242
J. Romero, K.-S. Lee, M. P ´
erez, M. A. Mateo, and T. Alcoverro
In general terms, Ca-bound P dominates in carbonate sediments, while Fe-bound P dominates in terrigenous sediments (Koch et al., 2001), and this explains the differences in P availability between these
two sediment types, which are discussed below (next
section). Although this summary is an oversimplification of the very complex P geochemistry in marine sediments, it can help as a basic view (see also
Marb` a et al., Chapter 6). Much remains to be investigated in this respect, and, specially, the net rates of
P sequestration in seagrass beds.
4. Refractory Material
While there has been intense study of the refractory biomacromolecules in terrestrial and freshwater environments (e.g. Derenne and Largeau, 2001),
in seagrass sediments this aspect has received little
attention. Moreover, what little information there is
refers to carbon fractions and not to nutrients associated to refractory particulate or dissolved organic matter. There is evidence on the abundance of
refractory compounds in seagrass sediments (Posidonia oceanica, Danovaro, 1996), and the quantitatively most important is probably lignin. Nevertheless, these compounds, formed by long chains of
phenolic compounds, are basically a carbon sink as
no nutrients, apart from C, are linked to them. However, both N and P appear (or are likely to appear)
bound to refractory organic compounds. In effect,
available data indicate that organic pools represent
a substantial part of sediment P (>50% in Florida
Bay, Koch et al., 2001), and most of it is probably in refractory forms. Concerning N, it has been
found recently that nitrogen from intrinsically labile
amino acids can be preserved in the sediments of
continental margins over thousands of years in refractory networks of peptide-like material (Grutters
et al., 2002). Since one-third of all the amino acids
found in sediments could be directly derived from
bacterial cell walls, given the relatively high bacterial activity observed in seagrass sediments (L´ opez
et al., 1995; Danovaro, 1996), it is reasonable to think
that this could be a relevant mechanism for nitrogen
immobilization in seagrass sediments.
Factors controlling the burial rate of nutrients
in these refractory pools are far from understood;
species-specific differences in biochemical composition, oxygen availability, bacterial activity (that can
be in turn nutrient-limited), origin of the material
(since part can be allochthonous, i.e. of terrestrial
origin, or from nearby ecosystems such as mangroves), and temperature are just some of the potential controls to be considered (Henrichs, 1993; L´ opez
et al., 1995; Mateo and Romero, 1997; Danovaro
et al., 2002; Mateo et al., Chapter 7). A very peculiar case of burial occurs when the seagrass belowground organs are stored with no or only slight decomposition, and accumulate as peat-like deposits.
This fact has been reported only for a very small
number of seagrass species. Such species are, as far
as we are aware, Thalassodendron ciliatum (Lipkin,
1979), Posidonia australis (Shepherd and Sprigg,
1976), and Posidonia oceanica (Boudouresque et al.,
1980; Romero et al., 1994; Mateo et al., 1997). Only
for this last species has the magnitude and dynamics
of the deposits been studied, and key aspects concerning their importance for carbon are summarized
elsewhere in this book (Mateo et al., Chapter 7).
The buried dead materials (rhizomes, leaf sheaths,
and roots) of P. oceanica have a nutrient content of
20–50% N and 5–20% P, compared with the original living material. This decrease in nutrient concentration takes place very quickly (3–5 yr, Romero
et al., 1992). No further changes seem to occur, since
the N and P concentration in material 10–20 yr old
does not differ from that found in material more than
1,000 yr old. Since bulk decay rates of this detritus
have been estimated to be extremely low, between
0.00008 and 0.00036 yr
−1 (Mateo et al., 1997), it is
concluded that nutrient release from it is very low,
and that these deposits are long-term nutrient sinks,
representing net losses of 0.8 and 0.044 g of N and P
m
−2 yr
−1 , or, roughly, 7% and 5% of the total plant
N and P, respectively, annual requirements for plant
growth.
Future efforts in the study of the burial of seagrass
production should be devoted to the characterization
of the refractory organic matter in general but, more
specifically to those compounds containing nitrogen
and phosphorus. Nevertheless, the general conclusion so far is that the amount of nutrients lost in
terms of burial is modest in terms of the ecosystem budget. However, the role of seagrass beds as
nutrient sinks should not be disregarded, especially
in the context of coastal waters facing problems of
eutrophication (see Marb` a et al., Chapter 6).
F. New and Recycled Production
in Seagrasses
In general terms nutrient budgets of seagrass ecosystem include an exchange term and a recycling term.
