The impact of a macroalgal mat on benthic biodiversity in Poole Harbour

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Marine Pollution Bulletin 53 (2006) 63–71

The impact of a macroalgal mat on benthic biodiversityin Poole Harbour

Martin Jones, Eunice Pinn *

School of Conservation Sciences, Bournemouth University, Fern Barrow, Poole, Dorset BH12 5BB, United Kingdom

Abstract

Blooms of macroalgal matting are increasingly common within temperate zones and are often comprised of opportunistic species suchas Ulva lactuca. Where this algae forms a dense mat, a stressful environment is created in the sediment below, influencing the invertebrateinfaunal assemblage. This study was conducted over a six month period during which a dense mat of U. lactuca developed and subse-quently dispersed. The algal mat was found to have a significant negative impact on species richness, abundance and biomass of the mac-roinfauna. However, a faunal community developed within the algal mat which contained several species not previously observed. Thiscommunity increased the abundance and diversity of the overall invertebrate assemblage. The results are discussed in relation to impactson the ecosystem as a whole.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Algal bloom; Macrofauna; Nutrients; Poole Harbour, Ulva lactuca

1. Introduction

Macroalgal blooms of opportunistic species belongingto the genera Ulva and Enteromorpha are becoming moreprevalent around the world (Morand and Briand, 1996;Pihl et al., 1999; Viaroli et al., 2001). Although bloomsof ephemeral green algae are a natural component ofestuarine habitats (Everett, 1991), they are becomingincreasingly more abundant and dense. These exceptionalblooms are thought to be indicators of anthropogenicallyinduced eutrophication or hypertrophiation (Everett,1991; Fletcher, 1996). Schramm and Nienhuis (1996) defineeutrophication as �the process of natural or man-madeenrichment with nutrient elements, mainly nitrogen andphosphorus, beyond the maximum critical level of theself-regulatory capacity of a given system for a balancedflow and cycling of nutrients� and hypertrophication (nutri-

0025-326X/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.marpolbul.2005.09.018

* Corresponding author. Present address: Joint Nature ConservationCommittee, Dunnet House, 7 Thistle Place, Aberdeen AB10 1UZ, UnitedKingdom. Fax: +44 1224 621488.

E-mail address: [email protected] (E. Pinn).

ent pollution) as �over-enrichment or excess supply ofnutrients beyond the maximum critical self-regulatory levelto an extent that detrimental processes cause irreversiblechanges in aquatic communities, as long as nutrient levelsare not reduced�.

Increased urbanisation and industrialisation has led to adramatic increase in nutrient inputs to aquatic systems overthe last 50 years (Deegan et al., 2002; De Jonge et al.,2002). From a global perspective, aquatic systems often re-spond differently from one another to eutrophication, withthe response primarily dependant on a variety of physicalattributes such as geomorphology and flushing time (DeJonge et al., 2002; Elliott and de Jonge, 2002). In general,however, opportunistic algal species can take advantageof increases in nutrient inputs, particularly nitrate andphosphate (Rosenberg et al., 1990; Peckol and Rivers,1996; Pihl et al., 1999). For example, Ulva and Enteromor-

pha can take up nutrients 4–6 times faster than slowergrowing perennial species (Pederse and Borum, 1997) lead-ing to blooms in the appropriate environment. In addition,they are better adapted to estuarine conditions than theircompetitors, with more efficient osmoregulation at reduced

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64 M. Jones, E. Pinn / Marine Pollution Bulletin 53 (2006) 63–71

salinities (Black and Weeks, 1972) and greater tolerance totemperature variation (Raffaelli et al., 1998).

Algal blooms represent an enormous stock of carbonand nutrients in the system (Morand and Briand, 1996),of which significant amounts of dissolved organic carboncan be leached from the algae into the water column (Tyleret al., 2001; Castaldelli et al., 2003). The presence of themats can lead to major changes in the biogeochemicalcycles (Morand and Briand, 1996; Valiela et al., 1997),which can modify food chains, faunal community structureand ecosystem processes (McComb et al., 1981; Hull, 1987;Sfriso et al., 1987, 1992; Lavery and McComb, 1991;Valiela et al., 1992, 1997; D�Avanzo and Kremer, 1994;Peckol et al., 1994; Raffaelli et al., 1998).

