Variation in seagrass height and density over a wide spatial scale: effects on common fish and decapods

This study examines whether relationships between abundances of fishes and decapods and the leaf height and density of seagrass determined from small scale field experiments within a seagrass bed were also evident at a wider spatial scale, i.e. among beds within a bay. Separate comparisons were made for common species of fishes and decapods associated with the seagrasses Zostera capricorni Aschers, and Posidonia australis Hook f. Of the 23 significant responses by species to leaf height or density in the small scale experimental study, 15 were not repeated at the wider scale. Abundances of species with inconsistent responses at the two scales were not correlated meaningfully with other major environmental variables such as sediment size, area of seagrass bed or distance from the ocean. We reconcile the high incidence of effects of leaf height and density within beds, and their scarcity among beds, with a model of how postlarvae settle into, and move within, seagrass beds. The model holds that (1) larvae of fishes and decapods are distributed patchily when ready to settle, (2) larvae do not discriminate among beds when they settle, and (3) individuals do not leave a bed soon after settling but do redistribute within the bed to select microsites favouring survival.