This study has shown that three intertidal estuarine shores based in the north-west of England differed significantly in terms of their macrofaunal species diversity and faunal biomass. Differences were also evident in the physical structure of the sediments. Despite these differences however, there were little differences apparent
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33 in the functional structure of the assemblages, as investigated through analysis of the distribution of functionally-important biological traits expressed within assemblages. Seasonal changes in sediment granulometry were observed within the study sites. Such seasonal changes are likely to be related to seasonal fluctuations in wave energy, being higher in winter than summer (Frostick & McCave 1979). In addition, seasonal changes to water temperature can also influence sedimentation with the sedimentation of mud particles particularly sensitive to temperature; this can cause finer sediments to be eroded over winter months, resulting in the coarser granulometries observed in spring (Anderson 1983; Chang et al. 2006). However, these observed changes were present only within the two sites located within the Dee estuary, at Thurstaston and West Kirby and not within the Morecambe Bay site at Warton Sands. This may be due to the fact that the Dee estuary shows a greater degree of sedimentary accretion than the area sampled within the south of Morecambe Bay and thus explains why seasonal differences in sediment granulometry were only observed within the Dee estuary (Mason & Garg 2001; Moore et al. 2009).
A number of differences in the taxonomic diversity of the study assemblages were observed, in addition to some differences to the functional diversity of these sites. All shores housed a species pool typical of a temperate estuarine intertidal habitat (e.g. de Deckere et al. 2001; Bolam & Fernandes 2003), with abundances at all three shores dominated by the gastropod Hydrobia ulvae and the polychaete Pygospio elegans. Cerastoderma edule dominated the biomass at all three sites, representing in excess of 50% of the total faunal biomass. Assemblages at Thurstaston within the Dee estuary contained a greater number of individuals across all taxa and also showed a greater diversity of taxa than those at West Kirby and Warton Sands. However, despite assemblages at Thurstaston housing a greater number of species and a greater faunal diversity, significant differences were not observed in the total biomass of fauna between the three assemblages.
Total faunal biomasses did differ by season, with assemblages sampled in autumn housing a greater biomass than those in spring. However, these differences were not reflected by seasonal changes to the total abundances of taxa or the diversity or
34 evenness of assemblages. That is, only the biomass distribution changed significantly between seasons; the total numerical abundances and identities of taxa did not change. This suggests that the observed differences were related either to the seasonal growth of individuals within assemblages, or the replacement of smaller individuals with larger individuals. Similar relationships have been observed elsewhere, for example Soares-Gomes & Fernandes (2005) found no seasonal changes in the density or diversity of bivalve assemblages in Brazil. Additionally, Dittman (1990) found no changes in the overall distribution of taxa between seasons, despite as was observed in the current study, the abundances of some individual taxa changing between seasons.
Assemblages at Thurstaston showed a greater taxonomic diversity and a more even distribution of taxa than the other two assemblages. This greater degree of biodiversity would traditionally be seen as an indicator that ecosystem functioning at Thurstaston is more robust than that at West Kirby or Warton Sands (Walker 1992). Under traditional biodiversity-ecosystem functioning (BEF) theory, increasing species richness is an indicator of a more robust functioning of the ecosystem (see Section 1.1). However, more recent contentions suggest that it is the diversity of ecologically-important biological traits within an assemblage that are more important drivers of ecosystem function than taxonomic diversity per se (McGill et al. 2006). However, with increasing taxonomic diversity, the likelihood of increasing functional diversity also grows due to the probability of gaining a taxon offering a trait or functional quality not already existent within an assemblage (Loreau 2000). This increase is not necessarily linear; therefore, despite highly significant differences in taxonomic diversity and evenness between the study assemblages, the observed differences in the distribution of biological traits were less apparent. That is, only a relatively low proportion of the variance in the trait-distribution data was explained by the sampling site. Instead, the factor explaining most of the difference in the distribution of traits was related to seasonal differences between assemblages. Such seasonal differences in the functional diversity of assemblages have been observed elsewhere (e.g. Bonada et al. 2007). The fact that there were evident seasonal differences in the distribution of functional traits and the total biomass of fauna within the benthos, but no significant changes in the taxonomic diversity or
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35 total species abundances suggests that it is the growth of a particular species set within the assemblages that is driving the change. The change in trait composition to being dominated by largely sedentary suspension feeders in autumn suggests that this difference is driven by the growth of bivalve taxa over summer months (Seed & Brown 1978).
Despite assemblages at Thurstaston housing a significantly greater degree of biodiversity than those at West Kirby and Warton Sands, no significant differences in assemblage biomass were observed. This suggests that, although the organic material contained within the three assemblages (i.e. assemblage biomass) did not differ significantly, there was the potential for the functional diversity of the assemblages to differ (Balvanera et al. 2006; Cardinale et al. 2006, Section 1.1). However, the minor differences in the distribution of traits between assemblages suggests that, at least in terms of the traits measured, the different assemblages present within the three study shores contain ecologically-similar species. Under the redundancy hypothesis (Walker 1992), the additional species present at Thurstaston would be seen as ‘ecological insurance’ and therefore serve to preserve ecological functioning in situations where the total number of species, or the abundances of dominant species, decline. That is, effects of the removal or severe reduction in the abundance/biomass of one species on ecological functioning will be shielded by the presence of functionally-similar species, hence allowing functional processes within the system to be maintained (Walker 1992; Walker 1995). This suggests that assemblages at Thurstaston are better-able to maintain ecological functioning in the face of introduced disturbances that may reduce species diversity, for example storm events, fishing impacts, pollution etc. than those at the less-diverse Warton Sands site. That is, the greater number of taxa at Thurstaston, despite offering no real functional uniqueness, in terms of supplying new biological traits not expressed by other taxa within the assemblage, means that this site shows a greater potential for ecological reliability in the face of species loss (Naeem 1998).
The apparently greater functional redundancy at Thurstaston may however be related more to the number and identity of traits that have been studied. It has been suggested that the competition for resources by benthic macrofauna is low, with the major controls relating to ‘top down’ predation pressure (Reise 1977; Virnstein 1977;
36 Evans 1983). Low competition between taxa for the available resources suggests that, with an increasing number of traits examined, more taxa will be shown to possess a unique suite of traits and thus a unique niche within the assemblage (Bremner et al. 2006a). Therefore, if the relative abundance of a taxon were to change, then there is the likelihood that the functional diversity and delivery of ecological functions would be affected. This supports the analysis of as large a collection of biological and ecological traits as is practical to obtain for investigations into ecological functioning. The ability of different shores to respond to a sudden reduction in the abundance of a biomass-dominant taxon will be explored further in Chapter 3, where experimental fishing for the biomass-dominant cockle
Cerastoderma edule will be carried out within assemblages at the species-rich Thurstaston and the relatively species-poor Warton Sands.
*chapter published Cesar & Frid (2009)