Multivariate analyses of South-American zoobenthic species — spoilt for choice

Abstract

28.1 Introduction and the underlying questions Defining spatial and temporal distribution patterns of a soft-bottom benthos community and its relationship with environmental factors has been a common task of many coastal marine ecologists. However, the choice of the most appropri-ate statistical tools for benthic data has been subject to considerable debate among researchers. Several research programmes aimed at studying the dynamics of benthic spe-cies and their environment, have been carried out at South American estuarine and coastal areas during the last few decades (Bemvenuti et al. 1978; Ieno and Bastida 1998; Lana et al.1989; Gimenez et al. 2005) focusing not only on the importance of commercial benthic species but also on the rapid habitat fragmentation and de-terioration (Elias 1992b; Elias and Bremec 1994) that have resulted from different levels of human impact and that have reduced the available feeding areas for birds. Samborombon Bay (Buenos Aires, province, Argentina) is an area of major importance in the life cycle of a large number of organisms that play key roles in the food web of the ecosystem (Ieno and Bastida 1998; Martin 2002). Bivalves, crustaceans and especially polychaetes that inhabit the inter-tidal and tidal flats represent an important link in the food chain from primary producers to predators such as resident and migratory birds and fishes. Samborombon Bay is used by mi-gratory nearctic and austral shorebirds from September to April; the main species preying on macrozoobenthos during the annual stop over migrations are the Red Knot {Calidris canutus rufa), the White-rumped Sandpiper {Calidris fuscicollis), the Hudsonian Godwit (Limosa haemastica), the American Golden Plover (Pluvi-alis dominica) and the Two-banded Plover (Charadrius falklandicus) (Myers and Myers 1979; Blanco 1998; Ieno et al. 2004). Direct observation and fecal and giz-zard analysis have shown that polychaetes along with decapod crabs are the most important items in the diet of these shorebird species during their stay at Sam-borombon Bay. The data analysed here, which have been introduced in Chapter 4, come from a benthic-monitoring programme in the autumn-spring period in 1997 at 30 stations from the inter-tidal mudflats of San Clemente Channel in the south of Samborom-504 28 Multivariate analyses of South-American zoobenthic species — spoilt for choice bon Bay. The area is characterised by a benthos displaying high species densities and low species diversity (Ieno and Bastida 1998). The monitoring plots (tran-sects) on San Clemente Channel were selected to represent the major macroben-thic habitats due to the overwhelming abundance of short-lived and fast-growing polychaete species (Ieno and Bastida 1998; Martin 2002). In the original study, the main goal was to determine the relationship between waders and their inter-tidal food supply. The sampling scheme was determined by the topography of San Clemente Creek as well as the feeding behaviour of the secondary consumers. The underlying question we aim to answer with this particular data set is whether the environmental variables (sediment composition) had any effect on the macrobenthic species data. We also want to determine whether there are differ-ences between transects and seasons and, in particular, whether the two transects close to the eastern part of the study area are different. The main advantage of these data is that only a few species (infaunal data) were monitored at a very low spatial scale. This makes the following statistical explanation and interpretation easier for the reader to understand. The aim of this chapter