Chapter 7 Carbon Flux in Seagrasses
169
period L i (sampled in the field), export (E i ) can be
calculated as the difference
E i = L
i − L i
(4)
Such an approach entails intensive field effort, requiring estimates of leaf input, litter decay, and litter
stocks throughout the year, and it possibly underestimates discontinuous export events. Nevertheless,
all the methods required are robust, easy to apply,
and integrate changes over long time. Therefore, this
should be a useful integrative approach for future
studies.
The few reports indicate that export can vary from
0 to 100% of total production (e.g. Bach et al., 1986;
Hemminga and Nieuwenhuize, 1990; Stapel et al.,
1996; Mateo and Romero, 1997; Ochieng and Erftemeijer, 1999; Hemminga and Duarte, 2000). This
large variability results from the high variability of
the intensity of physical energy in the bed, the major driving force (Josselyn et al., 1983; Bach et al.,
1986; Fry and Virnstein, 1988; Mateo et al., 2003;
Mateo and Rossi, submitted; Section II.A of Koch et
al., Chapter 8). Weather, tides, and the degree of bed
exposure (i.e. area of open water or fetch and openness to offshore waters) dictate this intensity. The
crucial role of physical energy is shown by supralittoral deposits in different ecosystems. The largest
accumulations of seagrass leaf litter cast on beaches
have been reported in a small Mediterranean exposed
bay (Tabarca Island, Alicante, Spain) for the species
P. oceanica (Mateo et al., 2003; Fig. 5, left and top
right); the distribution and height of the deposits
(‘banquettes’) accurately described the water energy
reaching the perimeter of the bay, with leaf litter accumulation in amounts from 18 to 500 kg of dry
wt.(m shoreline)
−1 , at both ends and in the center
of the bay, respectively (see also Kuo and den Hartog, Chapter 3 for P. australis examples). The authors estimated for the P. oceanica example that the
total supralittoral deposits represented 50.7, 71.0,
27.2, and 8.7% of the annual bed dry weight, carbon, nitrogen, and phosphorus production, respectively. They concluded, however, that the deposits
were only temporary sinks because the accumulated
material can eventually return to the water (Fig. 6).
At the other extreme, export figures for C. nodosa
leaf litter in a semi-enclosed estuarine bay (Alfacs,
Tarragona, Spain) were found to be almost negligible due to the rapid wave energy dissipation against
the embayment shore (the relative proportions being 0.26, 0.27, 0.27, and 0.16% of the annual bed
dry weight, carbon, nitrogen, and phosphorus production, respectively; Mateo and Rossi, submitted).
Some seagrass species have long, bulky leaves that
sink soon after shedding, whereas others produce
light, thin leaves that can float for long periods before sinking and are thus, more likely to be exported.
Zieman et al. (1979) provided the first example of
the importance of leaf buoyancy. They compared adjacent beds of the relatively broad-leaved turtle grass
(T. testudinum) with beds of the thin-leaved manatee grass (Syringodium filiforme), and showed that,
whereas turtle grass exported 1% of its leaf production, manatee grass exported 75%.
Export of below-ground parts is rarer and only
strong storms can carry significant amounts out of
the bed or throw them onto the beach (Bach et al.,
1986; Fig. 5, bottom right).
In many temperate systems, autumn is characterized by high absolute amounts of litter export
because many seagrass species shed most of their
leaf biomass at this time (Cebri´ an et al., 1997; Mateo and Romero, 1997; Hauxwell et al., 2003). Accordingly, Bach et al. (1986) surveyed leaf export
from an eelgrass bed in Phillips Island (NC, USA)
monthly over 1 year and found the greatest levels of
absolute export in late August, which was also the
period of maximum leaf shedding. However, ecologically meaningful export rates are those relative to
detritus production or to plant requirements. In a seasonal study of leaf litter export in a P. oceanica bed,
Mateo and Romero (1997) found that the highest export losses relative to detritus production occurred
from February–May although maximal litter stocks
were recorded during July–October.
The nutritional quality of seagrass leaf litter is
often strongly correlated with decomposition rates,
which in turn influence export (Mateo and Romero,
1997; P´ erez et al., 2001). Thus, seagrass leaf litter nitrogen content is often positively correlated
with decomposition rates although contradictory results abound (see Section III.C): if decomposition
is slow there is the greater likelihood of export or
burial (see Section IV.B.1). Two P. oceanica beds,
one off Medes Islands (NW Mediterranean, Spain)
and another off the Island of Ischia (Naples, Italy),
both located in open areas and at similar latitudes,
had a three-fold difference in export rates (higher
at Ischia). The effect of waves and currents affecting both beds seemed to be different, and the different export rates were most probably associated
169
period L i (sampled in the field), export (E i ) can be
calculated as the difference
E i = L
i − L i
(4)
Such an approach entails intensive field effort, requiring estimates of leaf input, litter decay, and litter
stocks throughout the year, and it possibly underestimates discontinuous export events. Nevertheless,
all the methods required are robust, easy to apply,
and integrate changes over long time. Therefore, this
should be a useful integrative approach for future
studies.
