Effects of the closure of a major sewage outfall on sublittoral, soft sediment benthic communities

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Marine Pollution Bulletin 52 (2006) 645–658

Effects of the closure of a major sewage outfall on sublittoral,soft sediment benthic communities

Julie Smith *, Susan E. Shackley

School of Biological Sciences, University of Wales Swansea, Singleton Park, Swansea, Wales SA2 8PP, United Kingdom

Abstract

The present study examines the benthos within western inner Swansea Bay (Wales, UK), for the period before, during and immedi-ately after the cessation of a major, sewage discharge from Mumbles Head. There have been significant improvements in seawater qualityand changes in the species composition of the benthic communities following the cessation of the sewage discharge. There has been anincrease in the diversity of deposit feeders, especially the amphipods, and a decrease in the diversity of the filter feeders, especially thepolychaetes. Changes are not attributable either to sediment organic matter content or to gross changes in sediment type, but are relatedto the significant reduction in suspended particulate organic material and sewage contaminants discharged to the Bay. A recovery modelhas been proposed to describe how soft sediment benthic communities in a shallow, sublittoral, high tidal energy environment respond tothe abrupt cessation of a major sewage discharge.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Swansea Bay; Seawater quality; Benthos; Sewage outfall; Recovery model

1. Introduction

Studies have investigated the effects of either dischargingrelatively untreated sewage into the marine environment(Carrasco and Carbajal, 1998; Cardell et al., 1999; Munizand Pires, 1999) via offshore, deep water outfalls (Dieneret al., 1995; Chapman et al., 1996; Taylor et al., 1998) orof discharging sewage, which has undergone upgradedtreatment (Read et al., 1982; Read et al., 1983; Dieneret al., 1995; Soltan et al., 2001). None have examined theeffects on benthic communities related to the complete ces-sation of a major, untreated sewage discharge.

Many studies support Pearson and Rosenberg (1978)model of an increase in number of species, biomass andabundance with organic enrichment, before declining withincreased organic load as anoxic conditions arise. Not allmarine benthic communities follow this model (Khan and

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doi:10.1016/j.marpolbul.2005.10.016

* Corresponding author. Tel.: +44 1792 229380; fax: +44 1792 295708.E-mail address: [email protected] (J. Smith).

Garwood, 1995; Karakassis et al., 1999), and often it is dif-ficult to distinguish between natural fluctuations andchanges brought about by organic enrichment (Buchananand Moore, 1986). There are instances where high organicmatter levels actually bring about an increase in macroben-thic density (Khan and Garwood, 1995) or diversity ofbiota remains high, and there is no collapse of a diversecommunity (Diener et al., 1995).

This paper examines how shallow water, soft sediment,sublittoral benthic communities respond to the cessationof a major sewage discharge in a high-energy environment.It provides a recovery model to describe how benthic com-munities in western inner Swansea Bay respond to a signif-icant reduction in total suspended particulate organicmaterial (POM) and decreased levels of sewage contami-nants discharged to the Bay, brought about by the commis-sioning of the new waste water treatment plant (SwanseaSTW) sited to the east of the Bay.

Swansea Bay is situated on the northern coastline of theBristol Channel (Fig. 1), with the second largest tidal range

mailto:[email protected]

Fig. 1. Location of sampling Stations 1 and 2, and the Mumbles Head sewage outfall (A) within western inner Swansea Bay (derived from AdmiraltyChart 1161).

646 J. Smith, S.E. Shackley / Marine Pollution Bulletin 52 (2006) 645–658

in the world (i.e. means of 8.5 m spring tides; 4.1 m neaptides) (Wilding and Collins, 1979; Shackley and Collins,1984). The tidal current takes the form of a rectilinear,reversing offshore flow with an anticlockwise eddy in thewestern inner part and an area of divergence on the easternside of the embayment (Collins et al., 1979). Sediments arepredominantly fine and medium sand in inner SwanseaBay with increasing proportions of mud occurring closeinshore to the west, as a result of Mumbles Head providingprotection from wave exposure and the shallow water slow-ing the tidal currents.

2. Materials and methods

2.1. Study site

The study sites, Stations 1 (51�34.60 0N, 03�58.50 0W) and2 (51�34.40 0N, 03�59.00 0W) (Fig. 1) are situated in westerninner Swansea Bay, at depths of 0.3 m and �0.8 m (rela-tive to CD) respectively, within an area of the Bay consid-ered to be most likely to be affected by sewage pollution,due to its close proximity to the Mumbles Head outfalland the pattern of currents in the Inner Bay. That is, anti-clockwise, semi-rotatory tidal currents produce an anti-clockwise eddy as water leaves the Bay on the ebb tidevia the Mumbles inner and outer sounds (Rimmer, 1987).Station 1 is within an area of substrate composed of muddysand, whilst Station 2 has a predominantly muddy sub-strate. Prior to the present study, the substrates of bothsites had remained unchanged for 2 years (Smith, 1997).

