This study aims at estimating the contribution of different seagrass species growing across an extensive range of deposition to inorganic carbonate and non-carbonate and organic sediment
Trang 1Sediment deposition and production in SE-Asia seagrass meadows
E Gaciaa,*, C.M Duarteb, N Marba`b, J Terradosb, H Kennedyc,
M.D Fortesd, N.H Trie a
Centre d’Estudis Avanc¸ats de Blanes (CSIC), Apartat de correus 118, 17300 Blanes, Spain b
Instituto Mediterra´neo de Estudios Avanzados (CSIC-UIB), C/Miquel Marque´s, 21, 07190-Esporles, Mallorca (Islas Baleares), Spain
c Marine Science Laboratories, University of Wales, Bangor, Anglesey LL59 5EY, Wales, UK d
Marine Science Institute, CS, University of The Philippines, Diliman, Quezon City 1101, Philippines e
Mangrove Ecosystem Research Division, Center for Natural Resources and Environmental Studies, The Vietnam National University,
No 7 Ngo 115 Nguyen Khuyen Street, Hanoi, VietNam Received 25 February 2001; received in revised form 8 April 2002; accepted 15 April 2002
Abstract
Seagrass meadows play an important role in the trapping and binding of particles in coastal sediments Yet seagrass may also contribute to sediment production directly, through the deposition of detritus and also the deposition of the associated mineral particles This study aims at estimating the contribution of different seagrass species growing across an extensive range of deposition
to inorganic (carbonate and non-carbonate) and organic sediment production Total daily deposition measured with sediment traps varied from 18.8 (2.0) g DW m2d1in Silaqui (Philippines) to 681.1 (102) g DW m2d1in Bay Tien (Vietnam) These mea-surements correspond to a single sampling event and represent sedimentation conditions during the dry season in SE-Asia coastal areas Enhalus acoroides was the most common species in the seagrass meadows visited and, together with Thalassia hemprichii, was present at sites from low to very high deposition Halodule uninervis and Cymodocea species were present in sites from low to medium deposition The mineral load in seagrass leaves increased with age, and was high in E acoroides because it had the largest and long-lived leaves (up to 417 mg calcium carbonate per leaf and 507 mg non-carbonate minerals per leaf) and low in
H uninerviswith short-lived leaves (4 mg calcium carbonate per leaf and 2 mg non-carbonate minerals per leaf) In SE-Asia seagrass meadows non-carbonate minerals accumulate at slower rates than the production of calcium carbonate by the epiphytic community, consequently the final loads supported by fully grown leaves were, as average, lower than calcium carbonate loads Our results show that organic and inorganic production of the seagrasses in SE-Asia represents a small contribution (maximum of 15%) of the materials sedimented on a daily base by the water column during the sampling period The contribution of the carbonate fraction can be locally significant (i.e 34% in Silaqui) in areas where the depositional flux is low, but is minor (<1%) in sites were siltation
is significant (i.e Umalagan and all the visited sites in Vietnam)
Ó 2003 Elsevier Science B.V All rights reserved
Keywords: deposition; seagrass; leaf production; sediment; siltation
1 Introduction
Seagrass meadows affect sedimentary dynamics by
promoting sedimentation and preventing resuspension
(Almasi, Hoskin, Reed, & Milo, 1987; Gacia, Granata,
& Duarte, 1999; Terrados & Duarte, 2000; Ward,
Kemp, & Boyton, 1984) Resuspension is prevented by
the network of rhizomes and roots that bind the
sediments (e.g Fonseca, 1989, 1996), and by the dissipation of energy by the plant canopies, which is also believed to be the main mechanism responsible for increased sediment deposition within seagrass beds (Eckman, Duggins, & Sewell, 1989; Fonseca & Fisher, 1986; Fonseca, Fisher, Zieman, & Thayer, 1982; Gambi, Nowell, & Jumars, 1990; Verduin & Backhaus, 2000) Seagrass may also contribute to sediment production directly, through the deposition of litter and associated mineral particles, but reports on this process are still few The accumulation of calcium carbonate produced
by seagrass leaf epiphytes has been quantified for some
* Corresponding author.
