Pergamon

PII: S0025-326X(98)00103-9

Marine Pollution Bulletin, Vol. 38, No. 2, pp. 102-108, 1999

© 1999 Elsevier Science Ltd. All rights reserved

Printed in Great Britain

0025-326X/99 $ - - see front matter

Benthic Community Re-adjustment following Dredging of a Muddy-Maerl Matrix S. DE GRAVE* and A. WHITAKER Aquatic Services Unit, Department of Zoology and Animal Ecology, National University of Ireland - - University College Cork, Lee Maltings, Prospect Row, Cork, Ireland

Changes in the benthic infaunal community structure at a location in south-west Ireland subject to dredging impacts were studied. Dredging of a muddy-maerl matrix takes place with a seasonally varying frequency of 2 -5 times per month. The benthic infauna at a currently dredged site was compared to a fallowed portion of the area, at which extraction ceased 6 months prior to the study. As virtually the entire licensed area is dredged in rotation, longer term recovery is not possible at the study site. In contrast to numerous other study, the benthic infauna at the dredged site was not depleted, but did exhibit higher levels of stress and a higher diversity than the fallowed site. A clear shift from the abundance of omniverous crustaceans in the dredged area to filter-feeding bivalves in the fallowing area was observed. These observations are thought to be linked to sediment mobility, the creation of spatial patchiness and the mobilisation of food resources. © 1999 Elsevier Science Ltd. All rights reserved

Keywords: subtidal benthos; suction dredging; maerl; Mollnsea; Crustacea; trophic groups.

Dredging of harbours, docks and navigation channels, in order to deepen access conditions, is a long estab- lished human-induced disturbance in the marine environment. Dredging for these purposes usually takes place over a protracted time period and is normally of a single occurrence, although maintenance dredging may be repeated at periodic intervals. The frequency of the latter is usually every few years, and may be of a smaller spatial scale or involve less material being removed than the original dredging

* C o r r e s p o n d i n g a u t h o r .

102

operation. A more recent development is the dredging of natural sea-bed resources, such as gravel for road aggregates (Millner et al., 1977) or sand for infilling (de Groot, 1979). These resources are more frequently dredged, and it can be postulated that different types of changes in both the faunal community structure and the sedimentary size spectra of the sediment matrix could occur.

A special case of sea-bed resources are in the form of maerl gravels, which in European waters occur extensively in Brittany and along the western seaboards of Ireland and Scotland (Briand, 1991). Maerl beds are

algal gravels formed by non-geniculate red algae (Bosence, 1983) and are composed of a number of distinct, but inter-grading, sedimentary facies, ranging from pure maerl banks to mixed mud or sand and maerl debris facies (Bosence, 1980). The chemical composition of maerl (Blunden et al., 1977) lends itself to a wide variety of uses: soil amelioration, agricultural and horticultural fertilisers, animal fodder additive, acid lake and pond treatment, biological denitrification, drinking water potabilisation, toxin elimination, pharmaceuticals, cosmetics, diabetics, bone surgery and even chemical processing in the nuclear industry (Briand, 1989), although the main use remains as a cost-effective source of calcium/magnesium soil additive in agriculture (Briand, 1991).

Although historically collection of maerl has been known since the 17th century in Ireland and the 18th century in Cornwall (Briand, 1991; Keary, 1994), present day extraction is on a very small scale in both Ireland and the United Kingdom. In contrast, extrac- tion is very intensive in Brittany (France) with as much as 500000 tonnes per annum being extracted by either grab, or pump dredging (Briand, 1991).

Volume 38/Number 2/February 1999

The impacts of dredging, in general, on benthic community structure have been relatively well documented, with the major direct, short-term impact being the destruction of benthic populations in the removed sediment (Morton, 1977). Other effects noted include enrichment of the surrounding area due to re-suspension of material and its consequent re- settlement (Poiner and Kennedy, 1984) and smothering of organisms by settling of fine material (Jones and Candy, 1981). Subsequent re-colonisation of the denuded substrate and surrounding area is very much a site specific process, with both the time scale involved and the resulting community being dependent upon local hydrodynamical and sedimentary conditions (Morton, 1977; van der Veer et al., 1985).

McCauley et al. (1977) propose that the term 'recovery' should not be employed when studying re-colonisation processes after cessation of dredging, as 'recovery' not only implies a return to prior abundance levels, but also a return to ecological pathways which may have taken years to develop. Drastic changes induced by dredging may alter these pathways signifi- cantly, and although the resulting community may return to its pre-dredging abundance levels, it may never return to its pre-dredging structure and internal integrity. Therefore they suggest using the term 're-adjustment', which is used throughout this contribution.