2.2. Sampling strategy

Stations 1 and 2 were sampled seasonally between Janu-ary 1998 and December 1999, using a 0.1 m2 Day grab.

Sampling occurred over high water Spring tides. As a resultof a pre-study �power analysis�, five replicate samples weretaken from Station 1 and three from Station 2.

2.3. Treatment of samples

A sediment sub-sample was collected from each of the 8replicate samples at sea and the residual sediment wassieved through a 1 mm mesh. Sieve debris with retainedfauna was preserved in 70% ethanol. On return to the lab-oratory, sediment samples were frozen and at a later datedefrosted and analysed. Fauna were removed from thesieve debris by hand sorting, identified to species levelwhenever possible (using texts by Lincoln, 1979; Haywardand Ryland, 1995) and their wet weights were recorded(biomass). Epifauna were identified and noted, but theywere not used in subsequent statistical analysis.

The trophic status of each species was determined usinginformation from the literature (Fauchald and Jumars,1979; Nybakken, 1993; Fish and Fish, 1996; Martinet al., 2000; Neal and Avant, 2004; Hentschel and Larson,2005). Species diversity was calculated using the Shannon–Wiener diversity index (H 0) (Shannon and Weaver, 1963),which incorporates both species richness (number of spe-cies in a community) and evenness (how evenly individualsare distributed amongst the species present) (Gray andPearson, 1982; Clarke and Warwick, 1994; Clark et al.,1997). Log base e was used in the calculation of the diver-sity indices.

Each replicate sediment sample was wet sieved througha 63 lm mesh in order to calculate the sand-to-mud ratio.The retained sand fraction was oven dried at 50 �C for24 h, and then the dried samples were mechanically gradedthrough a sieve stack of sieve sizes of 1000, 500, 250, 125and 63 lm.

J. Smith, S.E. Shackley / Marine Pollution Bulletin 52 (2006) 645–658 647

The difference-on-ignition (DOI) method of Luczaket al. (1997) was used to establish the mean organic mattercontent of each set of three replicate sediment samples.

2.4. Seawater quality

Surface seawater samples were collected from the studyarea and their bacteriological quality was analysed by Cityand County of Swansea (CCS). These data, made availableby CCS, were used to indicate changes in seawater qualityfollowing the closure of the outfall. Although changes inbacteriological quality did not impact directly on the ben-thos, they were considered to be a reliable indicator ofthe outfall-related changes occurring in the well-mixedwater column.

Mumbles outfall water quality data from the Environ-ment Agency was also analysed to assess if there was anysignificant changes in the total suspended particulate mat-ter (POM), nitrogen, ammonia, nitrate, nitrite, ortho-phos-phate, and in the heavy metals (i.e. cadmium, chromium,copper, nickel, lead and zinc) found in sewage sludge (Ger-lach, 1981; Clark et al., 1997; Naoum et al., 2001) that wasdischarged from the outfall over the study period. That is,for the period that the Mumbles Head outfall was in fulltime operation (January 1998), during a 50% reduction inoutfall discharges (May 1998), and immediately after thecessation of sewage discharge from Mumbles Head (Febru-ary 1999).

3. Results

3.1. Bacteriological

Surface seawater samples were analysed to determinewhether they were within the Bacteriological QualityRequirements for Bathing Water as defined through Man-datory and Guideline Standards (EU directive: 76/160/EEC—Table 1). Provided that water quality meets Guide-line Standards (EU Directive) over a bathing season andsuitable shore-based facilities are present, Blue Flag statusis then awarded to designated bathing beaches by the‘‘Keep Britain Tidy’’ Group/ENCAMS (Shackley, 1998).

Analyses show that Swansea Bay was generally withinMandatory Standards for Total and Faecal Coliforms in

Table 1Quality requirements for Bathing Water (76/160/EEC)

Mandatory Standards

Total Presumptive Coliform count 10,000/100 mlPresumptive E. coli (Faecal) count

(1 failure allowed in 20 samples)2000/100 ml

Guideline Standards

Total Presumptive Coliform count 500/100 mlPresumptive E. coli (Faecal) count

(4 failures allowed in 20 samples)100/100 ml

Faecal Streptococci

(2 failures allowed in 20 samples)100/100 ml

1998, but was rarely within Guideline Standards. Waterquality improved with the 50% reduction in sewage outfalldischarges (May 1998), in that only 18% of samples failedto reach Guideline Standards for Faecal Streptococci. Yetduring this period, 80% of samples still failed to reachGuideline Standards for Total Presumptive Coliformsand 85% failed Guideline Standards for E. coli (Faecal)Coliforms.