E-mail address: gacia@ceab.csic.es (E Gacia).
0272-7714/03/$ - see front matter Ó 2003 Elsevier Science B.V All rights reserved.
doi:10.1016/S0272-7714(02)00286-X
Trang 2tropical and temperate species (Bosence, 1989; Canals &
Ballesteros, 1997; Frankovich & Zieman, 1994; Heijs,
1984, 1985, 1987; Land, 1970; Patriquin, 1972; Romero,
1988; Walker & Woelkerling, 1988) and was shown to
be quantitatively significant In addition to production
of calcium carbonate on seagrass leaves, seagrass may
probably also capture significant amounts of particles
that might be passively trapped by on leaf surfaces due
to the binding capacity of exopolymeric substances
pro-duced by both the epiphytes and the plants themselves
However, this contribution of mineral particles
pas-sively trapped by seagrass surfaces to sediment
produc-tion is yet to be investigated
Here we quantify the contribution of some SE Asian
seagrass species growing under contrasting
environ-mental conditions to inorganic (carbonate and
non-carbonate) and organic sediment production The
seagrass stands were sampled across an extensive range
of deposition regimes in coastal areas from the
Philippines and Vietnam We separate the total
in-organic particles on leaf surfaces into carbonate and
non-carbonate minerals The carbonate minerals are
mainly produced in situ by the epiphyte community
whereas the non-carbonate minerals, mainly of
terres-trial origin, are passively adhered on to leaf surfaces We
then compare the mineral accumulation on the seagrass
(sediment production) to the total sediment deposition
(primary plus resuspended flux) measured during the
sampling event in the meadows investigated
2 Methods
A total of nine different sites corresponding to three
contrasting locations (two on the Western Philippine
coast and one on the coast of Vietnam; Fig 1) were
sampled to estimate total sediment deposition, seagrass
species composition, aboveground biomass and carbo-nate and non-carbocarbo-nate mineral accumulation on seagrass leaves Samples were collected during the dry season in one sampling event from 23 March to 7 April
1999 The Bolinao area (Panganisan, 16N 119E; Fig 1) in the Philippines, is characterized by mixed seagrass meadows growing along an S to N siltation gradient from the heavily silted mouth of the Alaminos river to the pristine reef lagoon of Santiago Island (Bach, Borum, Fortes, & Duarte, 1998; Le Jeune, 1995) Cape Bolinao area experiences a maximum semi-diurnal tidal range of 1 m Here three sites were sampled, Santa Barbara, Pislatan and Silaqui that followed a gradient from turbid to clear waters Ulugan Bay (Palawan, 10N
118E; Fig 1) in the Philippines contained mixed seagrass meadows outside mangrove forest (Fortes, 1988) Current patterns in the bay are dominated by prevailing wind and water surface’s tidal ebb and flow Over 10 rivers empty into Ulugan Bay altogether creat-ing a natural gradient of exposure to wave action and currents, freshwater input and geomorphology (Padilla, pers commun.) At this location two sites, Buenavista and Umalagan, were visited Buenavista is located on the eastern part of the bay and is subject to intense wave action and to a high degree of man made and natural disturbance Umalagan is located at the southern end of the bay and is characterized by highly turbid waters due to the sediment loads carried by six rivers that empty in the area Four sites were visited in the Central coast of Vietnam (12N 109E; Fig 1), where the hydrodynamic regime is characterized by high-energy environment due
to strong wind conditions and open nature of the coasts Two of the sites (My Giang I and My Giang II) had mixed seagrass meadows in an area with no river influence but recently exposed to heavily silt load from quarry exploitation and the activities of a port nearby At My Giang average tidal amplitude is of 2.3 m and current velocities fluctuate between 19 and 23 m s1 A third site (Bay Tien) was characterized by monospecific meadows
of Thalassia hemprichii exposed to heavy boat traffic and small sewage Mean tidal amplitude is of 1.4 m and current velocities fluctuate between 11 and 18 m s1 A fourth site (Dam Gia) was a pristine bay with meadows of Enhalus acoroidesunder slight influence of the rivers Cai and Be, discharging 12 km away from the bay Dam Gia has mean tidal range of 2.8 m and current velocities from
15 to 27 m s1 Total sediment deposition was measured within the plant canopies using sediment traps consisting of 20-ml cylindrical glass centrifugation tubes with an aspect ratio
of 5 (16-mm diameter), thus integrating the deposition over the deployment time without losses from resuspen-sion The tubes were attached by groups of 5–30 cm long stainless bars, and were separated 4 cm from each other (see details in Gacia et al., 1999) Triplicate sets of sediment traps (i.e 15 traps) were deployed once at each
Fig 1 Localization of the field sites Bolinao (Panganisan, NW Luzon
Island) and Ulugan Bay (Palawan) in The Philippines, and Nha Trang
in the Central Vietnam.