At the base of the mat, an anoxic gradient develops dueto decomposition of the algae (Wharfe, 1977; Reise, 1985;Hansen and Kristensen, 1997; Bolam et al., 2000). In addi-tion, the water within the mat can become super-saturated(Krause-Jensen et al., 1999) which leads to severe diurnalfluctuations in oxygen (D�Avanzo and Kremer, 1994). Bac-terial decomposition has been demonstrated to increasedramatically within both the sediment and the watercolumn (Nedergaard et al., 2002), leading to hypoxia andanoxia. This can be prolonged, resulting in the accumula-tion of sulphides due to the activity of sulphate-reducingbacteria (Viaroli et al., 1995, 2001; Castel et al., 1996).The result of these biochemical changes is a modificationof the macrofaunal community (Reise, 1985; Norkko andBonsdorff, 1996a), producing a reduction in diversity andabundance of the infaunal community (Raffaelli et al.,1991; Bolam et al., 2000; Osterling and Pihl, 2001; Franzand Friedman, 2002). Changes at the macrofaunal levelhave significant impacts higher up the food chain (Norkkoand Bonsdorff, 1996a; Raffaelli et al., 1998; Deegan et al.,2002). Isaksson et al. (1994) reported an impact on cod for-aging, whilst Cabral et al. (1999) and Raffaelli (1999) re-ported significant impacts on wildfowl foraging. Forhumans, macroalgal blooms can lead to severe economiclosses in areas exploited for fisheries or aquaculture (Cast-aldelli et al., 2003).

In recent years, macroalgal mats of Ulva lactuca havebecome increasingly common in Poole Harbour (Dorset,southern England). The harbour is recognised as an impor-tant area for wildlife, with many designations includingSpecial Site of Scientific Interest (SSSI), Special ProtectionArea (SPA), and Ramsar site (designated under the Con-vention on Wetlands of International Importance Espe-cially as Waterfowl Habitat). The harbour also containsa Royal Society for the Protection of Birds (RSPB) naturereserve at Arne and has important fish nursery and shellfishgrounds. Concerns have been expressed by the RSPB andEnglish Nature regarding the impact of the annual algalmats on the wildfowl using the harbour, particularly inrelation to prey availability. The aim of this study was toprovide an initial assessment of this impact by investigatingthe effect of the macroagal bloom on the invertebrate faunaof the harbour.

2. Methods and materials

The study area was sited within Holes Bay, an enclosedbay of Poole Harbour, southern England, which has lim-ited tidal flushing (Fig. 1). A marina is situated withinthe bay and, in addition, a water treatment plant dischargesinto the river which feeds the bay. This study was con-ducted from June to November 2002 at a permanentlymarked site measuring 50 m · 50 m. Due to the nature ofthe site, it was not possible to have a control position whereno algal mat development occurred within Holes Bay. Nei-ther was it possible to set up a control site outside HolesBay with similar environmental and hydrographic regimes.

At monthly intervals, 30 0.25 m2 randomly placed quad-rats were used to determine the percentage coverage of thealgae and its thickness. From July onwards, a single quad-rat-sized piece of mat (0.25 m2) was removed to record itsfaunal community. Thirty sediment cores were also ex-tracted on a monthly basis. A 75 mm diameter pipe withan internal plunger was used to extract cores to a depthof 150 mm. The sediment obtained was washed through a0.5 mm sieve and the macrofauna obtained recorded tospecies level where possible.

Differences in the assemblages between months were eval-uated with both multivariate and univariate statistical analy-ses. Initially, the invertebrate data, i.e., both the core andalgal mat samples combined, were analysed using centroids(the mean of the replicate cores (n = 30) combined with themat data). However, due to the different methods used to col-lect the data, the results were converted to abundance indicesbefore analysis was undertaken. The abundance indices wereadapted from Hawkins and Jones (1992) based on the SAC-FOR scale; with 1 being equivalent to rare (1–3 individuals),2 to occasional (4–8 individuals), 3 to frequent (9–14 individ-uals), 4 to common (15–24 individuals), 5 to abundant (25–49individuals), 6 to superabundant (50–99 individuals) and 7 toextremely abundant (P100 individuals). The Bray–Curtismeasure was used to calculate dissimilarities between monthsand cluster analysis undertaken (Plymouth Routines in Mul-tivariate Ecological Research; Clark, 1993). Species contrib-uting most differences among months were identified usingsimilarity percentages (SIMPER; Clark, 1993).