Bacteriological quality of water in the Bay improved sig-nificantly (p < 0.05) with the cessation of sewage dischargefrom Mumbles Head (February 1999), with all samplesmeeting Mandatory Standards for Total Presumptive andFaecal Coliforms, and Guideline Standards for FaecalStreptococci. Only 8% of samples taken in 1999 failed toreach Guideline Standards for Total Presumptive Coli-forms and 17% failed to meet Guideline Standards for Fae-cal Coliforms, with November and December 1999 beingthe months responsible for these failures.

3.2. Nutrients and heavy metals

Water quality data from the Environment Agencyshows that there was a significant reduction (p < 0.05) intotal suspended particulate organic matter (POM) dis-charged into Swansea Bay, but no overall changes werenoted for total nitrogen, nitrate, nitrite, ortho-phosphateand ammonia. Nevertheless, levels of total nitrogen andnitrates discharged from the Mumbles Head outfall andnew waste water treatment plant (Swansea STW) was gen-erally highest in autumn and winter.

More zinc was discharged from Mumbles Head outfallthan any other heavy metal, but over the study period zinclevels declined. In contrast, from May 1999 (outfall closed)onwards more chromium was discharged from SwanseaSTW than had been released from the Mumbles Head out-fall, but there was an overall decrease in the amount ofzinc, lead, copper, nickel and cadmium discharged fromSwansea STW in comparison to Mumbles Head outfall.

Bacteriological analyses of surface seawater and waterquality data (from the Environment Agency) show thatdespite the improvement in water quality, contaminationlevels are still high in the autumn and winter. One of themajor contributing factors appears to be the storm dis-charges of raw untreated sewage that are periodicallyreleased from the Mumbles Head outfall and Swansea STW.

The most important factors, for the present study, arethe significant decrease in total suspended particulateorganic matter (POM) and reduction in sewage contami-nants discharged into the Bay when the Mumbles outfallwas decommissioned, and the Swansea STW became fullyoperational (mean values of POM discharged at Mumbles�111 mg l�1, and at STW �23 mg l�1).

3.3. Sediment

Station 1 has a muddy sand substrate (81–97% sand: 3–19% mud), whilst Station 2 has a substrate consisting of

Fig. 2. Means and 95% confidence intervals of percentage of (a) sand, (b)mud, and (c) organic matter content of sediment obtained from Station 1during 1998 and 1999. (A) May 1998 (50% closure of Mumbles Headoutfall); (B) February 1999 (complete closure of Mumbles outfall).

Fig. 3. Means and 95% confidence intervals of percentage of (a) sand, (b)mud, and (c) organic matter content of sediment obtained from Station 2during 1998 and 1999. (A) May 1998 (50% closure of Mumbles Headoutfall); (B) February 1999 (complete closure of Mumbles outfall).


almost equal proportions of mud and sand (48–64% sand:36–52% mud). Sediment composition at Stations 1 (Fig. 2aand b) and 2 (Fig. 3a and b) remained relatively consistentover the study period.

Cumulative frequency curves for the sand fractions ofStations 1 and 2 shows that the sand was well sorted,and composed of fine and very fine sand. The two stationsdiffered in that the sand fraction at Station 1 was composedprimarily of fine sand (53–70% fine sand; 13–31% very finesand), whilst Station 2 had almost equal proportions of fine(42–54%) and very fine sand (36–41%). This is expected anddemonstrates that particle size of the sand fractiondecreases in the direction of decreasing exposure to waveaction (i.e. from Station 1 to 2).

Sediment at both stations, over the study period, may bedescribed as organically enriched (>1% organic matter con-

tent), with organic matter contents ranging from 1.6% to3.2% at Station 1 (Fig. 2c), and 4.7% to 6.7% at Station2 (Fig. 3c). Increased organic enrichment at Station 2, how-ever, is likely to be in relation to a decrease in exposure towave action.

A significant correlation (p < 0.05) was found betweenthe levels of organic matter and mud content in the sedi-ment. This confirms the relationship, that the greater theproportion of mud there is in marine sediment, the higherthe levels of organic matter at the stations and confirmsthat the organic matter content of the sediments is con-trolled by the same sedimentary processes controlling thedeposition of the mud-sized particles at these Stationsviz. the extent of exposure to wave action and tidal cur-rents. It seems, therefore, that, although the amount of par-ticulate organic matter available for deposition would be

Fig. 4. Means and 95% confidence intervals of the Shannon diversity (H 0)index for (a) Station 1 and (b) Station 2. (A) May 1998 (50% closure ofMumbles Head outfall); (B) February 1999 (100% closure of MumblesHead outfall).


directly controlled by the amount entering the system (e.g.sewage effluent discharge), the ultimate fate of that organicmaterial would depend on the locally operating physicaland dynamic processes.

