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Chemistry and Ecology 261

Downloaded By: [Furnari, G.] At: 10:31 24 May 2010 The results of these studies are in line with the findings of other articles on the Mediterranean
(e.g. [115–117]). Seagrass abundance seems to be generally controlled by abiotic limitations
more than by biotic activities (i.e. herbivory), and bottom-up effects are recognisable in food
webs where seagrasses limit the herbivory level because of their low nutritional quality. True
and exclusive herbivores are not clearly recognisable: although a few species undoubtedly ingest
and also assimilate seagrass tissues, they exploit other items to meet their nutrient requirements
mainly in terms of nitrogen. In the interaction seagrass–epiphyte–herbivores a major role seems
to be played by epiphytes, although the spatial and temporal variability of this process needs
further study.

Because seagrass beds function as nursery and feeding areas for many fish species of commer-
cial value, an important task is the recognition of the trophic role of these plants in supporting
ichthyofauna. Indeed, the basal energy supply for fish potentially originates from different organic
matter sources, mainly seagrass living tissues and detritus, phytoplankton, epiphytes on seagrass
leaves and rhizomes and epilithic macroalgae. Vascular tissues contribute significantly to sedi-
mentary organic matter in seagrass beds, especially in those sites with high plant density and fine
sediments and where seagrass detritus is channelled through sedimentary organic matter to higher
trophic levels in food webs.

One of the most important ecological services provided by seagrass ecosystems is certainly
the support to food webs. However, several areas of seagrass ecology still need to be explored
in greater depth to understand food web dynamics, the spatio-temporal variability of specific
processes (herbivory intensity, the role of detritus and detritivores, exportation of organic matter
and so on) and the rules and mechanisms that determine such variability.

10. Conclusion

Seagrasses are very common along Sicilian coasts where they play an essential naturalistic and
ecological role in the balance of the coastal zone. Although all the Mediterranean seagrasses have
been described, only P. oceanica and C. nodosa show significant coverage. Currently, H. stip-
ulacea spreads easily in sheltered areas although it does not compete strongly against natural
communities, while N. noltii is not at all widespread and Z. marina has been reported only in one
location.

Most studies have focused on P. oceanica meadows because of their wide distribution, complex-
ity and ecological importance. Studies on the ecology of this seagrass have looked at the structural
and functional attributes at different organisational levels (physiological, individual, population).
Further studies and environmental monitoring of Sicilian seagrass ecosystems are required in order
to provide integrated coastal zone management actions and protect and prevent environmental
damage to aquatic ecosystems. According to a European directive (WFD 2000/60/UE) and an
Italian law (152/2006), Biological Quality Elements (BQE) based on seagrass descriptors will,
in the immediate future, be considered as ecological indicators for the determination of the Eco-
logical Quality status (EcoQ) [118]. The aim will be to use phenological and lepidochronological
descriptors to identify and validate a multivariate synthetic biometric index for P. oceanica.

Although it is not always possible to drive overall conclusions for the different species and for
the different aspects which have been studied on the seagrass meadows along the Sicilian coasts,
we can assess that P. oceanica meadows are in a good health in respect to average Mediter-
ranean conditions. This is demonstrated by the high levels of productivity, leaf biometry and
flowering performance which have been recorded. Overall genetic diversity is also high, with
the exception of isolated meadows such as the Posidonia banks and the Stagnone of Marsala
[23,63]. Several factors may explain the health status of Sicilian meadows, such as relatively low
anthropogenic pressure, sedimentation rate and favourable temperature and photoperiod, linked