Trang 3site at the bottom of the plant canopy Capped traps
were carefully deployed by SCUBA divers, who removed
the caps after ensuring that any resuspension they may
have caused had settled The traps were retrieved after
a deployment period of 2–4 d
In the laboratory the content of the traps was filtered
through pre-weighted 25 mm GF/C filters and the dry
weight (DW) was assessed after desiccation for 24 h at
60C Determination of the organic carbon and
carbon-ate content on sedimented particles was done by separcarbon-ate
analysis of the total carbon content (ignition) and the
organic carbon (acidification to remove carbonate and
subsequent ignition) using an EUROPA CN analyzer
The amount of carbonate carbon was then obtained by
difference between the total and organic carbon
The abundance of the different species was measured
by counting the shoots of each species present in five
(625 cm2) quadrates randomly placed within the seagrass
meadow For the large species E acoroides, the shoot
density was estimated by counting the number of shoots
in 10–20 quadrates A total of eight shoots of the most
abundant species were randomly selected per site and
the leaves sorted by rank position within the shoot to
measure the organic and carbonate mass, and to
estimate the mineral load on the leaves
The organic content of the leaves was estimated from
the weight loss of dried leaves (60C for 24 h.) following
combustion at 450C for 2 h The carbonate load on the
leaves was calculated from the weight loss of the
resulting ashes after combustion (1000C for 1 h; Dean,
1974) This method was used instead of the CN used
for the material collected in the sediment traps because
of the large amount of material available and the
convenience that the samples could be processed during
the campaign, thereby avoiding loss of materials during
transportation and storage Four of the collected shoots
were used to estimate internal non-carbonate mineral
content in the leaves The leaves of these shoots were
cleaned by scraping epiphytes and other materials
accumulated on their surface The internal content of
minerals in the leaves was subtracted from the total
content to estimate the non-carbonate mass on the leaf
surface only We assumed that the internal content of
carbonates in the leaves was negligible
Daily mineral accretion rates on leaves (g leaf1d1)
were calculated as the average mineral load per leaf
divided by the leaf weight and leaf age in days The
values reported here are the result of averaging four
shoots per site Leaf age was calculated as the leaf rank
within the shoot multiplied by the leaf plastochrone
interval (i.e time elapsed between the appearance of two
consecutive leaves, in days), estimated retrospectively
(Duarte et al., 1994) The seasonal variability in vertical
internodal length imprinted on the vertical rhizomes of
5–10 living shoots collected at each meadow, and
quanti-fied under a stereo microscope, allowed calculation
of the average number of leaves produced per short shoot annually, and, therefore, estimation of the leaf plastochrone interval The leaf plastochrone interval used here for Bolinao species, however, was obtained from Vermaat et al (1995) Leaf plastochrone interval was not measured for E acoroides growing at My Giang However, because variability in LPI is rather small for the same species from different sites of the same location (Vermaat et al., 1995) we have assumed the LPI of E acoroidesand T hemprichii to be the same in My Giang than for the other two sites in Central Vietnam