3.4. Fauna

3.4.1. Station 1

Prior to the cessation of the sewage discharge fromMumbles Head, the benthic community at Station 1 waslargely composed of polychaetes, bivalves, amphipodsand ophiuroids, but was one in which polychaetes domi-nated (Table 2). Once there was a 50% reduction in outfalldischarges in May 1998, there was a significant change inthe species composition of the community i.e. polychaetesand bivalves became less dominant as the relative propor-tions of amphipods, gastropods and ophiuroids increased(Table 2). This trend continued following the decommis-sioning of the Mumbles Head sewage outfall (February1999), to the point that by the end of the study period,the community was composed largely of polychaetes,amphipods, bivalves and ophiuroids (Table 2), but was inwhich polychaetes and amhipods were equally dominant.

Fig. 4a shows the Shannon–Wiener diversity indices(H 0) seasonally from January 1998 to October 1999 for Sta-tion 1. They provide evidence of a significant fall in speciesdiversity at Station 1 from the period that the MumblesHead outfall was in full time operation (January 1998) tothe time that it was decommissioned (February 1999).The 2-sample t-test shows that the means of the Shan-non–Wiener diversity indices (H 0) differed significantlybetween 1998 and 1999 (p < 0.05) at the 95% confidenceinterval.

The hierarchical agglomerative method (Fig. 5) showsthat there were 1 major group (Group 4) and 3 minorgroups (Groups 1–3) of benthic species at the 45% similar-ity level, and the further division of Groups 1, 2 and 4 intosub-groups at 60% similarity. Within Group 4, Groups a–fconsist of samples obtained before (January 1998) and dur-ing the initial phase of the 50% reduction in outfall dis-charges (May 1998), whilst Groups g–i are those samplestaken during the latter phase of the 50% reduction in out-fall discharges (August 1998 and January 1999) and follow-ing the decommissioning of the Mumbles Head outfall

Table 2Percentage of individuals from the major taxa present at Station 1

ClassPolychaeta

ClassBivalvia

OrderAmphipoda

OrderDecapoda

ClassGastropoda

OM

January �98 68.1 19.4 8.8 0.0 0.0 0May �98 33.3 12.3 1.6 0.3 0.5 0August �98 25.9 9.1 11.6 0.2 0.0 0October �98 34.5 4.1 8.1 0.0 0.6 0January �99 34.6 4.3 8.6 0.0 0.1 0May �99 29.2 15.7 2.9 0.2 3.8 0August �99 17.5 7.0 24.0 0.8 0.0 0October �99 22.9 5.1 18.9 0.2 0.0 0

(February 1999) (Fig. 5). Seasonality is also reflected inthe samples, with Group 4h consisting of samples obtainedduring the summer of 1998 and 1999, and Group 4i com-posed largely of samples acquired during the winter andautumn months of 1998 and 1999.

The similarity percentage programme SIMPER (Clarke,1993) was used to investigate the contributions of individ-ual species to dissimilarities between the 1998 and 1999samples of winter (January), spring (May), summer(August) and autumn (October). The polychaetes Amphar-

ete acutifrons (Grube), Owenia fusiformis (Delle Chiaje),and Lanice conchilega (Pallas) (Fig. 6a), and the amphipodAmpelisca brevicornis (Costa) (Fig. 6b) were more abun-dant during the period that there was a 50% reduction inoutfall discharges (January 1999), than when the outfall

rderysidacea

ClassOphiuroidea

ClassCirripedia

PhylumPycnogonida

PhylumCnidaria

OrderTanaidacea

.0 1.9 0.0 0.5 0.9 0.5

.0 2.0 0.0 0.5 0.0 0.0

.0 3.1 0.0 0.1 0.0 0.0

.1 3.1 0.0 0.0 0.0 0.1

.0 2.3 0.0 0.0 0.1 0.1

.0 1.1 0.1 0.3 0.5 0.1

.1 0.7 0.0 0.0 0.0 0.0

.0 2.5 0.0 0.4 0.0 0.0

Fig. 5. Dendrogram for hierarchical clustering of the faunal abundances at Station 1, using group-average clustering of Bray–Curtis similarities based onfourth root-transformed abundances.


was fully operational (January 1998) (Table 3). It appearsthat by utilizing its two feeding methods (filter-feeder/sur-face deposit-feeder), O. fusiformis and L. conchilega wasable to adapt and flourish in the 50% reduction in outfalldischarge phase (October 1998) (Fig. 6a), but then declinedfrom this point onwards.

Ampharete acutifrons and A. brevicornis continued toflourish and were most prolific by the time that the outfallhad been decommissioned for over 6 months (August 1999)(Fig. 6a and b). Since both A. acutifrons and A. brevicornis

are deposit feeders, the results suggest that they increasedin abundance with the reduction in the levels of total sus-pended particulate organic matter (POM) to the area. Fur-thermore, since crustaceans are known to be more sensitiveto pollution than other marine taxa (Rand and Petrocelli,1985), the increased abundance of A. brevicornis, Atylusswammerdami (Milne Edwards) and Aora gracilis (Bate)once water quality improved, may confirm a less stressedenvironment (Fig. 6b).

