Seagrass meadows as a globally significant carbonate reservoir

Abstract

Abstract. There has been a 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 organic carbon (POC) stocks and accumulation rates and ignored the 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study offers the first assessment of the global PIC stocks in seagrass sediments using a synthesis of published and unpublished data on sediment carbonate concentration from 402 vegetated and 34 adjacent un-vegetated sites. PIC stocks in the top 1 m sediments ranged between 3 and 1660 Mg PIC ha-1, with an average of 654 ± 24 Mg PIC ha-1, exceeding about 5 fold those of POC reported in previous studies. 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 degree-1 of latitude (GLM, p < 0.0003). Using PIC concentration and estimates of sediment accretion in seagrass meadows, mean PIC accumulation rates in seagrass sediments is 126.3 ± 0.7 g PIC m-2 y-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 meter of sediment and accumulate between 22 and 76 Tg PIC y-1, representing a significant contribution to the carbonate dynamics of coastal areas. Despite that these high rates of carbonate accumulation imply CO2 emissions from precipitation, seagrass meadows are still strong CO2 sinks as demonstrates the comparison of carbon (POC and POC) stocks between vegetated and adjacent un-vegetated sediments.

An invasive foundation species enhances multifunctionality in a coastal ecosystem

“…Native oysters, seagrasses, and marshes typically have more stable biomass across seasons than Gracilaria, thereby providing more stable ecosystem functions. These native foundation species also have stronger legacy effects, as relic bivalves and macrophyte rhizomes can accumulate over time and thereby enhance ecological functions over longer time scales (63,64). Finally, in contrast to Gracilaria and other invasive seaweeds, native marshes, oysters, and seagrasses have not been shown to cause large-scale anoxia events and associated faunal die-offs.…”

Accumulation of Carbonates Contributes to Coastal Vegetated Ecosystems Keeping Pace With Sea Level Rise in an Arid Region (Arabian Peninsula)

“…In seagrass meadows, Mazarrasa et al () calculated a global mean C inorg stock (within the top 1 m of soil) of 654 ± 24 Mg C inorg /ha (ranging between 3 and 1,660 Mg C inorg /ha). The C inorg stocks in seagrass meadows of the Red Sea and Arabian Gulf are comparable to this global estimate, 570 ± 30 and 960 ± 75 Mg C inorg /ha, respectively (mean ± SE; Table ).…”

“…However, regarding SAR, organic carbon often only represents a small fraction of these soils (globally ~2–3% in seagrass and mangroves ecosystems; Fourqurean et al, ; Kristensen et al, ). Mineral deposits, siliciclastic and carbonate, generally account for the majority of the accreted substrate, contributing about 97–98% of the dry weight of seagrass and mangrove soils (Kristensen et al, ; Mazarrasa et al, ), and up to 50% in tidal marshes (Drake et al, ; Macreadie et al, ).…”

“…The accreted carbonate can originate from external sources: erosion of limestone and/or nearby habitats composed of carbonates (reefs and bioherms), and from autochthonous production by calcifying organisms inhabiting nearshore ecosystems (e.g., macroalgae, oysters, crabs, snails, and foraminifers; Por & Dor, ; Price et al, , ; Sournia, ). Recently, Mazarrasa et al () estimated that the inorganic carbon (C inorg , 12% of the calcium carbonate, CaCO 3 , weight) burial rate in seagrass beds may reach 22 to 75 Tg C inorg /year, mostly occurring in the intertropical areas (1.5% to 5% of the biogenic carbonate production of the global coastal ocean). These are regions of intense biomineralization by corals and algae, and in desert and semiarid climate regions, where terrigenous inputs are limited by the lack of surface runoffs, weathering of reefs and bioherms could be the principal source of sediment to the nearshore area.…”

Seagrass (Posidonia oceanica) seedlings in a high-CO2 world: from physiology to herbivory

“…These photosynthetic organisms form the basis of diverse coastal food webs having important ecological functions and offering crucial services for humans1. As seagrasses are highly productive, they act as a carbon sink234 and, as ecosystem engineers, they provide substratum for epiphytic plants and animals to settle5 and support coastal and offshore fisheries6. However, seagrasses are presently suffering a global loss associated to numerous human activities78.…”