The average daily rate of accumulation of carbonate (Ac) and non-carbonate (Anc) mineral (g m2d1) was assessed from the maximum load (LLc, LLnc) of fully grown leaves (g leaf1), combined with shoot densities (d, shoots1m2) and the daily shoot production of leaves (LP, d1) for the different species at the different sites:
Ac¼Xsp¼n sp¼1 LLc d LP
Anc¼Xsp¼n sp¼1 LLnc d LP
where n is the number of species present at any one site Because the leaf life span for these species ranges be-tween weeks and months (Hemminga & Duarte, 2000), these estimates, as well as those of leaf production, represent average estimates at these time scales, rather than estimates for individual days
3 Results 3.1 Sediment deposition The total daily deposition within the seagrass can-opy varied from minimum values of 18.8 (2.01)
g DW m2d1 in Silaqui (Bolinao) to maximum values
of 681.1 (102.4) g DW m2d1in Bay Tien (Nha Trang; Table 1) The deposition rates within the range studied were low to intermediate in the Philippines, and low
to very high in Vietnam The organic content of the deposition ranged from 4.9 0.22 to 8.0 0.80% of the DW and was particularly low in Buenavista, and relatively high in St Barbara and Pislatan Calcium carbonate represented a variable portion of the total inorganic deposition, ranging from 8.9 to 33.9% in Buenavista and My Giang II, respectively The rest of the material corresponded to non-carbonaceous min-erals, such as silicates, and various salts
3.2 Species composition and leaf production
E acoroides was the most common species in the seagrass meadows visited, and was present across the
Trang 4whole deposition gradient comprised by the sites studied
except in Bay Tien E acoroides was also the species with
the largest leaves, followed by Cymodocea serrulata,
Cymodocea rotundata, T hemprichii and finally Halodule
uninervis (Table 2) The leaves of E acoroides were
extremely large (2316 g DW leaf1) in Umalagan
(Pala-wan) compared to the average value (505 130.5
g DW leaf1) at other localities (Table 2) T hemprichii
and E acoroides were present from areas with high
to areas with low deposition, whereas H uninervis,
C serrulata and C rotundata were only present at sites
from low to medium deposition
The estimated net leaf production of the seagrass communities, including that of the epiphytes, oscillated between 0.26 and 4.34 g OW m2d1 (Table 3), com-parable to the range observed for seagrass meadows worldwide (Duarte, 1989; Duarte & Chiscano, 1999) For some meadows (e.g Silaqui, Pislatan, My Giang I and My Giang II) the values given here represent minimum estimates, because they only include the more abundant species For instance, only four species in Silaqui, representing 95% of the aboveground biomass (Vermaat et al., 1995) were included here, while there is
a total of seven different species in the meadow
Table 2
Species composition, shoot densities, maximum leaf biomass (organic content, OW; and dry weight, DW), maximum carbonate (CaCO 3 ) and non-carbonate mineral (non-CaCO 3 min) accumulation per leaf and annual average leaf plastochrone interval (LPI) of the different species of the meadows visited
Densities Biomass
Shoots (m2) SE
OW (mg leaf1)
DW (mg leaf1)
CaCO 3 (mg leaf1)
non-CaCO 3 min (mg leaf1) LPI (d)
Table 1
Total daily sediment deposition (S; g DW m 2 d 1 ), organic content (% o.m.) and calcium carbonate deposition (CaCO 3 ; g m 2 d 1 ) in seagrass meadows from the Philippines and Vietnam
S (g DW m2d1) Percentage of o.m CaCO 3 (g m2d1)
My Giang 1 (12.4N, 109.2E)
My Giang 2 (12.4N, 109.3E)
Values are mean standard error.
