Role of Posidonia oceanica bed structure in determining the diversity of associated macroinvertebrate assemblages
Seagrass beds are highly productive shallow-water marine ecosystems and serve as a habitat for numerous invertebratesa nd fishes, some of which constitute important fisheries species.S eagrassb eds play an important role in physical coastal dynamics, since they stabilise soft bottoms, while their leaf canopy attenuates strong wave action and currents, thereby, reducing coastal erosion. However, seagrass habitats are undergoing a decline worldwide, hence, there is an urgent need for data on the influence of bed fragmentation and changes in plant architecture on the diversity of associated biota. In the Mediterranean Sea, the endemic seagrass Posidonia oceanica forms extensive beds that have a high structural complexity and support a high biodiversity, making them one of the most important marine habitat types in the region. The present study investigated the influence of P. oceanica bed structure (seagrassb ed morphology and within-bed architecture) on the diversity of associatedm acrofauna.P ilot surveys were made to map the occurrence and spatial distribution of different types of P. oceanica beds present off the northeastern coast of the Maltese Islands, to enable selection of four appropriate study locations. A quantitative sampler, which samples the total macroinvertebrate assemblages of P. oceanica beds, was designed and tested to determine the appropriate area and number of sampling units required to achieve given levels of accuracy and precision. Using data gathered from the pilot survey, studies were made to examine for differences in P. oceanica plant architecture, and in the species richness and abundance of motile macroinvertebrates between continuous (non-fragmented) and reticulate (fragmented)b eds of the seagrassa, crosst hree different spatial levels: (a) large (severalk m), (b) medium (circa 100 in) and small (tens of metres). A separate study examined differences in the species richness and abundance of associated motile macro invertebrates between living matte (bearing living shoots) and dead matte (without living shoots) of P. oceanica. Results of univariate and multivariate analyses indicated that there was no consistent pattern of significant differences in plant architecture,a nd in the speciesr ichnessa nd abundanceo f associatedm otile macroinvertebratesb etween the two P. oceanica bed types. Significant differences in macroinvertebrate species richness and abundance were detected between P. oceanica beds at the large spatial level that were significantly related to seagrasse piphytesa nd mean sedimentg rain size. Significant differences in macroinvertebrate species richness and abundance were detected between P. oceanica beds at the medium spatial level that were significantly related to shoot biomass. Significant differences in the assemblage composition of macroinvertebrates associated with P. oceanica beds were detected by multivariate analysis at the large and small spatial levels, with the best explanation for the multivariate assemblage structure being a combination of environmental variables that included epiphytes and sediment grain size. The results of the study comparing the motile macroinvertebrates of living and dead matte showed that dead matte supported significantly higher species richness and abundance of associated motile macro invertebrates, and had a significantly different macroinvertebrate assemblage structure, compared to living matte. Furthermore, a greater number of species occurred exclusively in dead matte than in living matte. It was concluded that natural fragmentation of continuous to reticulate P. oceanica beds does not lead to changes in plant architectural differences, or to a reduction in diversity of associated macroinvertebrates, nor does complete loss of the foliar canopy necessarily lead to a reduced diversity of macrofauna. Local factors, including nutrient levels in the water colunm (which enhanced epiphytic growth on the seagrass), sediment grain size and the hydrodynamic regime, were more important in influencing the diversity of macrofauna associated with P. oceanica beds, than bed type, leading to significant spatial variation in the associatedm acroi nvertebrate assemblagesT. he study concluded that the relative importance of different factors in influencing the diversity of macro invertebrates associated with P. oceanica beds varied, depending on scale. The results are discussed in the light of the available published data on fragmentation of seagrass habitat and on current Euro-Mediterranean proposals for the conservation of different bed types of P. oceanica. Attempts to classify different bed types of P. oceanica in categories according to value should be based on adequate ecological data. Different bed types of P. oceanica all form part of a dynamic heterogeneousla ndscapew here transformation from one bed type to another may occur, but which does not necessarily lead to loss of biodiversity. Hence, equal effort should be directed at conserving and managing different bed types of P. oceanica.