Small-scale Structure of Infaunal Polychaete Communities in an Estuarine Environment: Methodological Approach

This study compares different methods for the estimation of minimal areas (viz. species/area curves, diversity/area curves, similarity/area curves, variance/mean ratio vs. area curves) as community structure descriptors. The comparisons are based upon two polychaete taxocoenoses from muddy and sandy habitats, located in a semienclosed shallow-water Mediterranean bay (Alfacs Bay, Ebro Delta, NW Mediterranean). The mud community appeared to be very homogeneous, with very low diversity. This community displayed high structural simplicity (related to various stress factors), and therefore, qualifies as a physically controlled community). The diversity index was stabilized for areas of 37 cm2, quantitative similarity (Kulczynski index) was higher than 0•7 for areas of 90 cm2, and density of individuals was stabilized for areas of 120 cm2. Therefore, an area of 120 cm2 is suggested as being representative of the community structure. However, it was impossible to define a qualitatively adequate sampling area (> 300 cm2). The sand community displayed high structural complexity, with high species richness and high diversity. This community was characterized by high environmental stability and high variability of microhabitats, as is frequent in biologically accomodated communities. The number of individuals became homogeneous for areas of 600-1000 cm2, diversity was stabilized around 300 cm2 and a Kulczynski similarity index of 0•7 was already attained at areas of 1000 cm2. Thus, a quantitatively representative sampling area of between 700 and 1000 cm2 was suggested. Moreover, the more general pattern of species distribution (with an important set of common species) was directly related to the relatively low qualitative minimal area (400 cm2).