Trang 53.3 Mineral accretion in leaves
The mineral load on the seagrass leaves progressively
increased for both carbonate and non-carbonate
mate-rial from the young to the old leaves for all the species
studied at all the sampling sites (Fig 2) In some cases,
particularly for E acoroides, a decrease in the mineral
load per leaf was found in the older leaves because the
leaf apices may break due to grazing from herbivores or
mechanical impact
E acoroides leaves supported the highest load of
carbonates, with values ranging from 26 to 417 mg
leaf1, and non-carbonate minerals, which ranged from
8 to 507 mg leaf1 (Table 2) Fully grown C rotundata
leaves also reached a high carbonate load of 17–
55 mg leaf1 and non-carbonate minerals ranged from
0.4 to 2 mg leaf1, followed by C serrulata (25–43 mg
carbonates per leaf and 0.9–13 mg non-carbonate
minerals per leaf), and T hemprichii (8–35 mg
carbon-ates per leaf and 1–18 mg non-carbonate minerals per
leaf; Table 2) Fully grown H uninervis leaves presented
the lowest load of carbonates (4 mg carbonates per leaf)
and non-carbonates mineral (2 mg non-carbonate
min-erals per leaf ) loads The average load of calcium
carbonate per leaf of E acoroides was significantly
higher (Anova p < 0:05) than for the rest of the species,
resulting in five times the average values for Cymodocea
species and close to one order of magnitude larger that
of T hemprichii (Fig 3) The load of non-carbonate
minerals was also significantly higher for E acoroides
compared to the other species (Anova p < 0:05), and
lower to that of calcium carbonate except for extremely
high values for E acoroides from Umalagan (Table 2)
The weight of calcium carbonate on seagrass leaves
widely exceeded that of non-carbonate minerals in most
of the sites except in Umalagan, Dam Gia and My
Giang where non-carbonated minerals doubled the
production of carbonates (Table 2)
Rates of mineral accretion for calcium
carbon-ate oscillcarbon-ated between 1.8 mg CaCO3g DW1d1 in
CaCO g DW1d1 in T hemprichii from My Giang I,
and the accretion rate of non-carbonate minerals ranged from 1.2 mg g DW1d1 in E acoroides from Pislatan
to 13.3 mg g DW1d1in H uninervis from Buenavista Mineral accretion rates were similar within species across sites (Anova, p > 0:7 and p > 0:6 for CaCO3 and non-CaCO3 minerals, respectively) but varied significantly among species (Anova, p < 0:005 for CaCO3 and Anova p < 0:05 for non-CaCO3 minerals; Fig 4) The accretion rate of calcium carbonate was significantly higher in T hemprichii compared to E acoroides The accretion of non-carbonate minerals was also significantly higher in H uninervis and T hemprichii than E acoroides
Estimates of the total leaf mineral accretion in the seagrass meadows ranged from 0.23 g m2d1 in Dam Gia Bay to 2.30 g m2d1in Pislatan (Table 3), of which carbonates represented, on average, 62.5 5.76% The production of calcium carbonate closely correlated to leaf seagrass production (r¼ 0:74; p < 0:01) while non-carbonate mineral accretion was independent of leaf production (r¼ 0:13; p > 0:5) The estimated accumu-lation of CaCO3 and non-carbonate minerals in the seagrass meadows studied was independent of the estimated total sediment deposition across the sites (r¼ 0:34 and r ¼ 0:14, respectively; p > 0:5)
4 Discussion The deposition rates reported for the SE-Asia seagrass meadows are high when compared to data reported from around the world (Table 4) The range of values observed in the Philippines are comparable to the rates reported for shallow coastal areas in the Pacific and Atlantic, while even the minimum values recorded
in Vietnam are very high (Table 4) This comparison should, however, be considered with caution, since our data only reflect a single short-term sampling event Furthermore, as we did not sample under extreme weather conditions that may enhance deposition (e.g heavy rainfall and typhoons) the range of values reported here should represent minimum estimates of
Table 3
The estimated net organic and inorganic production (seagrass plus epiphytes; g m 2 d 1 ) for the studied seagrass communities
Biomass (g OW m2d1)
CaCO 3 (g m2d1)
Non-CaCO 3 min (g m2d1)
Total mineral (g m2d1)
Trang 6Fig 2 Accumulation of carbonates (mg leaf1) and non-carbonate minerals (mg leaf1) in seagrass with time for the most abundant species of the different stations; Silaqui (black diamonds), Pislatan (black squares) and Santa Barbara (black triangles) in Bolinao (Philippines), Buenavista (empty squares) and Umalagan (empty triangles) in Palawan (Philippines) and Bay Tien (empty circles), My Giang I (+), My Giang II () and Dam Gia Bay (full dots) in Central Vietnam The plastochrone interval (time between the appearance of two consecutive leaves) is indicated in days.