The bivalve Mysella bidentata (Montagu) (Fig. 6c) dif-fered in that it was most abundant during the period thatthere was a 50% reduction in outfall discharges (August1998). This is likely considering that M. bidentata is a fil-ter-feeder, and so would have been most affected by thedecreased suspended particulate organic matter (POM)input. It should be noted, however, that the suddenincrease in numbers of Angulus tenuis (da Costa) andNucula nucleus (Linnaeus) at Station 1 in May 1999 andAugust 1999 respectively (Fig. 6c), appears to be due tothe bivalves having being transported in by storm activity(allochthonous) as they were largely the size of juvenile(1–4 mm) bivalves rather than post-larval (<1 mm) (Nor-kko et al., 2001). Nevertheless, there was an overall

increase in abundance of the deposit-feeder N. nucleus fol-lowing the decommissioning of the Mumbles Head sewageoutfall (Fig. 6c).

3.4.2. Station 2

Station 2 differed to Station 1, in that before the decom-missioning of the Mumbles Head sewage outfall, the ben-thic community was almost entirely dominated bypolychaetes (Table 4). With the 50% reduction in outfalldischarges in May 1998, polychaetes became less dominantas the relatively proportions of bivalves, amphipods andophiuroids increased (Table 4). This trend continued fol-lowing the decommissioning of the Mumbles Head sewageoutfall (February 1999), and resulted in a community stilldominated by polychaetes, but with an increased percent-age of amphipods, bivalves and ophiuroids (Table 4).

Fig. 4b shows the Shannon–Wiener diversity index (H 0)seasonally from January 1998 to October 1999 for Station2. The indices show an increase in species diversity from theperiod that the Mumbles Head outfall was in full timeoperation (January 1998) to the time that it was decommis-sioned (February 1999).

The hierarchical agglomerative method (Fig. 7) showsthat there were 1 major group (Group 2) and 1 minorgroup (Group 1) of benthic species at the 45% similaritylevel, and the further division of Groups 1 and 2 intosub-groups at 65% similarity. Group 2a consists of samplesobtained prior to the cessation of the sewage dischargefrom Mumbles Head, Groups 2b and 2d are those samplestaken during the 50% reduction in outfall discharges (May1998), whilst Groups 1a and 1b are samples from the per-iod following the decommissioning of the outfall (February1999) (Fig. 7). It should be noted, however, that in May

Fig. 6. Changes in the abundance of (a) polychaete, (b) amphipod and (c) bivalve species in response to decreasing organic input at Station 1. (A) 50%Reduction in outfall discharges; (B) Full closure of the outfall.


1999 sampling did not take place at the exact station posi-tion and so the resulting data should be treated with cau-tion for this particular month. Taking this into account,January and May 1998 are most separated from the othermonths sampled (Fig. 7).

The similarity percentage programme SIMPER (Clarke,1993) was used to investigate the contributions of individ-ual species to dissimilarities between the 1998 and 1999

samples of winter (January), summer (August) and autumn(October). The Sabellid polychaetes were the principal con-tributor to the dissimilarities between the winters and sum-mers of 1998 and 1999, whilst the bivalve Angulus tenuis

was chiefly responsible for the differences between theautumns of 1998 and 1999 (Table 5).

The filter-feeding Sabellids were the numerically domi-nant polychaetes before and during the 50% reduction in

Table 3Top 10 ranked species responsible for the observed dissimilarities betweensamples of winter (January), spring (May), summer (August) and autumn(October) of 1998 and 1999 at Station 1, and their average abundances(m�2) and individual contribution (%) to the average dissimilarity value

Species Averageabundance(1 m2)

% % Averagedissimilarity

January �98 January �99

Ampharete acutifrons 10 248 6.2 55.2Ampelisca brevicornis 8 182 5.7Owenia fusiformis 32 342 5.2Lanice conchilega 6 76 4.9Perioculodes longimanus 12 0 4.1Sabellid spp. 24 0 4.1Atylus swammerdami 0 26 3.7Angulus tenuis 16 36 3.6Modiolula phaseolina 10 14 2.9Phyllodoce groenlandica 4 14 2.7

May �98 May �99

Lanice conchilega 204 14 5.6 56.0Sabellid spp. 28 0 4.2Owenia fusiformis 34 292 4.0Ampharete acutifrons 6 62 4.0Ampelisca brevicornis 6 48 3.5Amphiura chiajei 28 0 3.5Ophiura ophiura 2 20 3.4Crepidula fornicata 6 72 3.2Spionid spp. 156 46 2.8Nucula nucleus 58 24 2.6

August �98 August �99

Lanice conchilega 320 34 4.5 41Mysella bidentata 104 0 4.4Heteroclymene robusta 146 22 3.4Aora gracilis 0 68 3.1Liocarcinus holsatus 6 36 2.9Ampharete acutifrons 166 518 2.9Ampelisca brevicornis 366 902 2.5Microprotopus maculatus 14 4 2.5Orbinia sp. 8 0 2.5Pectinariid spp. 0 8 2.4