Analysis of variance (ANOVA) was undertaken for theinfaunal abundance and biomass data. In addition, due toits dominance in the samples, the data for Nereis diversi-

color was considered on an individual basis. Prior to anal-ysis, data were tested for homogeneity of variances usingLevene�s test. Heterogeneous data were square root trans-formed (Underwood, 1997). This, however, did not removethe heterogeneity. Underwood (1997) and Pallant (2001)both report that for large balanced data sets, violationsin the assumption of homogeneity and normality areunlikely to affect the F ratio. It was, therefore, decidedto undertake the ANOVA using the non-transformeddata, with a more conservative probability of 0.01. AfterANOVA, Tukey HSD post-hoc tests were conducted toassess the cause of variations between months.

Fig. 1. Position of the Holes Bay survey area (aerial image of Holes Bay adapted from Environment Agency CASI image of Poole Harbour taken on 8thAugust 1998).

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M. Jones, E. Pinn / Marine Pollution Bulletin 53 (2006) 63–71 65

3. Results

3.1. Variation in mat coverage

Within two months of the first visit in June, a dense algalmat had developed. Mean coverage in June was 5.2%, thisincreased to 73.7% by July and to a maximum of 91.0% inAugust (Fig. 2). Thereafter, mean coverage declined,reducing to 3.8% in November when the survey finished.At its height the mat was up to 90 mm thick.

Jun Jul Aug Sep Oct Nov0

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Fig. 2. Development of the macroalgal mat at Holes Bay between Juneand November 2002 (error bars represent standard deviation).

3.2. The estuarine invertebrate community

A total of 18 invertebrate species were identified fromthe core and algal mat samples collected, although the dis-tribution and occurrence of these species varied over the

Table 1The Estuarine Community

June July August September October November

Nereis diversicolor C C, M C, M C C, M CNereis zonata CNereis pelagica C C, M CPerineris cultifera C CArenicola marina CNephtys caeca CCapitella capitata CHydrobia ulvae C C, M C, M C, M MTapes descussatus C M C C, M CLutraria lutraria C CCerastoderma edule C MGari costulata CLittorina tenebrosa M M MMactra stultorum MVenerupis senegalensis M CCarcinus maenus C, M MGammarus locusta MPeachia cylindrica C C, M C, M C, M C

C: infaunal community from core; M: faunal community from mat.


study period (Table 1). Initially, species richness increasedbetween June and July as the mat developed (Fig. 3A). The

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Fig. 3. Species richness of the invertebrate com

following month, it had rapidly reduced, despite a richfauna within the mat. Thereafter there was a slight increase

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munity (A) and cluster analysis results (B).

Table 3SIMPER dissimilarity results

Abundanceindex

Cumulativepercentage

July OctoberGammarus locusta 0 5 15.15Venerupis senegalensis 4 0 27.27Hydrobia ulvae 7 4 36.36Tapes decussatus 4 1 45.45Carcinus maenas 3 0 54.55Capitella capitata 0 3 63.64Mactra stultorum 3 0 72.73

July NovemberHydrobia ulvae 7 0 21.88Tapes decussatus 4 0 34.38Peachia cylindrica 3 0 43.75Cerastoderma edule 3 0 53.12Carcinus maenas 3 0 62.50Mactra stultorum 3 0 71.88

August/September October


in invertebrate species richness, which reduced again toreach its lowest point in November (Fig. 3A).

Cluster analysis revealed distinct differences between theNovember samples and all others (Fig. 3B). The species ob-served in October were also predominantly different. TheAugust and September were grouped together as the mostsimilar. Consequently, for the SIMPER analysis, themonths of August and September were combined into asingle group. The key species distinguishing the June andJuly samples were predominantly molluscs. July saw inincrease in the abundance of Hydrobia ulvae, Venerupis

senegalensis, Tapes decussatus, Cerastoderma edule andMactra stultorum (Table 2). The abundance of many ofthese species was reduced in August and September, withV. senegalensis, C. edule and M. stultorum being lost fromthe community (Table 2). Although, H. ulvae was observedat higher levels than the other invertebrate species, fromAugust/September to October, the abundance of H. ulvae

Table 2SIMPER dissimilarity results

Abundanceindex

Cumulativepercentage

June JulyHydrobia ulvae 2 7 17.86Venerupis senegalensis 0 4 32.14Tapes decussatus 1 4 42.86Cerastoderma edule 0 3 53.57Carcinus maenas 0 3 64.29Mactra stultorum 0 3 75.00

June August/SeptemberHydrobia ulvae 2 7 34.43Neris diversicolor 6 4 44.85Littorina tenebrosa 0 1 55.26Perneris cultiferia 2 0 65.57Nereis zonata 1 0 72.47