Trang 7the average deposition in the area at this time of the
year The average organic content of the sedimentary
flux (6.33 1.13% of DW) is low when compared to
published estimates in the literature (Table 4), and is
very close to that in the seagrass sediments (6.04 1.51%
of DW, Kamp–Nielsen, unpublished results) suggesting
a common origin Moreover, the general low organic
content of the material trapped suggests a significant
contribution of allochtonous inorganic material to total
deposition in shallow seagrass meadows of the
Philip-pines and Vietnam This can be explained by the
wide-spread environmental problem of siltation in SE-Asia
coastal areas (e.g Fortes, 1988; Go´mez, 1988), which
results in excessive sediment loads, as indicated by
our results, that lead to a deterioration of light (Bach
et al., 1998) and sediment (Terrados et al., 1998)
con-ditions with detrimental consequences for seagrass
communities
E acoroides, the species most resistant to siltation
(Duarte et al., 1997; Terrados et al., 1998) was the most
common species in the seagrass meadows visited both, in
the Philippines and Vietnam, and was present across the
deposition gradient This species is able to withstand
high deposition rates due to their long leaves (up to 2 m
long; Terrados et al., 1998; Vermaat et al., 1995) which
raise the canopy closer to the water surface thereby, minimizing the negative effects of the shading resulting from high silt loads in the water column (Bach et al., 1998) T hemprichii, which is believed to be particularly sensitive to siltation (Terrados et al., 1998; Vermaat
et al., 1997), was present at sites with very high deposi-tion such as in Bay Tien (Table 1) where strong tidal cur-rents advect significant amount of sediment across the meadow Sediment mobilization results in turbid waters but not necessarily into high burial rates Hence, T hemprichii in Bay Tien is able to survive there forming small highly rooted and compacted shoots to avoid uprooting from the currents, and with very low growth rates (Vermaat et al., unpublished data) due to the deteriorated light regime
Mineral load in seagrass leaves increased with age as expected from longer exposure to colonization by epi-phytes and a higher cumulative interception of sus-pended particles Consequently, the total accumulation
Fig 4 Box plots for average rates of mineral (carbonate and non-carbonate) accretion in mg mineral g DW leaf1 for the different species Boxes encompass 50% of the values, the line represents the median value, and the bars extend to the 95% confidence limits.
Fig 3 Box plots showing the range of carbonates and non-carbonate
minerals accumulated per leaf of the different seagrass species Boxes
encompass 50% of the values, the line represents the median value, and
the bars extend to the 95% confidence limits.
Trang 8of minerals was high in E acoroides, which had the
largest and longest-lived leaves (Table 2), and low in H
uninerviswith small and short-lived leaves (Table 2) The
rates of CaCO3accretion did not vary within the same
species across sites, which indicates sufficient and
non-limiting conditions for epiphyte development in any of
the areas visited The rates of mineral accretion were,
however, particularly low for the slow growing high
density leaf biomass of E acoroides compared to the fast
growing and leaf elongation T hemprichii (Fig 4)
Non-carbonate minerals accumulate at slower rates than the
production of calcium carbonate by the epiphytic
community (Fig 4) and, consequently the final loads
supported by fully grown leaves were, as average, lower
than calcium carbonate loads
Estimates of the production of calcium carbonate by
seagrass meadows display variable rates (Table 5); the
lowest recorded are those from Umalagan and Dam
Gia, because of the low community production,
followed by meadows of temperate oligotrophic waters
(Canals & Ballesteros, 1997) The maximum rates are
found in highly productive tropical meadows, such as
meadows of different species in Papua New Guinea
(Heijs, 1984, 1985, 1987), or meadows of Thalassia
testudinum in Bermudas (Patriquin, 1972) A
compila-tion of data from the literature showed that log
transformed data on calcium carbonate production and leaf biomass production were highly correlated for seagrass meadows worldwide (Fig 5; r¼ 0:71;
p <0:001) This observation agrees with our findings indicating that the net leaf production is a key factor enhancing the potential contribution of minerals to sediments by the seagrasses
The potential contribution of the seagrass meadows, including both the mineral and the organic production,
Table 4
Range and average bulk sediment deposition and organic content for different shallow coastal areas
g DW m2d1 Percentage of o.m.