October �98 October �99

Owenia fusiformis 408 40 7.0 38.2Heteroclymene robusta 36 12 4.1Maldanid spp. 0 14 3.7Glycera gigantean 4 22 3.6Capitellid spp. 0 8 3.5Cirratulid spp. 26 10 3.4Chamelea gallina 2 10 3.2Spisula subtruncata 6 14 3.2Amphiura chiajei 60 24 3.0Fabulina fibula 2 6 2.9

Table 4Percentage of individuals from the major taxa present at Station 2

ClassPolychaeta

ClassBivalvia

OrderAmphipoda

ClassOphiuroidea

January �98 98.9 0.9 0.0 0.0May �98 99.1 0.8 0.0 0.1August �98 98.6 0.9 0.0 0.3October �98 97.7 1.7 0.0 0.3January �99 95.2 4.4 0.0 0.5May �99 68.6 10.1 15.7 0.0August �99 89.4 8.1 1.8 0.3October �99 96.3 2.8 0.4 0.4


outfall discharges period, but once the new waste watertreatment works became operational in February 1999,both the Sabellids and Cirratulids (Fig. 8a) were succeededby the filter-feeder/surface deposit-feeder polychaete O.

fusiformis (Fig. 8a). While the deposit-feeding Cirratulidpolychaetes appear to have declined with the significantdecrease in total suspended particulate organic matter(POM) discharged into Swansea Bay, it appears that the

deposit-feedering polychaete A. acutifrons (Fig. 8a), theamphipod A. brevicornis (Fig. 8b) and the bivalve Nucula

nucleus (Fig. 8c), and the filter-feeder/surface deposit-feed-ing amphipod Corophium volutator (Pallas) (Fig. 8b) gainedfrom the change in environmental conditions.

It should be noted, however, that the sudden increase inthe bivalve Spisula subtruncata (da Costa) in August 1999(Fig. 8c), appears to be due to them having been broughtin by storm activity (allochthonous), as they suddenlyappeared in great numbers at this site and were largelythe size of juvenile (1–4 mm) bivalves rather than post-lar-val (<1 mm) (Norkko et al., 2001). Furthermore, despitethe bivalve Angulus tenuis being shown to be primarilyresponsible for the differences between October 1998 andOctober 1999 (Table 5), there was an overall decline inthe filter-feeding A. tenuis over the study period (Fig. 8c).

3.5. Trophic relationships

3.5.1. Station 1

Over the study period, the benthic filter-feeders wereout-competed by the deposit-feeders at Station 1, but therewas no overall difference in the percentage of carnivores(Fig. 9a). Filter-feeding spionid species (Fig. 6a) and bival-ves (Fig. 6c) appear to have been the dominant organismsprior to the cessation of the sewage discharge, whilst thespecies which can both filter-feed and surface deposit-feedi.e. the polychaetes L. conchilega and O. fusiformis weredominant (Fig. 6a) during the 50% reduction in outfall dis-charge period (May 1998). Following the decommissioningof the Mumbles Head sewage outfall, however, theseorganisms were replaced by the deposit-feeding polychaeteA. acutifrons (Fig. 6a) and amphipod A. brevicornis

(Fig. 6b). These findings suggest that the changes in trophicgroups are likely to be in response to reduced levels of totalsuspended particulate organic matter (POM) in the near-bed water column.

3.5.2. Station 2It appears that there were again changes in trophic

groups at Station 2, as in Station 1. This site, however, dif-fered, in that the benthic filter-feeders were out-competedby the filter-feeders/surface deposit-feeders (Fig. 9b), andthe deposit-feeders were most abundant during the 50%

Fig. 7. Dendrogram for hierarchical clustering of the faunal abundances at Station 2, using group-average clustering of Bray–Curtis similarities based onfourth root-transformed abundances.


reduction in outfall discharge period (i.e. August 1998). Nooverall difference was noted in the percentage of carnivores(Fig. 9b).

Filter-feeding Sabellid polychaetes were the numericallydominant species before and during the 50% reduction inoutfall discharges period, but both the Sabellids and Cirr-atulids (Fig. 8a) were succeeded by those species whichcan both filter-feed and surface deposit-feed i.e. the poly-chaete O. fusiformis (Fig. 8a) and amphipod C. volutator

(Fig. 8b), once the new waste water treatment worksbecame operational (February 1999). It appears that byutilizing two feeding methods, they were able to adaptand flourish with the changes in food supply (De Santa-Isa-bel et al., 1998). Nevertheless, it appears that the deposit-feedering polychaete A. acutifrons (Fig. 8a), the amphipodA. brevicornis (Fig. 8b) and the bivalve Nucula nucleus

(Fig. 8c) also gained from the change in environmentalconditions.

