bottoms of the Baltic Sea decreased adult numbers of P. elegans to zero, while Cha (in prep.) found that P. elegans density was lower in Enteromorpha- removed plots compared to weed plots. However, the possibility of the disturbance created by the physical removal of weed in the latter experiment having a negative effect on P. elegans cannot be overlooked. Hull (1987) found that the effect of macroalgal mats on the densities of P. elegans and other infaunal species of the Ythan estuary was algal-biomass dependent. With low (0.3kgFW/m2) and moderate (1kgFW/m 2) densities of Enteromorpha spp., P. elegans increased in numbers, presumably as a result of an increased detrital-food supply. At high densities (3kg FW/m2), P. elegans decreased in numbers. The decline in abundance of P. elegans under high weed biomass was thought to have been possibly due to increased siltation clogging P. elegans suspension-feeding mechanism (Hull, 1988) or the physical presence of the weed having a detrimental effect on the worm's feeding behaviour (Everett, 1994). Price and Hylleberg (1992) proposed that the effect was in fact due to a decreased food supply: the alteration in water flow due to the algal mats preventing a continuous settlement of debris onto the sediment surface and around animal tubes. Hull (1988) carried out an experiment in which the mechanisms of faunal changes due to macroalgal cover were investigated. He used treatments to which organic matter was added, and separate nylon filament treatments, to mimic the enrichment and physical effects of weed respectively. He concluded that the decline in P. elegans numbers was due to the enriching effect of weed mats rather than due to its physical presence. However, P. elegans has been found to be an opportunistic species and 104
- Page 68 and 69: formed patches less than 1m2 and th
- 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
- 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 142 and 143: This study did not set out to expli
- 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
increase in numbers under the favourable conditions afforded by the weed mats on<br />
Drum Sands. Larval settlement of C. capitata was possibly facilitated in the weed<br />
plots by a reduction in water flow, and its success within these plots may have been<br />
due to its ability to tolerate the reducing conditions while being able to feed on the<br />
increased detritus (Price and Hylleberg, 1982).<br />
Pygospio elegans showed a significant decline in abundance within the weed plots<br />
during the present experiment after 6 weeks and 20 weeks of E. prolifera cover. The<br />
responses of P. elegans abundance to weed cover has been shown to vary between<br />
experiments. For example, Bonsdorff (1992) found that drifting algal mats in the<br />
shallow sandy bottoms of the Baltic Sea decreased adult numbers of P. elegans to<br />
zero, while Cha (in prep.) found that P. elegans density was lower in Enteromorpha-<br />
removed plots compared to weed plots. However, the possibility of the disturbance<br />
created by the physical removal of weed in the latter experiment having a negative<br />
effect on P. elegans cannot be overlooked. Hull (1987) found that the effect of<br />
macroalgal mats on the densities of P. elegans and other infaunal species of the Ythan<br />
estuary was algal-biomass dependent. With low (0.3kgFW/m2) and moderate<br />
(1kgFW/m 2) densities of Enteromorpha spp., P. elegans increased in numbers,<br />
presumably as a result of an increased detrital-food supply. At high densities (3kg<br />
FW/m2), P. elegans decreased in numbers. The decline in abundance of P. elegans<br />
under high weed biomass was thought to have been possibly due to increased siltation<br />
clogging P. elegans suspension-feeding mechanism (Hull, 1988) or the physical<br />
presence of the weed having a detrimental effect on the worm's feeding behaviour<br />
(Everett, 1994). Price and Hylleberg (1992) proposed that the effect was in fact due to<br />
a decreased food supply: the alteration in water flow due to the algal mats preventing a<br />
continuous settlement of debris onto the sediment surface and around animal tubes.<br />
Hull (1988) carried out an experiment in which the mechanisms of faunal changes due<br />
to macroalgal cover were investigated. He used treatments to which organic matter<br />
was added, and separate nylon filament treatments, to mimic the enrichment and<br />
physical effects of weed respectively. He concluded that the decline in P. elegans<br />
numbers was due to the enriching effect of weed mats rather than due to its physical<br />
presence. However, P. elegans has been found to be an opportunistic species and<br />
104
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