Deployment Range Average Range Average depth (m) Area/bottom Reference
Cattaneo-Vietti, Cerrano, Danovaro, and Fabiano (1995)
Gre´mare, and Baudart (1995)
Bay of Calvi
(Corsica)
Fanals (Spain) Mediterranean 2–140 23.8 2.5–39 17 15 Posidonia oceanica Gacia and Duarte (2001)
(1999)
(1994)
Scofield (1994)
Mann (1975) Discovery Bay
(Jamaica)
Thompson (1974)
Data are from sediment traps deployed near the bottom Values in brackets indicate the maximum depth when traps were above the substrate.
Fig 5 Relationship between calcium carbonate production and leaf biomass production for seagrass meadows worldwide.
Trang 9to the estimated total deposition during the period
studied can be locally significant, up to 15% in Silaqui
and Pislatan where total deposition is low (Tables 1 and
3) However, this potential contribution is minor (<1%)
for sites receiving important deposition loads such as
Bay Tien, Umalagan and My Giang I, with an average
potential contribution of 0.36 0.12% of the
deposi-tion This low contribution of the plants to total
de-position explains why the estimated seagrass production
fails to explain the measured total deposition in the
meadows visited during the survey (Fig 6) For calcium
carbonate, however, the potential contribution of the
plants to the measured deposition is a bit higher, and
close to the 35% in very productive meadows of the
Philippines such as Silaqui These estimates however, do
not account for potential losses of seagrass biomass and
associated minerals due to grazing from herbivorous or
Table 5
Estimated production of calcium carbonate and seagrass leaf biomass for different seagrass communities
Community
Leaf production (g DW m2d1)
CaCO 3 production
Thalassia hemprichii 1.61–2.78 0.92–6.52 b Bootless Bay
(Papua New Guinea)
Tropical Heijs (1984)
(Papua New Guinea)
Tropical Heijs (1985)
(Papua New Guinea)
Tropical Heijs (1985)
(Papua New Guinea)
Tropical Heijs (1985)
(Papua New Guinea)
Tropical Heijs (1985)
(Papua New Guinea)
Tropical Heijs (1987) Amphibolis antarctica — 0.14–0.96 Shark Bay (Western Australia) Subtropical Walker and Woelkerling (1988)
Thalassia testudinum 0.16–2.50 0.15–2.85 Florida Bay, USA Subtropical Bosence (1989)
(Philippines)
Subtropical This study Enhalus acaroides–Cymodocea
serrulata
(Philippines)
Subtropical This study
Caulerpa prolifera–Cymodocea
nodosa
(Western Mediterranean)
Temperate Canals and Ballesteros (1997)
(Western Mediterranean)
Temperate Canals and Ballesteros (1997)
(Western Mediterranean)
Temperate Romero (pers commun.; 1988)
Daily rates have been recalculated from the literature.
a
DW (g) including ashes.
b
Estimated from published DW vs AFDW ratios.
Fig 6 Distribution of the different meadows sampled as function of estimated biomass production and measured total deposition.
Trang 10export phenomena, nor losses of sediment material from
resuspension, which may vary significantly among sites
In summary, the results presented here indicate a
significant production of both, organic and mineral
particles by some SE Asian seagrass meadows, which
support, even in the dry season, high loads of
non-carbonaceous, terrigenous sediments in silted coastal
areas The direct sediment production by seagrasses is,
however, small compared to the direct deposition of
materials sedimented by the water column, except for
carbonate deposition, for which the potential
contribu-tion of seagrass produccontribu-tion can be locally significant
Seagrass meadows can be, therefore, important sites of
carbonate production in the coastal areas Long-term
seasonal measurements, together with evaluation of plant
losses are however still needed in order to provide a
complete budget of the contribution of seagrass meadows
to sediment production in vegetated coastal areas
Acknowledgements
This research was funded by project PREDICT
(contract IC18-CT98-0292) of the INCO program of
the European Commission We are grateful to the
PREDICT colleagues for the great time and continuous
support during the 1999 fieldtrip HK wishes to thank The
Ministry of Education and Culture, Spain for support
during the preparation of this manuscript We also thank
three anonymous reviewers for their criticisms that
helped to improve a former version of the manuscript
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