Seagrass meadows as a globally significant carbonate reservoir

Abstract

Abstract. There has been growing interest in quantifying the capacity of seagrass ecosystems to act as carbon sinks as a natural way of offsetting anthropogenic carbon emissions to the atmosphere. However, most of the efforts have focused on the particulate organic carbon (POC) stocks and accumulation rates and ignored the particulate inorganic carbon (PIC) fraction, despite important carbonate pools associated with calcifying organisms inhabiting the meadows, such as epiphytes and benthic invertebrates, and despite the relevance that carbonate precipitation and dissolution processes have in the global carbon cycle. This study offers the first assessment of the global PIC stocks in seagrass sediments using a synthesis of published and unpublished data on sediment carbonate concentration from 403 vegetated and 34 adjacent un-vegetated sites. PIC stocks in the top 1 m of sediment ranged between 3 and 1660 Mg PIC ha−1, with an average of 654 ± 24 Mg PIC ha−1, exceeding those of POC reported in previous studies by about a factor of 5. Sedimentary carbonate stocks varied across seagrass communities, with meadows dominated by Halodule, Thalassia or Cymodocea supporting the highest PIC stocks, and tended to decrease polewards at a rate of −8 ± 2 Mg PIC ha−1 per degree of latitude (general linear model, GLM; p < 0.0003). Using PIC concentrations and estimates of sediment accretion in seagrass meadows, the mean PIC accumulation rate in seagrass sediments is found to be 126.3 ± 31.05 g PIC m−2 yr−1. Based on the global extent of seagrass meadows (177 000 to 600 000 km2), these ecosystems globally store between 11 and 39 Pg of PIC in the top metre of sediment and accumulate between 22 and 75 Tg PIC yr−1, representing a significant contribution to the carbonate dynamics of coastal areas. Despite the fact that these high rates of carbonate accumulation imply CO2 emissions from precipitation, seagrass meadows are still strong CO2 sinks as demonstrated by the comparison of carbon (PIC and POC) stocks between vegetated and adjacent un-vegetated sediments.

Key biogeochemical factors affecting soil carbon storage in <i>Posidonia</i> meadows

“…The SAR in seagrass meadows is mainly controlled by the canopy structure, which affects the trapping and retention of sediment particles (Gacia and Duarte, 2001;Peralta et al, 2008;Hendriks et al, 2010), the hydrodynamic energy, the availability of fine-grained suspended particles in the water column and the production of biogenic carbonates within the meadow (De Falco et al, 2000Mazarrasa et al, 2015). High plant biomass and density is associated with greater retention of particles (in particular, fine-grained sediments), lower hydrodynamic energy and higher production of biogenic carbonates within the meadow (De Falco et al, 2000), ultimately enhancing soil accumulation.…”

“…The rates of soil accumulation, the sediment structure and the biochemical composition of the organic matter buried may strongly influence C org accumulation and preservation, and are highly variable among seagrass meadows (De Falco et al, 2000;Kennedy et al, 2010;Duarte et al, 2013). Soil accumulation may be a function of the seagrass canopy structure (De Falco et al, 2000;Gacia and Duarte, 2001;Peralta et al, 2008;Hendriks et al, 2010), the availability of suspended particles to settle outside of the water column and the production of biogenic carbonates within the meadow (De Falco et al, 2000;Mazarrasa et al, 2015). If the accumulated sediments are fine, then they are likely to enhance the preservation of C org since they tend to limit oxygen exchange and redox potentials, which reduce remineralization (e.g., Keil and Hedges, 1993).…”

Reviews and syntheses: Hidden forests, the role of vegetated coastal habitats in the ocean carbon budget

“…Calcareous algae, such as coralline and Halimeda, have been long recognized to be important contributors to carbonate formation, with estimates of net calcification by calcifying algae being on the order of 20 Tg C yr −1 for Halimeda bioherms (Milliman and Droxler, 1996). The carbonate production in seagrass meadows was recently estimated at 20 to 75 Tg C yr −1 (Mazarrasa et al, 2015). There is no information on the carbonate deposition in mangrove or salt-marsh sediments, probably due to the belief that they are unlikely to accumulate carbonate.…”

Commentary: Evaluating the Role of Seagrass in Cenozoic CO2 Variations

“…Instead, the authors treat carbonate production as resulting in net CO 2 sequestration, which they add to the CO 2 sink capacity of seagrasses, when in reality it needs to be treated as a CO 2 source. Previous studies have pointed out that although Posidonia meadows (as well as other seagrass species; Mazarrasa et al, 2015) host significant CaCO 3 stocks and accumulation rates, it has been shown that calcification represents a global CO 2 source to the atmosphere (Smith and Gattuso, 2009), and therefore seagrass meadows (all species) could represent a significant net CO 2 source Serrano et al, 2012). A global estimate for the entire Mediterranean indicates that calcification in P. oceanica meadows could be responsible for the emission of 0.7 to 4.2 Tg C year −1 to the atmosphere .…”