oxygen and carbon metabolism of zostera muelleri across a depth gradient implications for resilience and blue carbon

Accepted Manuscript Oxygen and carbon metabolism of Zostera muelleri across a depth gradient – Implications for resilience and blue carbon Angus J.P Ferguson, Renee Gruber, Jaimie Potts, Aaron Wright, David T Welsh, Peter Scanes PII: S0272-7714(17)30011-2 DOI: 10.1016/j.ecss.2017.01.005 Reference: YECSS 5363 To appear in: Estuarine, Coastal and Shelf Science Received Date: 13 July 2016 Revised Date: December 2016 Accepted Date: January 2017 Please cite this article as: Ferguson, A.J.P., Gruber, R., Potts, J., Wright, A., Welsh, D.T., Scanes, P., Oxygen and carbon metabolism of Zostera muelleri across a depth gradient – Implications for resilience and blue carbon, Estuarine, Coastal and Shelf Science (2017), doi: 10.1016/j.ecss.2017.01.005 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain ACCEPTED MANUSCRIPT Oxygen and carbon metabolism of Zostera muelleri across a depth gradient – implications for resilience and blue carbon RI PT Angus J P Ferguson1*, Renee Gruber1,2, Jaimie Potts1, Aaron Wright1, David T Welsh3, Peter Scanes1 NSW Office of Environment and Heritage, Sydney, Australia University of Western Australia, Perth, Australia Griffith University, Gold Coast, Australia SC *Corresponding author Keywords: blue carbon, seagrass, carbon budget, metabolism, depth gradient M AN U Abstract 11 There is growing interest in the role that seagrasses play as ‘blue carbon’ stores or sinks, and their 12 potential to offset rising CO2 levels in the atmosphere This study measured primary aspects of the 13 carbon balance (biomass, community metabolism, dissolved organic carbon [DOC] fluxes, seston 14 trapping) across the depth gradient in a Zostera muelleri meadow during the seasonal biomass 15 minimum and maximum Over the annual estimation, the meadow was neither a sink nor source of 16 carbon, with inputs of seston (~58% of total inputs) balanced by exports of wrack and DOC The 17 carbon sink represented by wrack export depends on the nature of the environment where the 18 wrack accumulates; if it reaches subtidal sediments it will largely be remineralised over the annual 19 cycle, whereas between 14 – 26% of the wrack may be preserved if the material is exported to 20 terrestrial environments The fate of DOC exuded by seagrasses is unknown due to a lack of 21 knowledge about its composition and lability; however, a number of lines of evidence suggest that a AC C EP TE D 10 ACCEPTED MANUSCRIPT large fraction of DOC is mineralised The net community metabolism (NCM) of the meadow was 23 balanced, indicating that photosynthetic O2 production balanced community respiration and / or the 24 reoxidation of reduced compounds (sulphur and iron) in the rhizosphere We suggest that a 25 balanced NCM may be the preferred state for Zostera spp and may limit their occurrence in 26 environments where plants cannot balance the respiratory demand exerted by seston inputs There 27 was a close coupling between metabolism and biomass, which in turn is forced by antecedent light 28 over the preceding 120 days (the time integration window for antecedent light that best predicted 29 biomass) Increased metabolism with depth and seasonal variation in light is accompanied by a 30 decrease in the above ground:below ground biomass ratio (AGB:BGB) This trend is suggested to be 31 a morphological adaptation that balances the competing requirements of maintaining a neutral 32 plant carbon balance across enrichment and light gradients Our results suggest that Zostera 33 muelleri may be most important as a ‘blue carbon’ store (i.e carbon stored as biomass standing 34 stock), which is therefore vulnerable to degradation if seagrasses are lost 35 Introduction 36 Seagrass meadows are important and iconic coastal systems that provide a range of ecosystem 37 services including serving as a food source and habitat for other species; stabilising subtidal 38 sediment; and constituting a store of carbon (Heck et al 2003, Emmett Duffy 2006, Waycott et al 39 2009 , Kennedy et al 2010) Recent concern over rising atmospheric carbon dioxide levels due to 40 climate change has prompted greater scientific focus on the ‘blue carbon’ sink potential represented 41 by seagrasses (Duarte et al 2010) (Fourqurean et al 2012) Seagrass meadows store carbon in living 42 biomass (‘stores’) and as buried refractory seagrass detritus (‘sinks’), such as rhizome mats in 43 Posidonia meadows (Duarte et al 2011) Many seagrasses exist in sediments with low organic 44 carbon (Ferguson et al 2016), suggesting they may store carbon primarily in living biomass, 45 therefore the significance of their role as ‘blue carbon’ stores depends on their continued survival 46 (Lavery et al 2013a) Seagrasses are under increasing pressure globally due to multiple stressors AC C EP TE D M AN U SC RI PT 22 ACCEPTED MANUSCRIPT associated with human development (Orth et al 2006), and global declines have already resulted in 48 substantial loss of carbon storage (McLeod et al 2011) 49 Central to determining the carbon balance of seagrass meadows is a knowledge of the net 50 community metabolism (NCM), defined as gross primary production (GPP) less community 51 respiration (CR) over the diel period, which can also be expressed as a ratio of GPP to CR (P:R) 52 Assessments of NCM are commonly based on the measurement of O2 and/or CO2 fluxes over a diel 53 period (Duarte et al 2010)) Net autotrophic systems (P:R >1) fix more carbon than is released by 54 respiration and provide the conditions for a potential carbon sink, as long as that carbon is buried 55 either within the meadow or elsewhere Balanced NCM (NCM=0, P:R =1), implies that all 56 allochthonous and autochthonous carbon inputs are balanced by microbially mediated 57 remineralisation of the labile organic matter (OM) plus burial of refractory material In the absence 58 of significant burial of OM within the meadow, a balanced NCM implies that it functions primarily as 59 a store of carbon in living biomass Net heterotrophic metabolism (NCM