The present study reports on the benthic community inhabiting a muddy-maerl sediment matrix, which has been dredged on a frequent basis for at least the previous 5 years. In addition, the Present dredging process is unique in that long-term cessation of dredging is not envisioned until the resource is depleted. Short-term fallowing of the order of months does take place in certain areas, allowing for a poten- tial temporary re-adjustment of the benthic community.

Methods

The studied extraction area is located off the north- eastern tip of Lonehort Point on Bere Island, Bantry Bay, Co. Cork (Fig. 1). Most of the sediment in the Sound separating Bere Island from the mainland is composed of fine material, with coarse material only being present at the western end of the sound. The extraction area is situated on a shallow shelf at 1-3 m below Chart Datum (CD). On the northern edge it steeply slopes down to the main channel bed at 16-20 m CD. Although on the land-ward edge of the extraction area a narrow strip of living maerl bed occurs, the majority of the substrate within the extrac- tion area consists of varying proportions of mud/sand and maerl debris. Extraction frequency varies season- ally from 2 to 5 times per month. Dredging tracts are run along the east-west axis of the area. A total of 1000 tonnes per annum are extracted which, based on subsurface profiling and the ratio of maer! to other sedimentary fractions, amounts to less than 1% of the total volume of the deposit (J. Walshe, pers. comm.).

Two locations were selected for study, with one location continuously extracted prior to sampling ('Dredged' stations), whilst at the other location extrac- tion stopped 6months prior to sampling ('Fallow' stations).

Benthic macrofaunal samples were collected with a 0.025 m 2 Van-Veen grab. Ten replicate samples were collected from each location, actual sampling positions within each location were randomly selected. The material was processed through a sieve with a mesh size of 0.5 mm and the retained fraction was fixed in 4% neutral formalin to which Rose Bengal was added. Animals were sorted out of the residue by eye, identi- fied to family level (see Warwick, 1988a,b) and counted; only Nemertea were not identified to family level. Previous, unpublished research by the authors on

Fig. 1 Location of study area. Licensed extraction area is indicated by shaded rectangle.

103

the effects of dredging and spoil dumping has demon- strated that family level identification is sufficient to determine changes in the composition of the benthic infauna.

Based on the literature data, taxa were assigned to one of the following trophic categories: omnivores/ predators, filter feeders, grazers, deposit feeders. If more than one feeding mode exists within a family, a further identification to species level was carried out before assigning trophic status. Oligochaeta and Nemertea were considered as deposit feeders and omnivores/predators, respectively.

Granulometric samples were obtained following the same sampling procedure as for the faunal samples, with five replicate samples collected from each location. Particle size distribution was evaluated according to the wet sieving methods outlined by Buchanan (1984) and expressed as %Coarse ( > 2 mm), % Sand (2 mm < > 62 gm) and % Silt-clay (<62 tam). Based on the easily distinguishable maerl thalli, a further visual separation of the coarse fraction into maerl debris and other material (mainly molluscan shell debris) was carried out.

Results

In terms of the sediment particle size distribution, no statistically significant difference was found between the two treatments (Table 1). Nevertheless, a small increase in the proportion of fines, and a resulting minor decrease in the coarser fraction at the fallowing stations is evident in the data.

Only nine out of a total of 60 taxa encountered exhibited a significant difference in density between the two treatments (Table 2). Comparatively speaking, higher densities of Leuconidae, Philinidae, Scalibreg- matidae and Scrobiculariidae were encountered in the Fallow station grouping, whilst the Dredged station group is characterised by higher densities of Melitidae, Nereidae, Phoxocephalidae, Polyonidae and Sigalionidae.

In addition to 58 family level taxa, the potentially multi-family taxa Nemertea and Oligochaeta were

TABLE 1

Particle size distribution. Mean values (-+ 1 standard deviation) of percentage contribution of granulometric fraction per treatment,

with results of one-way ANOVA.

Fraction Fallow Dredged F~.a

Coarse Maerl debris 24.45 _+ 8.28 37.65 + 13.61 3.43 n~ Non-maerl debris 1.72 -+ 1.12 2.67 +_ 0.75 2.48 n~

Sand 14.96 -+ 1.79 12.80 _+ 1.77 0.74 n~ Silt-clay 58.87 _+ 4.35 46.88 ± 5.69 2.79 n~

**p <0.05, *p <0.05, "~ non significant.

104

Marine Pollution Bulletin

TABLE 2

Mean densities per 0.025 m 2 (_+1 standard deviation) for infaunal family taxa which exhibited significant differences per treatment, with

results of one-way ANOVA.