4. Discussion

Reduction in bacterial input to Swansea Bay followingthe full closure (February 1999), of the major, untreatedsewage discharge at Mumbles Head was predicted giventhe high level of sewage treatment at the new waste watertreatment plant (Swansea STW). That this occurred overthe summer months, in the surface water samples in wes-tern inner Swansea Bay, is a direct indication of the influ-ence of the Mumbles outfall on the quality of the entirewater column. Suspended particulate organic matter(POM) would have been directly important to filter (andpossibly detritus) feeders (Grant et al., 1997; Riisgard

and Larsen, 2000) and so indirectly to the food web, andits absence is likely to have led to community changes,but not to changes in sediment organic matter content.Reduction in heavy metal contamination will reduce theecotoxicological effects over time (Clark et al., 1997), butthis aspect does not form part of the present study.

Benthic recovery processes vary considerably and aredependent on the type of stress, and the temporal and spa-tial scales of disturbance (Johnson and Frid, 1995; Karak-assis et al., 1999; Gray et al., 2002). For example, signs ofdisturbance were discovered 14 years after the cessation ofsewage sludge disposal in the Firth of Clyde (Scotland)(Moore and Rodger, 1991; as cited by Karakassis et al.,1999), whilst it took 5 years for the macrobenthos of aSwedish fjord to recover from high pollution levels (cessa-tion of pulp mill effluent) (Rosenberg, 1973; as cited byAtkins and Jones, 1991). In contrast, recovery from theimpact of fish farming is relatively rapid and may takeplace over months rather than years, since fish feed andfaeces are more labile than most sewage and spatial scalesof impact are smaller (Karakassis et al., 1999). Takingthese findings into account, it was postulated that therecovery process in western inner Swansea Bay wouldeither be a rapid, short-term (over months) or more long-term (over years) reversal of the pollution effects of organicenrichment.

The proposed recovery model is based on the notionthat, since the Mumbles outfall had been in operation,largely unaltered, over many decades, the soft sedimentbenthic communities in the receiving area of westerninner Swansea Bay would have been in equilibriumwith that existing, organically enriched environment. This

Table 5Top 10 ranked species responsible for the observed dissimilarities betweensamples of winter (January), spring (May), summer (August) and autumn(October) of 1998 and 1999 at Station 2, and their average abundances(m�2) and individual contribution (%) to the average dissimilarity value

Species Averageabundance (1 m2)

% % Averagedissimilarity

January �98 January �99

Sabellid spp. 13,687 1760 14.5 48.3Cirratulid spp. 463 40 9.3Cirratulus filiformis 50 0 8.1Owenia fusiformis 80 760 7.2Modiolula phaseolina 17 0 6.5Nephtys hombergi 27 27 5.7Ampharete acutifrons 0 27 5.5Chaetozone setosa 13 0 4.6Eteone sp. 13 0 4.6Retusa obtuse 13 0 4.5

August �98 August �99

Sabellid spp. 7160 787 11.1 36.8Angulus tenuis 0 8 0 8.1Tharyx marioni 13 160 7.6Cirratulid spp. 2240 453 6.8Corophium volutator 0 107 6.0Spisula subtruncata 40 493 5.8Ampelisca brevicornis 0 53 5.5Chamelea gallina 0 40 4.8Ophiura ophiura 40 13 4.8Owenia fusiformis 2506 5840 4.6

October �98 October �99

Angulus tenuis 0 67 15.3 32.5Owenia fusiformis 867 4107 13.9Nephtys hombergi 40 160 9.6Cirratulid spp. 67 13 9.3Nucula nucleus 67 93 7.4Amphiura chiajei 13 27 7.4Tharyx marioni 53 0 6.3Sabellid spp. 2707 1573 5.4Ampelisca brevicornis 0 27 5.3Crepidula fornicata 13 0 4.9


pre-closure equilibrium would have been disturbed by thephased decrease (50% May 1998, 100% February 1999) insuspended particulate organic matter (POM) and sewagecontaminant input from the Mumbles Head outfall. Thisinitial phase is effectively a reaction by the benthos to anenvironmental stress, where the likely effect of the stresswould be decreased species diversity and increased domi-nance (Pearson and Rosenberg, 1978; Gray, 1989; Rosand Cardell, 1992; Hall et al., 1996). This initial phase ofrecovery (disturbance) would have continued at least untilthe cessation of the discharge and most likely for some per-iod thereafter.

An intermediate phase would then occur of increasingspecies diversity and decreasing dominance, with changesin the species composition as the fauna respond to thechanged environmental conditions. This increasing, or atleast changing, diversity and decreasing dominance couldoccur gradually or in a series of steps when species numbers

and abundances could be seen to fluctuate quite rapidlyover several months.

Once the changed environmental factors stabilisedagain, i.e. with low inputs of particulates, a possible relateddecrease in the organic enrichment of the sediments andreduced levels of sewage contaminants, then the benthiccommunities would also stabilise and attain a new equilib-rium. Species composition of the communities in the finalrecovery phase would differ from the pre-closure communi-ties and would show increased diversity and decreaseddominance. If the change in organic enrichment was greatenough these new, recovered communities may have com-pletely different faunal assemblages from those of the origi-nal, organically enriched communities.