June OctoberGammarus locusta 0 5 26.32Capitella capitata 0 3 42.11Nereis diversicolor 6 4 52.63Hydrobia ulvae 2 4 63.16Perneris cultiferia 2 0 73.68

June NovemberNereis diversicolor 6 4 14.29Hydrobia ulvae 2 0 28.57Perneris cultiferia 2 0 42.86Venerupis senegalensis 0 2 57.14Nereis pelagica 1 0 64.29Nereis zonata 1 0 71.43

July August/SeptemberVenerupis senegalensis 4 0 18.59Cerastoderma edule 3 0 32.54Mactra stultorum 3 0 46.48Tapes decussatus 4 1 58.08Carcinus maenas 3 1 67.42Nereis diversicolor 6 4 74.41

0: absent; 1: 1–3 individuals; 2: 4–8 individuals; 3: 9–14 individuals; 4:15–24 individuals; 5: 25–49 individuals; 6: 50–99 individuals; 7: >100individuals.

Gammarus locusta 0 5 27.04Capitella capitata 0 3 43.26Hydrobia ulvae 7 4 59.41Cerastoderma edule 0 2 70.22

August/September NovemberHydrobia ulvae 7 0 39.91Peachia cylindrica 2 0 51.35Venerupis senegalensis 0 2 62.78Tapes decussatus 1 0 71.41

October NovemberGammarus locusta 5 0 26.32Hydrobia ulvae 4 0 47.37Capitella capitata 3 0 63.16Cerastoderma edule 2 0 73.68

0: absent; 1: 1–3 individuals; 2: 4–8 individuals; 3: 9–14 individuals; 4:15–24 individuals; 5: 25–49 individuals; 6: 50–99 individuals; 7: >100individuals.

dramatically reduced. Differences between the August/Sep-tember and October samples included the addition of Gam-

merus locusta and Capitella capitata to the invertebratecommunity, and C. edule returned (Table 3). From Octoberto November there was a large scale loss of species from thesamples including H. ulvae, C. edule, T. decussatus, G. loc-usta, and Peachia cylindrica (Table 3).

3.3. Infaunal community of the cores

Infaunal species richness was generally very low in HolesBay, with a maximum of 1.5 per core with a standarddeviation of 0.9 observed in June. Species richness thendeclined to 0.6 ± 0.7 per core in August as the mat devel-oped (Fig. 4A). Thereafter species richness remained at areduced level, with the lowest value being recorded inNovember (0.6 ± 0.6 per core). Using ANOVA, thesedifferences were found to be statistically significant(p < 0.001, f[5,174] = 7.212). Post-hoc comparisons usingthe Tukey HSD test, indicated that June did not differ fromJuly, but it did differ from the remaining samples, whilstJuly differed from August and November.

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Fig. 4. Variation in infaunal species richness (A), abundance (B) andbiomass (C) (error bars represent standard deviation).

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Fig. 5. Variation in the abundance (A) and biomass (B) of Nereis

diversicolor (error bars represent standard deviation).

Table 4Faunal abundance within the mat (numbers per 0.25 m2)

June July August September

Nereis diversicolor 3 4 2Nereis pelagica 1Hydrobia ulvae 147 253 62 19Tapes descussatus 24 3Cerastoderma edule 5Littorina tenebrosa 3 2 5Mactra stultorum 12Venerupis senegalensis 13Carcinus maenus 9 6Gammarus locusta 46Peachia cylindrica 8 6 4


Infaunal abundance declined steadily from a maximumof 3.7 ± 2.2 per core in June to a minimum of 0.9 ± 1.2per core in September (Fig. 4B). October showed a slightincrease, which decreased to the lowest value recorded inNovember, (0.8 ± 0.9 per core). ANOVA revealed thesedifferences to be significant (p < 0.001, f[5,174] = 14.229).Post-hoc testing showed that June was different to anyother month, and July differed from September andNovember.

Infaunal biomass peaked in July with 0.55 ± 0.74 g percore, whilst August had the lowest infaunal biomass,0.15 ± 0.37 g per core. In October and November, infaunalbiomass appeared to increase to values close to those ob-served at the start of the study (0.43 ± 0.52 per core and0.34 ± 0.52 per core, respectively) (Fig. 4C). These varia-tions were found to be statistically significant (p < 0.01,f[5,174] = 3.143). Post-hoc comparisons indicated that Julywas significantly different from the August and Septembersamples.

