Chapter 17 Biology of Posidonia
399
production (2–15% of the leaf production: Velimirov, 1984; Cebrian et al., 1996; Havelange et al.,
1997; Pergent et al., 1997b; Alcoverro et al., 1997a;
Cebrian and Duarte, 2001). Polychaetes, isopods,
amphipods, decapods, and molluscs, are important
links between the primary producers and the final
consumers, as they transfer most energy from algal epiphytes and transform significant amount of
leaf detritus (Buia et al., 2000). The Posidonia system is less efficient than other seagrass ecosystems
(e.g. Zostera noltii, Cymodocea nodosa) in channeling primary production to higher trophic levels
(Danovaro et al., 2002).
Isopods directly consume living leaf tissue in Australian species of Posidonia (Brearley and Walker,
1995, 1996). In P. oceanica beds, Gambi et al. (1997)
and Guidetti et al. (1997) described polychaetes boring dead tissue in the remains of leaf bases persisting
along rhizomes but also they attack live leaves and
removed a small percentage (1–4%) of the sheath
production (Guidetti, 2000). In some areas, polychaetes graze 17–41% of the scale biomass and so
have strong ecological implications in enhancing
scale fragmentation and decay by microbial activity (Gambi, 2000).
The role of Posidonia detritus in coastal food webs
remains little studied, with few investigations conducted in southeastern or southern Australia on the
fate of seagrass or macroalgal debris. The importance of seagrass debris presumably lies primarily in
chemical leachates being utilized by bacteria, fungi,
microalgae, and protozoa, and by the increasing proliferation of microbes on detrital particles as fragmentation occurs, rather than on the direct ingestion
of seagrass material (Fenchel and Harrison, 1976;
Fenchel, 1977; Mateo et al., Chapter 7). More protein has been found in surficial microbes than in the
decaying seagrass debris with which they are associated (Zimmerman et al., 1979).
In the Mediterranean Sea, decaying P. oceanica
leaves, with a low proportion of rhizomes, sheaths
and seaweed, form the ‘debris’ (Velimirov et al.,
1981). A fraction of dead leaf material enters the
food web, depending on the quantity of material exported inshore or offshore (10–95%) (Romero et al.,
1994; Mat` eo et al., 1997; Pergent et al., 1997b; Gacia
et al., 2002). As described by Walker et al. (2001),
the terminology in seagrass literature can cause confusion, the following different terms should be used:
wrack, for transported recognizable leaves; litter, for
fragmented material; broken down dead roots, rhizomes, hard to recognize leaves (<0.5 mm) are detritus. P. oceanica does not support all consumer nutrition and the microbial growth on leaf material seems
to be the major source of nutrition for macroconsumers.
About 50 species of fish are encountered in
P. oceanica beds, classified as resident (56%), as
transient (22%), or as occasional species (22%).
The most important families were Labridae, Scorpenidae, Serranidae, and Centracanthidae which account for 41% of the number of species and 87% of
the total biomass (Harmelin-Vivien, 1982, 1989).
Studies on the food web structure in Posidonia
beds (by the direct observation of dietary linkages
through gut content analysis or by indirect analysis dietary tracers: lipids and other biochemical
markers (Klumpp and Van der Valk, 1984) and
radioisotopes have not been conducted systematically in a range of marine habitats around Australia, so the overall contribution of seagrass, seagrass detritus, and seagrass epiphytes, relative to
other sources of organic matter, to coastal food
webs remains unknown. In Australia, very few attempts have been made to quantify the transfer of
Posidonia production into coastal food webs (sensu
Robertson, 1984), or to determine the relative importance of Posidonia production compared to primary
production generated by other major plant groups
(phytoplankton, macroalgae, benthic microalgae,
mangroves).
In the Mediterranean Sea, the relative importance
of the different sources in the diet of the main
consumers in P. oceanica meadows have been approached by the analysis of carbon and nitrogen stable isotopes and by determination of gut contents
(Dauby, 1989, 1995; Dauby et al., 1995; Havelange
et al., 1997; Jennings et al., 1997; Lepoint et al.,
2000; Pinnegar and Polunin, 2000; Vizzini et al.,
2002; Mateo et al., Chapter 7). Plankton and seaweed (including epiphytes) constitute the main fraction of the diet of primary consumers (herbivores,
suspension- and detritus feeders). Stable isotopes
demonstrate that direct seagrass grazers assimilate a
non-negligible amount of ingested seagrass carbon:
about 20% for Sarpa salpa (Havelange et al., 1997).
The carbon fraction originating from seagrasses appears extremely low and only dominates in the diet of
very few species, e.g. psammivorous holothurians.
Grazers are rarely exclusively plant consumers
(e.g. Mazzella and Russo, 1989). For example, the
crustacean Achaeus cranchii grazes algae on P.
