186
M. A. Mateo, J. Cebri ´
an, K. Dunton, and T. Mutchler
and with sediment microalgae apparently sometimes
accounting for the highest part of the productivity
(18–56%). Areas which need much attention are
the role of below-ground parts as producers, consumers or storage organs, canopy structure, and light
dynamics.
B. Fate
The preliminary review attempted in this chapter
shows that overall, most seagrass production is decomposed within the bed (∼65%) while the rest
is exported (∼15%), grazed (<10%), or accumulates in a refractory pool (∼10%). Detritus nutrient content may be a controlling factor on decomposition rates, alongside the oxygen status of the
sediment. The importance of certain chemical constituents (e.g. lignin and tannins) of seagrass detritus
in influencing microbial attack still needs to be explored in almost all seagrass species.
Seagrass leaf nutrient content and the intensity
of herbivory is a controversial topic and studies are
needed of how different field spatial (i.e. cultivated
patches vs. entire beds) and temporal scales (i.e. seasonality) affect the relationship. Along with these
manipulations, specific assimilation rates for the major grazers need to be determined because most grazing rates given in the literature may represent large
overestimates of the actual carbon flux from producers to consumers.
Given our lack of understanding of mechanisms
that affect food web dynamics and the current threat
of eutrophication to seagrass systems, it is recommended that the role of nutrients in determining the
fluxes of carbon and nitrogen be investigated. Both
surveys of natural abundances of stable isotopes and
isotope addition experiments will be useful in these
investigations.
The lack of a method that adequately covers the
entire ecosystem and integrates export rates over
time has been identified as the main reason behind
the scarcity of export estimates.
C. Budgets and Sinks
Based on the relatively limited information available, seagrass ecosystems appear to be net producer
systems, with P:R ratios ranging from 1 to 4.9; this
range is mainly a consequence of the quantitative importance of export and refractory accumulation and
needs detailed investigation. If true, it would confirm
the role of seagrass-dominated areas as food sources
for downstream ecosystems. At the same time, we
need more efforts to study single ecosystem components to avoid losing important information of bed
functioning.
The percentage of the production accumulated as
refractory materials has often been shown to be modest; however, the high productivity of seagrasses results in a substantial absolute amount of carbon fluxing to this pool. In the context of a world with an
increase in atmospheric CO 2 , global estimates suggest that seagrass ecosystems may be relevant carbon
sinks, not only at a local scale, but also in a biospheric
context; 0.08 PgC year
−1 , representing 20% as much
as for phytoplanktonic communities.
Acknowledgments
This chapter has been improved by the critical comments of Jack Middelburg, Tony Larkum, and two
anonymous referees. The authors are grateful to
Brian Fry and Mike Sullivan for their valuable comments on stable isotopes and seagrass food webs.
Heather Alexander contributed to improve the linguistic quality of the text. Part of the original material presented in this chapter has been obtained
and prepared in the framework of the project PALEOMED, financed by the Spanish Ministry of Science and Technology (project reference: BOS200202247).
References
Alcoverro T, Romero J, Duarte CM and L´ opez NI (1997) Spatial
and temporal variations in nutrient limitation of seagrass Posidonia oceanica growth in the NW Mediterranean. Mar Ecol
Prog Ser 146: 155–161
Alcoverro T, Manzanera M and Romero J (1998) Seasonal and
age-dependent variability of Posidonia oceanica (L.) Delile
photosynthesis and respiration. J Exp Mar Biol Ecol 230: 1–
13
Alcoverro T, Manzanera M and Romero J (2001) Annual
metabolic carbon balance of the seagrass Posidonia oceanica: The importance of carbohydrate reserves. Mar Ecol Prog
Ser 211: 105–116
Bach S, Thayer G and LaCroix M (1986) Export of detritus from
eelgrass (Zostera marina) beds near Beaufort, North Carolina,
USA. Mar Ecol Prog Ser 28: 265–278
Bay D (1982) In-situ study of the metabolism of 3 intra littoral
benthic communities of the Gulf of Aqaba. Comptes Rendues
des Seances de l’Academie des Sciences Serie III Sciences de
la Vie 294: 463–466
M. A. Mateo, J. Cebri ´
an, K. Dunton, and T. Mutchler
and with sediment microalgae apparently sometimes
accounting for the highest part of the productivity
(18–56%). Areas which need much attention are
the role of below-ground parts as producers, consumers or storage organs, canopy structure, and light
dynamics.
