Importance of local changes in leaf height and density to fish and decapods associated with seagrasses

Abstract

The height and density of seagrass leaves were manipulated to about one-third normal levels to test whether associated fish and decapods were affected. The experiments tested for effects of height, density and their interaction, and were run separately in single beds of Zostera capricorni Aschers, and Posidonia australis Hook f. to determine if effects were consistent for each seagrass. Abundances of individual species were often affected significantly by the manipulations. In Zostera, abundances of six species decreased when leaves were shortened, whereas abundances of three species increased, and seven species decreased, when leaves were thinned. In Posidonia, abundances of four species decreased when leaves were shortened, while abundances of two species increased and five decreased when leaves were thinned. Significant height × density interactions did not occur for any of the 13 species analysed from Zostera, but did occur for two of the 12 species from Posidonia. Several species, in both seagrasses, did not respond to the manipulations. Of the six species that were analysed in both seagrasses, four responded in the same way to height and density, and a fifth species responded consistently to height. There were significant effects on the number of individuals in feeding guilds. However, effects on guilds masked (opposite) effects, and the lack of effects, for component species, and showed little consistency between Zostera and Posidonia. This was also true for effects on the total number of individuals. The data do not support an existing model predicting responses of species richness and abundance to changes in physical complexity of seagrasses.