The few reports indicate that export can vary from
0 to 100% of total production (e.g. Bach et al., 1986;
Hemminga and Nieuwenhuize, 1990; Stapel et al.,
1996; Mateo and Romero, 1997; Ochieng and Erftemeijer, 1999; Hemminga and Duarte, 2000). This
large variability results from the high variability of
the intensity of physical energy in the bed, the major driving force (Josselyn et al., 1983; Bach et al.,
1986; Fry and Virnstein, 1988; Mateo et al., 2003;
Mateo and Rossi, submitted; Section II.A of Koch et
al., Chapter 8). Weather, tides, and the degree of bed
exposure (i.e. area of open water or fetch and openness to offshore waters) dictate this intensity. The
crucial role of physical energy is shown by supralittoral deposits in different ecosystems. The largest
accumulations of seagrass leaf litter cast on beaches
have been reported in a small Mediterranean exposed
bay (Tabarca Island, Alicante, Spain) for the species
P. oceanica (Mateo et al., 2003; Fig. 5, left and top
right); the distribution and height of the deposits
(‘banquettes’) accurately described the water energy
reaching the perimeter of the bay, with leaf litter accumulation in amounts from 18 to 500 kg of dry
wt.(m shoreline)
−1 , at both ends and in the center
of the bay, respectively (see also Kuo and den Hartog, Chapter 3 for P. australis examples). The authors estimated for the P. oceanica example that the
total supralittoral deposits represented 50.7, 71.0,
27.2, and 8.7% of the annual bed dry weight, carbon, nitrogen, and phosphorus production, respectively. They concluded, however, that the deposits
were only temporary sinks because the accumulated
material can eventually return to the water (Fig. 6).
At the other extreme, export figures for C. nodosa
leaf litter in a semi-enclosed estuarine bay (Alfacs,
Tarragona, Spain) were found to be almost negligible due to the rapid wave energy dissipation against
the embayment shore (the relative proportions being 0.26, 0.27, 0.27, and 0.16% of the annual bed
dry weight, carbon, nitrogen, and phosphorus production, respectively; Mateo and Rossi, submitted).
Some seagrass species have long, bulky leaves that
sink soon after shedding, whereas others produce
light, thin leaves that can float for long periods before sinking and are thus, more likely to be exported.
Zieman et al. (1979) provided the first example of
the importance of leaf buoyancy. They compared adjacent beds of the relatively broad-leaved turtle grass
(T. testudinum) with beds of the thin-leaved manatee grass (Syringodium filiforme), and showed that,
whereas turtle grass exported 1% of its leaf production, manatee grass exported 75%.
Export of below-ground parts is rarer and only
strong storms can carry significant amounts out of
the bed or throw them onto the beach (Bach et al.,
1986; Fig. 5, bottom right).
In many temperate systems, autumn is characterized by high absolute amounts of litter export
because many seagrass species shed most of their
leaf biomass at this time (Cebri´ an et al., 1997; Mateo and Romero, 1997; Hauxwell et al., 2003). Accordingly, Bach et al. (1986) surveyed leaf export
from an eelgrass bed in Phillips Island (NC, USA)
monthly over 1 year and found the greatest levels of
absolute export in late August, which was also the
period of maximum leaf shedding. However, ecologically meaningful export rates are those relative to
detritus production or to plant requirements. In a seasonal study of leaf litter export in a P. oceanica bed,
Mateo and Romero (1997) found that the highest export losses relative to detritus production occurred
from February–May although maximal litter stocks
were recorded during July–October.
The nutritional quality of seagrass leaf litter is
often strongly correlated with decomposition rates,
which in turn influence export (Mateo and Romero,
1997; P´ erez et al., 2001). Thus, seagrass leaf litter nitrogen content is often positively correlated
with decomposition rates although contradictory results abound (see Section III.C): if decomposition
is slow there is the greater likelihood of export or
burial (see Section IV.B.1). Two P. oceanica beds,
one off Medes Islands (NW Mediterranean, Spain)
and another off the Island of Ischia (Naples, Italy),
both located in open areas and at similar latitudes,
had a three-fold difference in export rates (higher
at Ischia). The effect of waves and currents affecting both beds seemed to be different, and the different export rates were most probably associated