Fallow Dredge Fl.a

Crustacea-Cumacea Leuconidae 54.60±40.63 10.50_+11.28 10.93"* Crustacea-Amphipoda Melitidae 0.60 ± 1.35 6.00 ± 7.45 5.082* Phoxocephalidae 0.50+_0.71 13.70-+9.71 18.39"* Polychaeta Nereidae 0.10 -+ 0.32 1.30 ± 1.34 7.62* Polyonidae 0.30 -+ 0.95 2.80 + 3.33 5.22* Scalibregmatidae 6.20 _+ 4.61 2.40 + 1.65 6.02* Sigalionidae 0.20 _+ 0.42 1.40 -+ 1.65 4.98* Mollusca-Gastropoda Philinidae 1.50_+2.01 0.10+0.32 4.72* Mollusca-Bivalvia Scrobiculariidae 37.30 _+ 21.56 11.00 _+ 7 . 6 4 13.22**

* *p < 0.05, *p < 0.05, '~ non significant.

present in the majority of samples (Table 3). Families which occurred at only one or two stations were considered rare with no definable distribution and were eliminated from Detrended Correspondence Analysis (DCA, see Hill and Gauch, 1980) which was performed on the resulting 40x20 species-station matrix. A logarithmic transformation, ln(x+l) was used to reduce the statistical weight of abundant species relative to those less abundant. From the DCA plot, a relatively tight clustering of the fallowing stations (Fallow, Fig. 2) is apparent, with the exception of a single station, which due to the much higher abundance of Capitel- lidae occupies an isolated position (Fig. 2). Overall, the dredged stations (Dredged, Fig. 2) are more widely scattered than the Fallow stations, this being indicative of more stressed conditions (Warwick and Clarke, 1993). The eigenvalues associated with the first two axes of the DCA ordination cumulatively accounted for 27.3% of the variability in the data, with the remaining two axes only accounting for a further 9.7%. The position of the two treatments is significantly different from each other along the first DCA axis (one-way ANOVA, Fl.ls 27.961, p<0.001), but not along the second DCA axis (F1,18 0.026).

Differences in community structure between the two treatments were further explored with a series of primary and derived diversity and evenness indices (Table 4). No significant difference was found in terms of either the total number of individuals (Ind) or the total number of species (S) between the two treatments (Table 4). In contrast, the derived diversity indices N1 and N2 (Hill, 1973) and the evenness indices El0 (Heip and Engels, 1974) and E2~ (Alatalo and Alatalo, 1977) exhibited elevated levels in stations of the Dredged group. Equally, the dominance index SI (Simpson, 1949) exhibited lower levels at the Dredged stations.

Volume 38/Number 2/February 1999

TABLE 3

Taxa encountered, with number of stations per treatment they were encountered

in which

Fallow (n = 10) Dredged (n = 10)

Polychaeta Ampharetidae 7 7 Capitellidae 2 2 Cirratulidae 7 6 Flabelligeridae l 3 Glyceridae 1 2 Hesionidae 1 1 Lumbrineridae 2 - - Magelonidae 2 2 Maldanidae 2 - - Nephthyidae 7 10 Nereidae 6 1 Opheliidae 1 - - Orbiniidae - - 1 Pectinariidae 1 2 Phyllodocidae 5 5 Polyonoidae 6 3 Sabellidae - - 1 Scalibregmatidae 9 9 Serpulidae 1 - - Sigalionidae 6 2 Spionidae 3 2 Syllidae 3 - - Terebellidae 7 8

Crustacea Ampeliscidae 5 7 Aoridae 9 4 Atylidae 3 1 Bodotridae - - 1 Caprellidae 10 10 Corophiidae 2 - - Diastylidae 2 1 Gnathiidae 1 - - Isaeidae 6 2 Leptognathidae 5 3 Leuconidae 10 10 Leucothoidae - - 1 Lyssianassidae 1 - - Melitidae 6 2 Nannastacidae 5 4 Paguridae 6 - - Phoxocephalidae 9 4 Portunidae 7 4 Stenothoidae 1 1

Mollusca Cardiidae 2 - - Cerithidae 1 - - Corbulidae 6 7 Montacutidae - - 1 Mytilidae 1 - - Nuculiidae 2 1 Phillinidae 1 5 Retusidae 4 5 Rissoidae 3 - - Scrobiculariidae 10 10 Thyasiridae - - 1 Turritelidae 2 - - Veneridae 1 2

Echinodermata Echinidae 1 - - Amphiuridae 2 3 Oligochaeta 6 5 Nemertea 6 4 Sipunculida 3 1

Ax 2

I

+ ©

©

• •

O © • ©

Ax 1 ~ L

Fig. 2 Detrended Correspondence Analysis based on reduced family density matrix. Position of sampling stations along the first two DCA axes (Dredged station group: open circle; Fallow station group: filled circle).

