Journal of the Royal Society of Western Australia, 96: 81-83, 2013 Comparison of carbon stores by two morphologically different seagrasses * M ROZAIMI1-2*, O SERRANO1'3, P S LA VERY1 1 School of Natural Sciences and Centre for Marine Ecosystems Research, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027, Australia. 2 School of Environmental and Natural Resource Sciences, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia. 3 The UWA Oceanslnstitute, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. f Corresponding author G3 [email protected] INTRODUCTION not confounded by habitat types potentially influencing the depositional environment of allochthonous carbon, The recent emphasis on global change has Intensified this study compared estuarine meadows of both species. research in the carbon sequestration potential of seagrass Both tidal and swell exchanges dominate WI while OH, ecosystems (Nellemann et al. 2009). Most estimates on SR and HE are wave dominated. Three sediment cores seagrass carbon storage are, however, derived from were each collected from the four sites in 2012 by either studies of a few species and habitats. Lavery et al. (2013) manually hammering core barrels or by vibracoring showed an 18-fold difference in carbon stores among (Vibecore-D, SDI) into the sediments. These cores were Australian seagrasses highlighting the importance of sliced, dried and weighed for dry bulk-density analysis. inter-habitat variability in carbon stocks. One implication Alternate slices of ground and acidified (1 M HC1) from this is that the factor of seagrass species may play a subsamples were encapsulated and analysed for total key role in the variation in carbon stores beneath those organic carbon (OC) and stable carbon isotope studied meadows. Different seagrass species have composition values (813C) at the UC Davis Stable Isotope inherently dissimilar traits, well summarised by the Facility (continuous flow isotope ratio mass spectrometer seagrass functional-form model in Carruthers et al. analyser, Sercon). For testing differences in sedimentary (2007). This functional-form model differentiates OC content among different seagrass species and sites, a seagrasses based on morphological plasticity, rhizome two-way nested ANOVA (SPSS 19) was applied (species persistence and occurrences in varying depositional as fixed factor, site nested within species) and post-hoc environments, among other traits. A further extension of (Tukey) tests were further applied to assess differences this model is suggested such that the seagrass species among sites within the same species. may shift from low to high transitions of standing crop biomass between extreme ends of the model. Of interest, the two seagrass genera Halophila and Posidonia are RESULTS placed at opposite ends of the functional-form model. Halophila has a small biomass with less persistent The sampling resulted in different lengths for all cores. rhizomes while Posidonia has the opposite traits.We After corrections for sediment compression during coring hypothesise that meadows of smaller, ephemeral (Glew et al. 2001), core lengths ranged from 170 cm to 250 seagrasses with low standing crop will have less cm. To allow comparisons, OC characteristics in all accumulated carbon compared to the larger and more sediment cores were standardised for the top 170 cm- persisting forms with higher productivity and biomass. thick deposits. In the Posidonia cores, localised Estuarine seagrass habitats dominated by Halophila ovalis agglutination of plant detrital matter was observed. This and Posidonia australis were studied to compare the total coarse Posidonia detritus was more abundant in OH cores. carbon stocks and origin of the preserved carbon. The top 10 cm of Halophila cores contained low amounts of coarse plant detrital matter becoming less evident until the 25 cm depth and was absent below this level. Mean METHODS OC content was higher in P. australis sites (mean±SE; 2.03+0.19% at OH and 1.12+0.08% at WI) compared to H. Four seagrass meadows were studied: P. australis from ovalis sites (0.34±0.15% at HE and 0.16±0.03% at SR). Tire Oyster Harbour (OH: 34°58'58.3"S, 117°58’29.9"E) and mean OC content in 170 cm-thick deposits was 6-fold Waychinicup Inlet Estuary (WI: 34°53'35.9"S, higher in P. australis sites (1.58±0.21%) compared to H. 118°19’57.7"E), and H. ovalis from Swan River Estuary ovalis sites (0.25±0.07%; ;><0.05). The OC content was (SR: 32°00'46.1"S, 115047'43.5"E) and Harvey Estuary (HE: significantly higher in P. australis OH cores compared to 32°38'00.3"S, 115°38'51.7"E). To allow viable comparisons those in WI (p<0.01), while the OC content among H. ovalis sites was similar (p>0.05). 8UC values of sedimentary organic matter in Posidonia meadows at OH * Extended abstract of a paper presented at the Royal Society of were more positive (-9.89%o to -14.0%o) than in WI Western Australia Postgraduate Symposium 2013 held at (-13.0%o to -19.5%o). The 5,3C signatures in both Halophila Murdoch University on 12 October 2013. sites followed similar trends in all cores (ranging from © Royal Society of Western Australia 2013 -16.0%o to -21.5%o). 