B. Fate
The preliminary review attempted in this chapter
shows that overall, most seagrass production is decomposed within the bed (∼65%) while the rest
is exported (∼15%), grazed (<10%), or accumulates in a refractory pool (∼10%). Detritus nutrient content may be a controlling factor on decomposition rates, alongside the oxygen status of the
sediment. The importance of certain chemical constituents (e.g. lignin and tannins) of seagrass detritus
in influencing microbial attack still needs to be explored in almost all seagrass species.
Seagrass leaf nutrient content and the intensity
of herbivory is a controversial topic and studies are
needed of how different field spatial (i.e. cultivated
patches vs. entire beds) and temporal scales (i.e. seasonality) affect the relationship. Along with these
manipulations, specific assimilation rates for the major grazers need to be determined because most grazing rates given in the literature may represent large
overestimates of the actual carbon flux from producers to consumers.
Given our lack of understanding of mechanisms
that affect food web dynamics and the current threat
of eutrophication to seagrass systems, it is recommended that the role of nutrients in determining the
fluxes of carbon and nitrogen be investigated. Both
surveys of natural abundances of stable isotopes and
isotope addition experiments will be useful in these
investigations.
The lack of a method that adequately covers the
entire ecosystem and integrates export rates over
time has been identified as the main reason behind
the scarcity of export estimates.
C. Budgets and Sinks
Based on the relatively limited information available, seagrass ecosystems appear to be net producer
systems, with P:R ratios ranging from 1 to 4.9; this
range is mainly a consequence of the quantitative importance of export and refractory accumulation and
needs detailed investigation. If true, it would confirm
the role of seagrass-dominated areas as food sources
for downstream ecosystems. At the same time, we
need more efforts to study single ecosystem components to avoid losing important information of bed
functioning.
The percentage of the production accumulated as
refractory materials has often been shown to be modest; however, the high productivity of seagrasses results in a substantial absolute amount of carbon fluxing to this pool. In the context of a world with an
increase in atmospheric CO 2 , global estimates suggest that seagrass ecosystems may be relevant carbon
sinks, not only at a local scale, but also in a biospheric
context; 0.08 PgC year
−1 , representing 20% as much
as for phytoplanktonic communities.
Acknowledgments
This chapter has been improved by the critical comments of Jack Middelburg, Tony Larkum, and two
anonymous referees. The authors are grateful to
Brian Fry and Mike Sullivan for their valuable comments on stable isotopes and seagrass food webs.
Heather Alexander contributed to improve the linguistic quality of the text. Part of the original material presented in this chapter has been obtained
and prepared in the framework of the project PALEOMED, financed by the Spanish Ministry of Science and Technology (project reference: BOS200202247).
References
Alcoverro T, Romero J, Duarte CM and L´ opez NI (1997) Spatial
and temporal variations in nutrient limitation of seagrass Posidonia oceanica growth in the NW Mediterranean. Mar Ecol
Prog Ser 146: 155–161
Alcoverro T, Manzanera M and Romero J (1998) Seasonal and
age-dependent variability of Posidonia oceanica (L.) Delile
photosynthesis and respiration. J Exp Mar Biol Ecol 230: 1–
13
Alcoverro T, Manzanera M and Romero J (2001) Annual
metabolic carbon balance of the seagrass Posidonia oceanica: The importance of carbohydrate reserves. Mar Ecol Prog
Ser 211: 105–116
Bach S, Thayer G and LaCroix M (1986) Export of detritus from
eelgrass (Zostera marina) beds near Beaufort, North Carolina,
USA. Mar Ecol Prog Ser 28: 265–278
Bay D (1982) In-situ study of the metabolism of 3 intra littoral
benthic communities of the Gulf of Aqaba. Comptes Rendues
des Seances de l’Academie des Sciences Serie III Sciences de
la Vie 294: 463–466