The difference between the two treatments is not only reflected in the family composition of both groups, but also in the distribution of major taxonomic (Fig. 3) and trophic groups (Fig. 4). Comparatively more Crustacea and Oligochaeta are present in the Dredged group, whilst Mollusca (especially Bivalvia) are more numerous in the Fallow group. A small increase in the numbers of Polychaeta is also evident in the Fallow group (Fig. 3). In terms of trophic groups, omnivores/ predators are far more numerous in the Dredged group, with in addition the numerical abundance of filter feeders being reduced in this station group (Fig. 4).

TABLE 4

Primary and derived diversity and evenness indices (_+ I standard deviation) per treatment, with results of one-way ANOVA. For

abbreviations see text.

Fallow Dredged F, .8

lnd 184.44 +- 87.18 122.70 ± 67.64 3.01 "s S 17.11 +_3.89 21.70 +_ 7.30 2.82 "~ SI 0.24 +_ 0.06 0.10 +- 0.03 41.14" * N1 6.73 +- 1.66 13.23 +-4.43 17.09'* N2 4.41 _+ 1.13 11.02+_4.23 20.57** Elo 0.36_+0.06 0.60+_0.12 29.35** E2I 0.59 _+0.08 0.81 _+0.14 17.76"*

**p < 0.005, *p < 0.05 n~non significant.

105

%

I00

8O

60

40

20

i:i:i:!:i:i:i:i:i:i:i:i:i:i:{i:i:i:i:i:i:i:i~:i

0 Fallow Dredged

Fig. 3 Relative numeric proportions of major taxonomic groups (Polychaeta: black; Crustacea: white; Mollusca: vertical hatching; Oligochaeta: horizontal hatching; other: dots).

% I00

80

60

40

20

Nil

/ /

illl

Fallow Dredged Fig. 4 Relative numeric proportions of trophic groups (Omnivores/

Predators: black; Filter feeders: white; Grazers: vertical hatching; Deposit feeders: horizontal hatching).

106

Marine Pollution Bulletin

Discussion

Suction dredging typically results in narrow furrows being cut into the sediment below the original bed level (McCauley et al., 1977). Due to the small area of the suction head and the low number of passes, relatively undisturbed 'hummocks' of undredged material are left behind. Over a period of time these furrows are filled with sediment brought into the area by tidal action (van der Veer et al., 1985), re-settling of the original sediment from overflow of the dredging vessel and suspended material in the water column due to the dredging action, and slumping of the sides of the furrows (McCauley et aI., 1977). Unless dredging has resulted in changed hydrographic conditions, the resultant sediment matrix will be similar to the original one (van der Veer et al., 1985), although some cases of large-scale shifts to finer sediment types have been documented (Kaplan et al., 1975; Jones, 1981). In the presently studied area, only a slight, non-statistically significant shift towards a somewhat finer sediment was observed in the fallowing area, possibly caused by a reduction of current velocity in the furrows.

Essentially, the removal of the surface layer of the sea-bed results in a defaunation of the dredged furrows. The adjacent patches of undisturbed sediment ('hummocks') remain relatively uninfluenced, although these may be subjected to heightened settlement of suspended material, resulting from ship overflow and suspended material in the water column (Johnson, 1981). The presence of these relatively undisturbed hummocks of undredged material has high biological significance in the eventual re-adjustment of the indig- enous benthic community within the dredged area, as viable adult organisms capable of repopulating the dredged furrows are left behind (McCauley et aI., 1977). This re-populating process results from migra- tion of adults, and reproduction and larval recruitment (Maurer et al., 1981a). Both these pathways can origi- nate from adjacent undisturbed areas or residual populations on the hummocks (McCauley et al., 1977; Maurer et al., 1981a). In addition, sediment transport due to slumping of furrow walls can transport a large number of individuals, if subsequent vertical migration through the disturbed sediment matrix is successful (Maurer et al., 1981a,b, 1982).