81 Journal of the Royal Society of Western Australia, 96(2), December 2013 DISCUSSION OC content trends and 813C values through the cores, OH meadows contained double OC stocks of mainly Comparison of carbon stocks between species seagrass-derived OC compared to WI. Subsequent radiocarbon dating of the cores from WI (unpubl. data; The OC stocks in 170 cm-thick deposits beneath P. accelerator mass spectrometry, Australian Nuclear australis meadows were 6-fold higher than in H. ovalis Science and Technology Organisation) showed that at sites. The difference in OC accumulation between the two similar stratigraphic levels, the organic matter age varied different seagrass species can be partially explained by by -1000 calibrated years before present. A plausible the higher biomass and productivity of P. australis explanation for this variation in tire chronostratigraphy meadows (1300-2100 g DW m'2 and 0.23 g DW nr2 d'1, among cores is a scouring event leading to major respectively) compared to the H. ovalis meadows (76 g sediment reworking at WI resulting in a loss of buried DW nr2 and 0.01 g DW m3 d1: Duarte & Chiscano 1999). OC through erosion. Sediment scouring may also occur In addition, Posidonia species invests larger amounts of in estuaries with higher degree of tidal and swell carbon into below-ground organs (1220 g DW m'2) influence, or during extreme events, and thus could affect compared to small seagrasses such as Halophila (21 g DW the sedimentary OC and 8'3C characteristics. m'2), which invests more energy in rapid clonal propagation resulting in high turnover rates. The below¬ ground organs of seagrasses are more recalcitrant than CONCLUSIONS above-ground organs, and therefore are more likely to end buried in situ as detrital matter, particularly for P. The comparison of OC stocks and 813C signatures of australis rhizomes which can grow 20-50 cm beneath the sedimentary organic matter in the two morphologically sediment surface. Only in the event of major scouring different seagrass species demonstrated significant would these below-ground tissues be uprooted and variations in both the carbon accumulation potential as exported. Furthermore, the dense and relatively high well as the origin of buried carbon among the seagrasses. canopy of P. australis stabilises the sediment, enhancing Meadows of the larger P. australis accrue 6-fold more OC sediment deposition and reducing re-suspension, and than those of the smaller H. ovalis species. Seagrass- increasing tire likelihood of OC stocks being preserved. derived organic matter forms the bulk of those higher This contrasts with H. ovalis meadows with a sparse and stores while allochthonous organic matter is the major low canopy. Below-ground living organs are ephemeral contributor in the Halophila sites. However, within sites growing at 5-10 cm below the sediment surface. Any of the same species, further variations in carbon detrital matter produced by H. ovalis is thus more likely characteristics may be exhibited due to natural ecological to be scoured and allochthonously transported rather processes. The results of this study clearly confirm that than remain buried in situ. both species and habitat may contribute to variation in the OC stored in seagrass meadows and that more Source of carbon comprehensive scrutiny of the factors accounting for The 813C values of sedimentary organic matter in P. those variations are required to improve global estimates australis sites (mean = -13.82%0 for both sites) are of Blue Carbon storage in seagrass meadows. indicative of a strong influence of P. australis-derived input. The reported 513C values of P. australis organs range from -9.9%o to -11.9%o (Hemminga & Mateo 1996), ACKNOWLEDGEMENTS consistent with those observed in the deposits. In This work was supported by a Hodgkin Trust Top Up contrast, 813C values at H. ovalis sites (mean = -19.16%o) Scholarship, an ECU Postgraduate Research Scholarship are indicative of low amounts of seagrass detritus and a UKM Scholarship for Postgraduate Study awarded contributing to the sedimentary organic pool. The to MR. Dating analyses were supported by the Australian reported 813C values of H. ovalis organs range from -6.4%o Institute for Nuclear Science and Technology (Project to -15.5%o (Hemminga & Mateo 1996), much more Award No; ALNGRA12011P) at ANSTO. We appreciate enriched than those we observed in the sediments. The the field- and lab-work assistance from colleagues from S13C values of allochthonous sources of organic matter ECU's Centre for Marine Ecosystems Research. Findings (i.e. seston, algae and terrestrial organic matter) are more from this study will contribute to the research output of negative than seagrass isotopic signatures, in the range the CSIRO Coastal Carbon Cluster. of -13%o to -29% (Smit et al. 2005; Loneragan el al. 1997) and may account for the relatively depleted values in the H. ovalis sediments. Previous studies have commented on REFERENCES the ability of the Halophila canopy to effectively trap allochthonous carbon (Fonseca 1989) and our findings are Carruthers T J B, Dennison W C, Kendrick G A, Waycott M, consistent with an earlier study which found that H. Walker D I & Cambridge M L 2007. Seagrasses of south-west ovalis had the second highest OC accumulation after P. Australia: a conceptual synthesis of the world's most diverse australis meadows, possibly due to the nature of the and extensive seagrass meadows. 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