DSpace at VNU: Sediment deposition and production in SE-Asia seagrass meadows

Estuarine, Coastal and Shelf Science 56 (2003) 909–919 Sediment 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 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 Ngo 115 Nguyen Khuyen Street, Hanoi, VietNam b Received 25 February 2001; received in revised form 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 (Ỉ2.0) g DW mÿ2 dÿ1 in Silaqui (Philippines) to 681.1 (Ỉ102) g DW mÿ2 dÿ1 in Bay Tien (Vietnam) These measurements 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 uninervis with short-lived leaves (4 mg calcium carbonate per leaf and 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 ( 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 mÿ2 dÿ1 in Dam Gia Bay to 2.30 g mÿ2 dÿ1 in Pislatan (Table 3), of which carbonates represented, on average, 62.5 Ỉ 5.76% The production of calcium carbonate closely correlated to leaf seagrass production (r ¼ 0:74; p < 0:01) while noncarbonate mineral accretion was independent of leaf production (r ¼ 0:13; p > 0:5) The estimated accumulation of CaCO3 and non-carbonate minerals in the seagrass meadows studied was independent of the estimated total sediment deposition across the sites (r ¼ 0:34 and r ¼ 0:14, respectively; p > 0:5) 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 914 E Gacia et al / Estuarine, Coastal and Shelf Science 56 (2003) 909–919 Fig Accumulation of carbonates (mg leaf ÿ1) and non-carbonate minerals (mg leaf ÿ1) 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 (Â) and Dam Gia Bay (full dots) in Central Vietnam The plastochrone interval (time between the appearance of two consecutive leaves) is indicated in days E Gacia et al / Estuarine, Coastal and Shelf Science 56 (2003) 909–919 915 Fig 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 the average deposition in the area at this time of the year The average organic content of the sedimentary flux (6.33 Ỉ 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 Ỉ 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 Philippines and Vietnam This can be explained by the widespread 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) conditions 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 m long; Terrados et al., 1998; Vermaat et al., 1995) which Fig Box plots for average rates of mineral (carbonate and noncarbonate) accretion in mg mineral g DW leaf ÿ1 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 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 deposition such as in Bay Tien (Table 1) where strong tidal currents 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 epiphytes and a higher cumulative interception of suspended particles Consequently, the total accumulation 916 E Gacia et al / Estuarine, Coastal and Shelf Science 56 (2003) 909–919 Table Range and average bulk sediment deposition and organic content for different shallow coastal areas g DW mÿ2 dÿ1 Percentage of o.m Average Deployment depth (m) Area/bottom — — 20 1.25–10 5.2 3.8–112 24.4 — 6–24 — 14 15 18 Banyuls (France) Bay of Calvi (Corsica) Fanals (Spain) Po River Delta Mediterranean 0.1–316 22.8 Mediterranean 0.04–9.8 2.3 5.4–22 — 12 — 18 36 Mediterranean Mediterranean 2–140 23.8 0.8–88.7 8.3 2.5–39 — 17 — 15 21 (23) Sweden Baltic sea 0.5–116 7.2 — — 20 (32) Kiel Bight Chesepeake Bay Nova Scotia Baltic sea N.W Atlantic N.W Atlantic 0.1–79.9 4.6 23–115 51.4 9–108 37.4 — — — — — — 18 (20)