In the present study a well-developed infaunal community was encountered in the dredged area. This is in contrast to many studies which report a near complete defaunation in active dredging areas (Johnson, 1981). The small annual volume of extracted material in relation to the total deposit in the area and the fact that the created hummocks are presumed to be of a highly transitory nature, due to the high propor- tion of fines in the sediment matrix, may play a signifi- cant role in maintaining the community structure.

Volume 38/Number 2/February 1999

Certainly, previous studies of dredging impact on

communit ies inhabiting fine sediments have shown that

benthic re-adjustment is a fairly rapid process

(McCauley et al., 1977; Connor and Simon, 1979; Bonvicini Pagliai et al., 1985). Nevertheless, high varia- bility in numerical abundance of the consti tuent

species, as observed in the ordinat ion plot, does indicate higher levels of stress, which may be linked to

dredging-induced sediment mobility (see Ztihlke and

Reise, 1994). The higher level of diversity in the dredged area, as compared to the fallowing area, is thought to be linked to a mobilisation of resources by

the dredging operat ions (Poiner and Kennedy, 1984) and the creation of spatial patchiness (McCauley et al.,

1977). The results of t h e multivariate communi ty structure

analysis clearly indicate that the faunal communi ty at the fallowing area is distinct f rom the dredged area and

is thought to represent a re-adjustement of communi ty structure. In the absence of pre-dredging infaunal

communi ty data of the entire area and the lack of a suitable reference site, due to the unusual sedimentary composit ion, it is not clear whether this represents a

partial or even full re turn to pre-dredging conditions. Changes in the relative contr ibution of the major

taxonomic groups following dredging abatement have

been noted before (Johnson and Nelson, 1985; Hily and G16marec, 1990). However, these do not appear to follow a consistent pattern, and are presumably habitat

and communi ty specific. In addition, it has been noted that certain taxonomic groups appear to be more affected by dredging than other groups (Morton, 1996),

al though this may again be habitat, location and/or communi ty specific. Al though only infrequently

studied, similar changes in the relative contribution of trophic groups are known to be a result of dredging operat ions (Johnson and Nelson, 1985). In the present study, a clear shift f rom the abundance of omniverous

crustaceans in the dredged area to filter-feeding

bivalves in the fallowing area was observed. This is p resumed to be related to the relative instability of the sediment, the mobilisation of food resources and the

increased suspended solid loadings due to dredging operations.

Dr H. K. A. Wilkins is acknowledged for assistance with sample processing and for constructive criticism on an earlier version of the manuscript. Mr J. Walshe kindly provided information regarding the size of the resource; Celtic Seaminerals Ltd provided funding for the field work aspect of the study. This work was supported under contract IR.95.MR.019 of the Marine Research Measure (Opera- tional Programme for Fisheries 1994-1999), administered by the Marine Institute, and part funded by the European Union's Regional Development Fund.

Alatalo, R. and Alatalo, R. (1977) Components of diversity: multivariate analysis with interaction. Ecology 58, 900-906.

Blunden, G., Farnham, W. F., Jephson, N., Fenn, R. H. and Plunkett, B. A. (1977) The composition of ma~rl from the Glenfin Islands of southern Brittany. Bot. Mar. 20, 121-125.

Bonvicini Pagliai, A. M., Cognetti Varriale, A. M., Crema, R., Curini Galletti, M. and Vandini Zunarelli, R. (1985) Environmental impact of extensive dredging in a coastal marine area. Mar. Poll. Bull. 16, 483-488.

Bosence, D. W. J. (1980) Sedimentary facies, production rates and facies models for recent coralline algal gravels, Co. Galway. Geol. J. 15, 91-111.

Bosence, D. W. J. (1983) The occurrence and ecology of recent rhodoliths - - a review. In Coated Grains, ed. T. M. Peryt, pp. 225-242. Springer Verlag, Berlin.

Briand, X. (1989) Le Lithothamne: tradition d'hier et agroehimie de demain. Oc~anis 15, 693-739.

Briand, X. (1991) Seaweed harvesting in Europe. In Seaweed Resources in Europe: Uses and Potential, ed. M. D. Guiry and G. Blunden, pp. 259-308. John Wiley and Sons, Chichester.

Buchanan, J. B. (1984) Sediment analysis. In Methods for the Study of Marine Benthos, ed. N. A. Holme and A. D. McIntyre, pp. 41-65. Blackwell Scientific Publications, Oxford.

