SPATIAL STRUCTURE OF BIVALVES IN A SANDFLAT - SCALE AND GENERATING PROCESSES

A survey was conducted during the summer of 1994 within a fairly homog eneous 12.5 ha area of sandflat off Wiroa Island, in Manukau Harbour, New Zealand, to identify factors controlling the spatial distributions of the two dominant bivalves, Macomona liliana Iredale and Austrovenu s stutchburyi (Gray), and to look for evidence of adult-juvenile inter actions within and between species. Most of the large-scale spatial st ructure detected in the bivalve count variables (two species, several size classes of each) was explained by the physical and biological var iables. The results of principal component analysis and spatial regres sion modelling suggest that different factors are controlling the spat ial distributions of adults and juveniles. Larger size classes of both species displayed significant spatial structure, with physical variab les explaining some but not all of this variation. Smaller organisms w ere less strongly spatially structured, with virtually all of the stru cture explained by physical variables. The physical variables importan t in the regression models differed among size classes of a species an d between species. Extreme size classes (largest and smallest) were be st explained by the models; physical variables explained from 10% to a bout 70% of the variation across the study site, Significant residual spatial variability was detected in the larger bivalves at the scale o f the study site. The unexplained variability (20 to 90%) found in the models is likely to correspond to phenomena operating at smaller scal es, Finally, we found no support for adult-juvenile interactions at th e scale of our study site, given our sampling scale, after controlling for the effects of the available physical variables. This is in contr ast to significant adult-juvenile interactions found in smaller-scale surveys and in field experiments. Our perception of adult-juvenile int eractions thus depends on the scale of study. (C) 1997 Elsevier Scienc e B.V.