A decreasing trend in the abundance of N. diversicolor

was evident from June (3.0 ± 0.9 per core) through to Sep-

tember (0.6 ± 0.8 per core) (Fig. 5A). Thereafter, abun-dance remained low. These differences were significant(p < 0.001, f[5,174] = 14.566). A post-hoc comparison indi-cated that June differed form all other months. July differedfrom September, October and November. The biomass ofN. diversicolor, however, did not follow a similar patternand was quite variable. It was highest in June(0.24 ± 0.21 g per core) and July (0.27 ± 0.38 g per core)and lowest in August (0.14 ± 0.37 g per core) and Septem-ber (0.09 ± 0.13 g per core) (Fig. 5B). These differenceswere not found to be significant (p > 0.01, f[5,174] = 1.823).

3.4. Faunal community of the algal mat

From July through to October, a single piece of 0.25 m2

mat was removed at monthly intervals to assess the faunalassemblage that developed within the algal mat. Many of


the species observed were similar to those found in theearly cores, although several were new, e.g., Littorina ten-

ebrosa and M. stultorum. Eleven species were recorded intotal from the mat, with Hydrobia ulvae being by far themost abundant (numbers ranged from 19 to a maximumof 253 per 0.25 m2) (Table 4). Other species were observedto be abundant in particular months, e.g., T. decussatus inAugust and G. locusta in September (Table 4).

4. Discussion

Over the six month period of observations, markedchanges were observed in the invertebrate community ofHoles Bay, southern England. Due to the lack of a suitablecontrol, any changes observed in the invertebrate fauna cannot be causally linked to mat development. However, thisstudy was conducted during the spring/summer period,when the faunal community would normally be expectedto exhibit the highest levels of species diversity and abun-dance of any time of year (Souza and Gianuca, 1995; Tuyaet al., 2001; Rueda and Salas, 2003). It is, therefore, likelythat the changes observed in the community are associatedwith the development of the algal mat and its subsequentdispersal with the onset of winter.

As the mat developed, there was an initial increase ininfaunal diversity and abundance. However, this rapidlydeclined as the mat became more dense. Similar observa-tions associated with the development of macroalgalblooms have been made by Lopes et al. (2000) and Bolamet al. (2000). The changes occurring in the benthic commu-nity in relation to macroalgal blooms are extremely com-plex and result from the interaction of many factors(Hull, 1988; Raffaelli et al., 1998). These factors includereduced current velocity, which leads to siltation, andreduced oxygen exchange, anoxia and the production oftoxic hydrogen sulphide (Hull, 1987; Norkko, 1998; Bolamet al., 2000). Macroalgal mats have also been demonstratedto act as a physical barrier or enhancement to larval settle-ment and as a barrier to predation of infaunal species(Hull, 1987; Olafsson, 1988; Bolam et al., 2000). In addi-tion, Osterling and Pihl (2001) proposed that the effect ofa macroalgal bloom on the benthic community was relatedto tolerance, feeding mode and mobility of the macro-faunal species present.

The effects of macroalgal blooms are often similar tothose resulting from organic enrichment. Carbon andnitrogen levels have been found to be three times higherin sediment covered with an algal mat than unvegetatedareas (Pihl et al., 1999). In addition, as observed in the cur-rent study, there is also often an increase in opportunisticspecies such as C. capitata (Thrush, 1986; Bolam et al.,2000; Lopes et al., 2000).

N. diversicolor is a typical estuarine species. Norkko andBonsdorff (1996b) found an increase in abundance and sizeof this species under macroalgal mats and suggested that itstolerance to hypoxia and sulphide along with its role as aninfaunal predator enabled it to feed on stressed macro-

fauna. Osterling and Pihl (2001) also reported an increasein N. diversicolor amongst algal mats. In contrast, Lopeset al. (2000) reported an initial increase in the species, butthereafter the impact of the algal bloom was negative whilstLewis et al. (2003) reported a decline in the species. In thecurrent study, during development and subsequent dis-persal of the macroalgal mat, the abundance of this poly-chaete declined steadily, although the early biomassmeasurements did not decline. This may indicate thatinitially N. diversicolor gained from the impact of the maton other species through increased successful predationopportunities. However, as mat development progressedand the environment became more stressed, N. diversicolor

itself was negatively impacted by the environmentalchanges associated with the algal mat.