That is, the total need of nutrients required for
J. Romero, K.-S. Lee, M. P ´
erez, M. A. Mateo, and T. Alcoverro
In general terms, Ca-bound P dominates in carbonate sediments, while Fe-bound P dominates in terrigenous sediments (Koch et al., 2001), and this explains the differences in P availability between these
two sediment types, which are discussed below (next
section). Although this summary is an oversimplification of the very complex P geochemistry in marine sediments, it can help as a basic view (see also
Marb` a et al., Chapter 6). Much remains to be investigated in this respect, and, specially, the net rates of
P sequestration in seagrass beds.
4. Refractory Material
While there has been intense study of the refractory biomacromolecules in terrestrial and freshwater environments (e.g. Derenne and Largeau, 2001),
in seagrass sediments this aspect has received little
attention. Moreover, what little information there is
refers to carbon fractions and not to nutrients associated to refractory particulate or dissolved organic matter. There is evidence on the abundance of
refractory compounds in seagrass sediments (Posidonia oceanica, Danovaro, 1996), and the quantitatively most important is probably lignin. Nevertheless, these compounds, formed by long chains of
phenolic compounds, are basically a carbon sink as
no nutrients, apart from C, are linked to them. However, both N and P appear (or are likely to appear)
bound to refractory organic compounds. In effect,
available data indicate that organic pools represent
a substantial part of sediment P (>50% in Florida
Bay, Koch et al., 2001), and most of it is probably in refractory forms. Concerning N, it has been
found recently that nitrogen from intrinsically labile
amino acids can be preserved in the sediments of
continental margins over thousands of years in refractory networks of peptide-like material (Grutters
et al., 2002). Since one-third of all the amino acids
found in sediments could be directly derived from
bacterial cell walls, given the relatively high bacterial activity observed in seagrass sediments (L´ opez
et al., 1995; Danovaro, 1996), it is reasonable to think
that this could be a relevant mechanism for nitrogen
immobilization in seagrass sediments.
Factors controlling the burial rate of nutrients
in these refractory pools are far from understood;
species-specific differences in biochemical composition, oxygen availability, bacterial activity (that can
be in turn nutrient-limited), origin of the material
(since part can be allochthonous, i.e. of terrestrial
origin, or from nearby ecosystems such as mangroves), and temperature are just some of the potential controls to be considered (Henrichs, 1993; L´ opez
et al., 1995; Mateo and Romero, 1997; Danovaro
et al., 2002; Mateo et al., Chapter 7). A very peculiar case of burial occurs when the seagrass belowground organs are stored with no or only slight decomposition, and accumulate as peat-like deposits.
This fact has been reported only for a very small
number of seagrass species. Such species are, as far
as we are aware, Thalassodendron ciliatum (Lipkin,
1979), Posidonia australis (Shepherd and Sprigg,
1976), and Posidonia oceanica (Boudouresque et al.,
1980; Romero et al., 1994; Mateo et al., 1997). Only
for this last species has the magnitude and dynamics
of the deposits been studied, and key aspects concerning their importance for carbon are summarized
elsewhere in this book (Mateo et al., Chapter 7).
The buried dead materials (rhizomes, leaf sheaths,
and roots) of P. oceanica have a nutrient content of
20–50% N and 5–20% P, compared with the original living material. This decrease in nutrient concentration takes place very quickly (3–5 yr, Romero
et al., 1992). No further changes seem to occur, since
the N and P concentration in material 10–20 yr old
does not differ from that found in material more than
1,000 yr old. Since bulk decay rates of this detritus
have been estimated to be extremely low, between
0.00008 and 0.00036 yr
−1 (Mateo et al., 1997), it is
concluded that nutrient release from it is very low,
and that these deposits are long-term nutrient sinks,
representing net losses of 0.8 and 0.044 g of N and P
m
−2 yr
−1 , or, roughly, 7% and 5% of the total plant
N and P, respectively, annual requirements for plant
growth.
Future efforts in the study of the burial of seagrass
production should be devoted to the characterization
of the refractory organic matter in general but, more
specifically to those compounds containing nitrogen
and phosphorus. Nevertheless, the general conclusion so far is that the amount of nutrients lost in
terms of burial is modest in terms of the ecosystem budget. However, the role of seagrass beds as
nutrient sinks should not be disregarded, especially
in the context of coastal waters facing problems of
eutrophication (see Marb` a et al., Chapter 6).
F. New and Recycled Production
in Seagrasses
In general terms nutrient budgets of seagrass ecosystem include an exchange term and a recycling term.
That is, the total need of nutrients required for