Connor, W. G. and Simon, J. L. (1979) The effects of oyster shell dredging on an estuarine benthic community. Estuar. Coast. Mar. Sci. 9, 749-758.

de Groot, S. J. (1979) An assessment of the potential environmental impact of large-scale sand-dredging for the building of artificial islands in the North Sea. Ocean Mgmt 5, 211-232.

Heip, C. and Engels, P. (1974) Comparing species diversity and evenness indices. J. Mar. BioL Assoc. UK 54, 559-563.

Hill, M. O. (1973) Diversity and evenness: a unifying notation and its consequences. Ecology 54, 427-431.

Hill, M. O. and Gaueh, H. G. (1980) Detrended correspondence analysis, an improved ordination technique. Vegetatio 42, 47-58.

Hily, C. and G16marec, M. (1990) Dynamique successionneUe des peuplements de fonds meubles au large de la Bretagne. Ocean. Acta 13, 107-115.

Johnson, G. (1981) Estuarine dredge and fill activities: a review of impacts. Environ. Mgmt 5, 427-440.

Johnson, R. O. and Nelson, W. G. (1985) Biological effects of dredging in an offshore borrow area. Fl. Scientist 48, 166-187.

Jones, G. (1981) Effects of dredging and reclamation on the sediments of Botany Bay. Aust. a( Mar. Freshwater Res. 32, 369-377.

Jones, G. and Candy, S. (1981) Effects of dredging on the macrobenthic infauna of Botany Bay. Aust. J. Mar. Freshwater Res. 32, 379-398.

Kaplan, E. H., Welker, J. R., Kraus, M. G. and McCourt, S. (1975) Environmental effects of hydraulic dredging in estuaries. Mar. Biol. 32, 193-204.

Keary, R. (1994) Carbonate Sediments of the Irish Coast. Geological Survey of Ireland, Dublin.

Maurer, D., Keck, R. T., Tinsman, J. C. and Leathern, W. A. (1981a) Vertical migration and mortality of benthos in dredged material: Part I - - Mollusca. Mar. Envir.on. Res. 4, 299-319.

Maurer, D., Keck, R. T., Tinsman, J. C. and Leathern, W. A. (1981b) Vertical migration and mortality of benthos in dredged material: Part II - - Crustacea. Mat'. Environ. Res. 5, 301-317.

Maurer, D., Keck, R. T., Tinsman, J. C. and Leathern, W. A. (1982) Vertical migration and mortality of benthos in dredged material: Part III - - Polychaeta. Mar. Environ. Res. 6, 49-68.

McCauley, J. E., Parr, R. A. and Hancock, D. R. (1977) Benthic infauna and maintenance dredging: A case study. Water Res. 11, 233-242.

Millner, R. S., Dickson, R. R. and Rolfe, M. S. (1977) Physical and biological studies of a dredging ground off the East coast of England. ICES CM 1977/E:48, 1-11.

Morton, B. (1996) The subsidiary impacts of dredging (and trawling) on a subtidal benthic molluscan community in the southern waters of Hong Kong. Mat: Poll Bull. 32(10), 701-710.

Morton, J. W. (1977) Ecological effects of dredging and dredge disposal: a literature review. Tech. Papers. US Fish Wildl. Serv. 94, 1-33.

Poiner, I. R. and Kennedy, R. (1984) Complex pattern of change in the macrobenthos of a large sandbank following dredging. I. Community analysis. Mar. Biol. 78, 335-352.

107

Marine Pollution Bulletin

Simpson, E. H. (1949) Measurement of diversity. Nature, London 163, 688.

van der Veer, H. W., Bergman, M. J. N. and Beukema, J. J. (1985) Dredging activities in the Dutch Wadden Sea: effects on macrobenthic infauna. Neth. J. Sea Res. 19, 183-190.

Warwick, R. M. (1988a) Analysis of community attributes of the macrobenthos of Frierfjord/Langesundfjord at taxonomic levels higher than species. Mar. Ecol. Prog. Set. 46, 167-170.

Warwick, R. M. (1988b) The level of taxonomic discrimination required to detect pollution effects on marine benthic communities. Mar. Pollut. Bull. 19, 259-268.

Warwick, R. M. and Clarke, K. R. (1993) Increased variability as a symptom of stress in marine communities. J. Exp. Mar. Biol. Ecol. 172, 215-226.

Ztihlke, R. and Reise, K. (1994) Response of macrofauna to drifting tidal sediments. Helgol. Meeresunters. 48, 277-289.

108