The effects of seagrass (Zostera japonica) canopy structure on associated fauna: a study using artificial seagrass units and sampling of natural beds

The importance of seagrass canopy to associated fauna was assessed by compa ring the species richness, abundance and diversity of the epi- and infaunal macroinvertebrate assemblages in a seagrass (Zostera japonica Ascherson an d Graebner) bed and the adjacent unvegetated area in Hong Kong. Seagrass co ver had significant effects on the composition and abundance of the associa ted fauna and the amount of detritus accumulated on the sediment surface. D etritus abundance was significantly higher in the seagrass bed, and was pos itively correlated with both the above- and belowground biomass of Z. japon ica. Both the abundance and species richness of the epi- and infauna were s ignificantly positively correlated with the belowground biomass of the seag rass and detritus standing crop. Macrofaunal species richness was higher (1 18) in the seagrass bed than the adjacent unvegetated areas (70), with a hi gher degree of similarity between the infauna than the epifauna of the two habitats. While all species recorded from the unvegetated areas were found in the seagrass bed, 48 species occurred only in the seagrass-covered areas . Species richness of epifauna was significantly higher in the seagrass bed , but there was no difference between infaunal species of the two habitats. On the contrary, faunal (epi- and infauna) abundance was significantly hig her in seagrass areas. The seagrass bed also supported species of small tel linid bivalves previously not recorded from Hong Kong. Artificial seagrass units (ASUs, 0.2 m(2)) with four combinations of leaf density and leaf leng th and a control (bare sand) were placed at short distances from natural pa tches of Z. japonica. The composition, abundance and biomass of the epibent hos associated with the ASUs and the control were recorded after 3 months i n the field. While species richness did not differ among the treatments. to tal abundance of epibenthos was significantly higher in the high density-lo ng leaves (HL) treatment than in the control. Results of a discriminant ana lysis using log-transformed abundance data suggest that the gastropod Clith on oualaniensis, the mussel Musculista senhousia and the crab Thalamita sp. were important species distinguishing the assemblages in the various treat ments. All the three species were significantly more abundant in the HL tre atment than in the low density-short leaves (LS) treatment and the control. By contrast, there was no significant difference in the biomass of the epi fauna, but discriminant analysis again separated the five treatments based on the composition of the biomass, with the same three species identified a s the most important discriminating species. The species richness and abund ance of the epifauna associated with the ASUs were similar to the adjacent unvegetated areas, but significantly lower than in the Zostera patches. The physical canopy structure of Z. japonica beds increased the abundance of t he epibenthos, potentially through provision of canopy and indirectly throu gh trapping of detritus. (C) 2001 Elsevier Science B.V. All rights reserved .