Everett (1994) found that bivalves such as Macoma

balthica decreased in abundance under macroalgal mats.More recently, Osterling and Pihl (2001) reported an impacton surface deposit feeders and suspension feeders, includingCerastoderma spp. In contrast, Hull (1988) and Bolam et al.(2000) reported greater numbers of bivalves. In the currentstudy, although mollusc abundance initially increased inresponse to the macroalgal mat, latterly it declined quitesteeply with almost all bivalve species being lost from thesystem by November. Bolam et al. (2000) proposed thatthese differences relate to algal biomass, with bivalve num-bers only declining at higher biomass levels. The findings ofthe present study indicate that this is likely to be so.

Associated with the macroalgal bloom, was the develop-ment of a faunal community within the mat containingsome species not previously observed. Everett (1994) andCabral et al. (1999) reported similar findings. As observedin the current study, Bolam et al. (2000) also reported anincrease in gammerids associated with the mat. In particu-lar, however, it was H. ulvae that benefited from thepresence of the macroalgal mat in Holes Bay. Similarobservations have been made previously (Soulsby et al.,1982; Norkko et al., 2000). It is likely that some H. ulvaemigrated from the sediment, as observed by Osterlingand Pihl (2001). However, the numbers observed in themat were significantly greater than observed within the sed-iment at the start of the present study, indicating additionalhydrographic movement of H. ulvae within the system.

As the biomass of the macroalgal bloom increases, pro-portions of the mat become photosynthetically inactive andrespiratory processes dominate, resulting in a net carbonrelease (Peckol and Rivers, 1996). Rates of decompositioncan be extremely high, leading to a continuous input of or-ganic matter and release of nutrients to the system thatmay exceed the external load from land run-off (Pihlet al., 1999). Where water exchange is limited, organic mat-ter may accumulate leading to the formation of a basicpool of nutrients for future algal development (Pihl et al.,1999). In this way, macroalgal blooms may be self-regener-ating (Norkko and Bonsdorff, 1996b). However, Hansenand Kristensen (1997) proposed that the benthic faunacould actually accelerate the return to a healthy ecosystem


and re-establish pre-pollution conditions through bioturba-tion and active turnover of the sediment. In the case ofHoles Bay, however, where water turnover is limited, man-agement of anthropogenically derived nutrient sources willbe necessary to reduce the possibility of macroalgal bloomsoccurring before the activities of the benthic fauna canhave any dramatic effect.

Any long term altering of the benthic community couldhave a cascading effect on ecosystem function (Franz andFriedman, 2002). A shift to ephemeral green algae fromperennial brown algae or other habitat types will result inlarge scale alterations to the complexity of the ecosystemand changes in the invertebrate community. In some areasthis has already had an effect higher up the food chain, e.g.,reduced settlement and recruitment of plaice and cod(Isaksson et al., 1994; Wennhage and Pihl, 1994) and thefeeding patterns of birds (Cabral et al., 1999; Raffaelli,1999; Lewis et al., 2003). It will be necessary to undertakedetailed surveys in Holes Bay before the effect of the annualmacroalgal bloom can be assessed. It is likely, however,that the bloom will impact on the important wildfowl pop-ulations of the area.

In summary, patchy macroalgal mats may be beneficialto the benthic community, resulting in increased diversity(Pihl et al., 1996; Raffaelli et al., 1998). However, as ob-served in the current study, large extensive mats tend tolead to an impoverished benthic assemblage, often withaltered species composition (Norkko and Bonsdorff,1996a). Consequently, the larger the area covered by amacroalgal bloom, the more pronounced the ecological ef-fects are likely to be. The long term effects of macroalgalmat development will be determined, in part, by the spatialdistribution of the mats and the macrofaunal assemblagepresent (Raffaelli, 1999). Changes in the benthos will leadto alterations higher up the trophic web, usually a reducedabundance and diversity of larger organisms that feed onthe benthos (Deegan et al., 2002). In addition, hypoxia,often associated with eutrophication, reduces the capacityof the system to transfer carbon to high trophic levels, withthe system becoming increasingly dominated by microbialloops rather than transfer to consumers (Baird et al.,2004). Consequently, it is important that the causes of mac-roalgal blooms are well understood and controlled so thatfuture unnatural blooms are limited. Alternatively, thedegradation of the benthos is likely to leave a devaluedestuarine ecosystem that maybe incapable of performingmany of the functions taken for granted.

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The impact of a macroalgal mat on benthic biodiversity in Poole Harbour

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