399
production (2–15% of the leaf production: Velimirov, 1984; Cebrian et al., 1996; Havelange et al.,
1997; Pergent et al., 1997b; Alcoverro et al., 1997a;
Cebrian and Duarte, 2001). Polychaetes, isopods,
amphipods, decapods, and molluscs, are important
links between the primary producers and the final
consumers, as they transfer most energy from algal epiphytes and transform significant amount of
leaf detritus (Buia et al., 2000). The Posidonia system is less efficient than other seagrass ecosystems
(e.g. Zostera noltii, Cymodocea nodosa) in channeling primary production to higher trophic levels
(Danovaro et al., 2002).
Isopods directly consume living leaf tissue in Australian species of Posidonia (Brearley and Walker,
1995, 1996). In P. oceanica beds, Gambi et al. (1997)
and Guidetti et al. (1997) described polychaetes boring dead tissue in the remains of leaf bases persisting
along rhizomes but also they attack live leaves and
removed a small percentage (1–4%) of the sheath
production (Guidetti, 2000). In some areas, polychaetes graze 17–41% of the scale biomass and so
have strong ecological implications in enhancing
scale fragmentation and decay by microbial activity (Gambi, 2000).
The role of Posidonia detritus in coastal food webs
remains little studied, with few investigations conducted in southeastern or southern Australia on the
fate of seagrass or macroalgal debris. The importance of seagrass debris presumably lies primarily in
chemical leachates being utilized by bacteria, fungi,
microalgae, and protozoa, and by the increasing proliferation of microbes on detrital particles as fragmentation occurs, rather than on the direct ingestion
of seagrass material (Fenchel and Harrison, 1976;
Fenchel, 1977; Mateo et al., Chapter 7). More protein has been found in surficial microbes than in the
decaying seagrass debris with which they are associated (Zimmerman et al., 1979).
In the Mediterranean Sea, decaying P. oceanica
leaves, with a low proportion of rhizomes, sheaths
and seaweed, form the ‘debris’ (Velimirov et al.,
1981). A fraction of dead leaf material enters the
food web, depending on the quantity of material exported inshore or offshore (10–95%) (Romero et al.,
1994; Mat` eo et al., 1997; Pergent et al., 1997b; Gacia
et al., 2002). As described by Walker et al. (2001),
the terminology in seagrass literature can cause confusion, the following different terms should be used:
wrack, for transported recognizable leaves; litter, for
fragmented material; broken down dead roots, rhizomes, hard to recognize leaves (<0.5 mm) are detritus. P. oceanica does not support all consumer nutrition and the microbial growth on leaf material seems
to be the major source of nutrition for macroconsumers.
About 50 species of fish are encountered in
P. oceanica beds, classified as resident (56%), as
transient (22%), or as occasional species (22%).
The most important families were Labridae, Scorpenidae, Serranidae, and Centracanthidae which account for 41% of the number of species and 87% of
the total biomass (Harmelin-Vivien, 1982, 1989).
Studies on the food web structure in Posidonia
beds (by the direct observation of dietary linkages
through gut content analysis or by indirect analysis dietary tracers: lipids and other biochemical
markers (Klumpp and Van der Valk, 1984) and
radioisotopes have not been conducted systematically in a range of marine habitats around Australia, so the overall contribution of seagrass, seagrass detritus, and seagrass epiphytes, relative to
other sources of organic matter, to coastal food
webs remains unknown. In Australia, very few attempts have been made to quantify the transfer of
Posidonia production into coastal food webs (sensu
Robertson, 1984), or to determine the relative importance of Posidonia production compared to primary
production generated by other major plant groups
(phytoplankton, macroalgae, benthic microalgae,
mangroves).
In the Mediterranean Sea, the relative importance
of the different sources in the diet of the main
consumers in P. oceanica meadows have been approached by the analysis of carbon and nitrogen stable isotopes and by determination of gut contents
(Dauby, 1989, 1995; Dauby et al., 1995; Havelange
et al., 1997; Jennings et al., 1997; Lepoint et al.,
2000; Pinnegar and Polunin, 2000; Vizzini et al.,
2002; Mateo et al., Chapter 7). Plankton and seaweed (including epiphytes) constitute the main fraction of the diet of primary consumers (herbivores,
suspension- and detritus feeders). Stable isotopes
demonstrate that direct seagrass grazers assimilate a
non-negligible amount of ingested seagrass carbon:
about 20% for Sarpa salpa (Havelange et al., 1997).
The carbon fraction originating from seagrasses appears extremely low and only dominates in the diet of
very few species, e.g. psammivorous holothurians.
Grazers are rarely exclusively plant consumers
(e.g. Mazzella and Russo, 1989). For example, the
crustacean Achaeus cranchii grazes algae on P.