The timescale for full recovery to occur and for newequilibrium communities to be established is likely to takeseveral years to allow for migration and several spat settle-ment periods (Eagle, 1975). The initial phase is likely to beshort, months only, whilst the intermediate, disturbed com-munity structure phase when diversity increases and dom-inance decreases would bridge the time interval.

Data obtained from the present study substantiates theinitial and intermediate phases hypothesised in the postu-lated recovery pattern. That is, a decline in the filter-feed-ing, spionid species and bivalves, and the filter feeding/surface deposit feeding polychaetes O. fusiformis and L.

conchilega at Station 1 appear to have occurred, with thereduction of the suspended particulate organic matter(POM) input after outfall closure. This resulted in an initialsignificant decline in species diversity, which equates to theinitial phase in the proposed recovery model. Diversityindices also suggest that it is only in the autumn of 1999,8 months after the closure of the Mumbles Head outfall,that there appears to be an approach of the intermediatephase of the recovery model at this particular site.

The intermediate phase is reflected in the deposit-feedingpolychaete A. acutifrons and amphipod A. brevicornis seem-ingly becoming more abundant at Station 1 with the reduc-tion in suspended particulate organic matter (POM). Theincreased abundance of amphipods suggests that as the lev-els of POM and sewage contaminants decreased, the envi-ronment became less stressed, since amphipods are moresensitive to pollution (environmental stress) than other mar-ine species (Rand and Petrocelli, 1985; Arvai et al., 2002;Cesar et al., 2004; Riba et al., 2004).

Data indicates the final phase in the recovery model wasapproached at Station 2. That is, from high environmentalstress and communities with low diversity, through a mod-erately stressed phase, to one of low environmental stresswhere the communities have relatively high species diver-sity. These findings are in agreement with studies carriedout by Read et al. (1982, 1983), who found that duringtheir three year study to assess the environmental impactof changed effluent content produced by a new sewagetreatment scheme for Edinburgh (Scotland), there was asignificant increase in water quality, and species diversityand abundance in the sublittoral benthos.

Fig. 8. Changes in the abundance of (a) polychaete, (b) amphipod and (c) bivalve species in response to decreasing organic input at Station 2. (A) 50%reduction in outfall discharges; (B) Full closure of the outfall.


Analyses, however, indicate that there has not beencomplete recovery at Station 2, since the water qualityand the state of the benthos deteriorated in the autumnof 1999. This is expected as the Mumbles Head outfall con-tinues to discharge storm water overflow during periods ofvery high rainfall.

A decline in filter feeders was also apparent at Station 2,as was observed at Station 1. The filter feeding Sabellid poly-chaetes that were prolific at Station 2 when the outfall wasoperational, were replaced by the filter feeding/surface

deposit feeding polychaete O. fusiformis, and amphipod C.

volutator with the reduction in suspended particulate organicmatter (POM). This shows that a different faunal assemblagewas gradually replacing the original, organically enrichedcommunity that was present before the decommissioningof the Mumbles Heads outfall. The appearance of C. voluta-

tor and A. brevicornis at Station 2, once less stressed condi-tions occurred, agrees with the findings at Station 1, and isfurther evidence that amphipods appear to becoming moreabundant as the environment becomes less stressed.

Fig. 9. Changes in the occurrence of different trophic groups at (a) Station 1 and (b) Station 2, to decreasing organic input over the study period. (A) 50%reduction in outfall discharges; (B) Full closure of the outfall.


5. Conclusions

This investigation shows that there have been significantimprovements in seawater quality (Blue Flag Awardachieved for the first time at Bracelet Bay, west of SwanseaBay, for the 2002 bathing season) and changes in the speciescomposition of the benthic communities within westerninner Swansea Bay following the cessation of the MumblesHead sewage discharge. These changes are likely to berelated to the significant reduction in total suspended par-ticulate organic material (POM) and decreased levels ofsewage contaminants discharged to the Bay, brought aboutby the commissioning of the new waste water treatmentplant (Swansea STW). Data obtained from the presentstudy substantiates the initial, intermediate and final phaseshypothesised in the postulated recovery model, with thetimescale for the full recovery expected to take several years.

Acknowledgements

We wish to thank K. Naylor, boatman of the school�svessel, �Noctiluca�. Thanks also to H. Morgan (City and

County of Swansea) for providing bacteriological seawaterquality data, the Environment Agency for supplyingwater quality data, D.V. Smith, A. Smith and M.R. Lin-tern for their help in sampling, and to the anonymous ref-eree for valuable comments on the earlier version of themanuscript.

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Effects of the closure of a major sewage outfall on sublittoral, soft sediment benthic communities