Community structure and intertidal zonation of the macroinfauna in intermediate sandy beaches in temperate latitudes: North coast of Spain

Abstract

Nineteen intermediate exposed sandy beaches, located along the northern coast of Spain, were sampled during the summer 1999. Data from 10 of the beaches, located at the eastern part of this coast, were previously reported to evaluate environmental factors affecting benthic macrofauna. Data from nine of the beaches, located at the western part of this coast, were included to compare community structure and intertidal zonation of the macroinfauna on intermediate sandy beaches in temperate latitudes. Morphodynamic parameters such as Dean's parameter (Ω), Beach State Index (BSI) and relative tide range (RTR) were estimated at each beach. Beach length, width, intertidal slope, medium grain size, sorting, swash amplitude and wave characteristics were also analyzed. The highest macroinfaunal densities and biomass occurred at the mid and lower shore levels of each beach. Crustaceans, mainly cirolanid isopods, were the dominant group found on these beaches, whereas molluscs were the least representative. In general, the relationship between community structure and beach morphodynamics was similar to that found for the macroinfauna worldwide suggesting that macroinfauna in intermediate sandy beaches is affected, in the same way, by the physical processes associated with different beach types. Histograms and kite diagrams representing the intertidal distribution of the macroinfauna and multivariate analysis were used to show the zonation pattern on these exposed beaches. Intertidal slope values and beach profile pattern were found similar in all the beaches sampled. We hypothesized that this particular beach profile could influence the pattern of macroinfauna zonation. All the 19 beaches have two zones in common: the supralittoral zone of air breathers present on all shores at and above the drift line and the littoral zone extending from the drift line down the midshore to just above the water table outcrop. Ordination analyses identified two possible zones within the lower beach levels on seven of the beaches, but this cannot be clearly established. The Monte Carlo permutation test was used to select beach slope, length and wave height as the best predictor variables of macroinfaunal characteristics and it seems that the species most affected by the main variables showed the clearest zonation on the beaches.

Introduction

Sandy beaches are the most dynamic of soft bottom habitats (McLachlan et al., 1996) and dominate the world's temperate and tropical shorelines (Davies, 1972). Despite their uniform appearance and comparative poverty, intertidal zones of exposed beaches harbour a marine fauna of great ecological diversity. Crustacea, Polychaeta and Bivalvia rank among the most common macroinfaunal taxa (Brown and McLachlan, 1990), while Nematoda, Harpacticoidea, Plathelmintha and Oligochaeta dominate meiofaunal groups on sandy beaches (McLachlan, 1980).

Macrofaunal zonation on sandy beaches is a distinctive and well-described phenomenon of intertidal zones (McLachlan and Jaramillo, 1995); several attempts have been made to construct zonation schemes for sandy beach macroinfauna. Two general zonation schemes have been commonly used to determine distributions of organisms on sandy beaches: Dahl (1952) defined three biological zones in terms of a typical crustacean fauna inhabiting each zone, and Salvat (1964) defined four physical zones. These zones can be recognized by the species found, and Dahl's zonation pattern can easily been superimposed on Salvat's scheme. This scenario only represents the position during low tide and, because of the highly mobile fauna, such zones would not be expected to have sharp boundaries and, in fact, they often overlap (McLachlan, 1983, Degraer et al., 1999).

It has been shown that beach type can be used as a reliable predictor of species richness, abundance and biomass of the macroinfauna (e.g. McLachlan, 1990, Jaramillo and McLachlan, 1993, McLachlan et al., 1993). Swash characteristics, which are distinctive for each type of beach (e.g. McArdle and McLachlan, 1992), may also influence the community structure of the macroinfauna (e.g. Jaramillo et al., 1993). Slope is closely related to swash climate, which becomes less harsh as the beach face slope flattens (McArdle and McLachlan, 1992). Environmental changes, associated with the morphodynamic gradient, seem to have a relevant consequence on the intertidal macroinfauna and on the zonation patterns. The relationship between species richness and beach morphodynamics is supported by one common generalization in sandy beach ecology: the macroinvertebrates decreasing along a morphodynamic gradient from the dissipative to the reflective conditions (sensu Short and Wright, 1983) (e.g. Defeo et al., 1992, Jaramillo and McLachlan, 1993, McLachlan et al., 1996, Brazeiro, 1999). Intermediate beaches belong to the morphodynamic sandy beach type produced by moderate to high waves, fine to medium sand, and long wave periods with intermediate surf zones characterised by bars, troughs and rip currents (Wright and Short, 1983). The importance of understanding the ecology of intermediate sandy beaches derives from the fact that they are the most common morphodynamic beach type (Short, 1996, Alongi, 1998) and one of the most extended intertidal systems worldwide.

The pioneering studies carried out on intermediate beaches in northern Spain found no relationships between morphodynamic state and species richness, biomass or abundance of macroinfauna (Rodil and Lastra, 2004) and meiofauna (Rodríguez et al., 2003). In larger geographical studies, including sandy beaches from areas with different oceanographic conditions and other variables such as chlorophyll-a concentration in the water column could be a key factor explaining macrofaunal patterns. A key physical characteristic typical of these beaches was a profile with a steep foreshore followed by a flat lower shore, also found on Scottish beaches (Eleftheriou and Nicholson, 1975) and on the Northwest coast of Spain (de la Huz, pers. comm.). This profile creates a reflective character for the upper part of these beaches, while the lowest part of the shores exhibits dissipative properties. We hypothesized that this particular profile could give rise to a zonation other than that expected for intermediate sandy beaches with a more regular or monotonic profile; at the same time, the idea that such beaches may support more species due to the variation in swash climate across the beach profile could be investigated. In fact, this factor should be predicted to be more important for species that interact directly with the swash than for those that inhabit the supralittoral dry zones. Clear population responses to beach type have not been observed in supralittoral macroinfauna (Contreras et al., 2003, Defeo and Martínez, 2003), which seems to be less influenced by the swash climate. To examine this hypothesis, results were analyzed from 19 intermediate sandy beaches, located in the northern coast of Spain, to evaluate the zonation of intertidal macrofauna and compared with results from other intermediate beaches worldwide. This paper provides a description of faunal zonation, composition and density of the macroinfauna on intermediate morphodynamic beaches along the northern shoreline of the Iberian Peninsula. Changes in community structure along the beach profile and factors influencing these patterns are also considered.