Oyster reefs as carbon sources and sinks

Abstract

Carbon burial is increasingly valued as a service provided by threatened vegetated coastal habitats. Similarly, shellfish reefs contain significant pools of carbon and are globally endangered, yet considerable uncertainty remains regarding shellfish reefs' role as sources (+) or sinks (-) of atmospheric CO While CO release is a by-product of carbonate shell production (then burial), shellfish also facilitate atmospheric-CO drawdown via filtration and rapid biodeposition of carbon-fixing primary producers. We provide a framework to account for the dual burial of inorganic and organic carbon, and demonstrate that decade-old experimental reefs on intertidal sandflats were net sources of CO (7.1 ± 1.2 MgC ha yr (µ ± s.e.)) resulting from predominantly carbonate deposition, whereas shallow subtidal reefs (-1.0 ± 0.4 MgC ha yr) and saltmarsh-fringing reefs (-1.3 ± 0.4 MgC ha yr) were dominated by organic-carbon-rich sediments and functioned as net carbon sinks (on par with vegetated coastal habitats). These landscape-level differences reflect gradients in shellfish growth, survivorship and shell bioerosion. Notably, down-core carbon concentrations in 100- to 4000-year-old reefs mirrored experimental-reef data, suggesting our results are relevant over centennial to millennial scales, although we note that these natural reefs appeared to function as slight carbon sources (0.5 ± 0.3 MgC ha yr). Globally, the historical mining of the top metre of shellfish reefs may have reintroduced more than 400 000 000 Mg of organic carbon into estuaries. Importantly, reef formation and destruction do not have reciprocal, counterbalancing impacts on atmospheric CO since excavated organic material may be remineralized while shell may experience continued preservation through reburial. Thus, protection of existing reefs could be considered as one component of climate mitigation programmes focused on the coastal zone.

The role of microphytobenthos in soft‐sediment ecological networks and their contribution to the delivery of multiple ecosystem services

“…It provides new habitat for other species (Gutiérrez et al, ), and can mitigate against pollutants (Casado‐Coy, Martinez‐Garcia, Sanchez‐Jerez, & Sanz‐Lazaro, ). These benefits support incentives to conserve and restore shellfish beds as potential carbon sinks (Fodrie et al, ). The transfer of carbon offshore via the migration of juvenile fish and invertebrates (Dahlgren et al, ; Vasconcelos, Reis‐santos, Costa, & Cabral, ) and through the metazoan food web is also critical for the survival of many fish and bird species.…”

“…Up to 1.83 kg of CO 2 can be fixed for 1 kg of microalgae biomass (Chisti, ) with much of this energy channelled into metazoan and microbial food webs (Maher & Eyre, ; Moerdijk‐poortvliet et al, ). While the high productivity of these ecosystems often results in fixed carbon being consumed and respired back to the atmosphere, for every 0.6 mol of CO 2 respired as much as 1 mol of carbon is bound in the shell of resident bivalves (Fodrie et al, ).…”

A global review of the ecosystem services provided by bivalve aquaculture

“…Therefore, the calcification process is considered by some to be a source of atmospheric CO 2 (Fodrie et al . ). Other authors argue that the C stored in shell represents a long‐term sink.…”

“…During the calcification process carbon dioxide is formed (Ca 2þ þ 2HCO À 3 ðaqÞ CaCO 3 þ H 2 O þ CO 2 ), so potentially leading to an increase in pCO 2 in surface waters and evasion of CO 2 to the atmosphereespecially in the shallow well-mixed coastal waters where shellfish are typically farmed. Therefore, the calcification process is considered by some to be a source of atmospheric CO 2 (Fodrie et al 2017). Other authors argue that the C stored in shell represents a long-term sink.…”

effects of invasive crabs on benthic community structure and dynamics in the Gulf of Maine

“…In marine and terrestrial systems, foundation species are recognized for their disproportionately large role in facilitating associated faunal communities by creating complex habitats that ameliorate physical stress and provide refuge from predators and competitors (Dayton 1972, Angelini et al 2011. In marine systems, biogenic habitats such as seagrass beds, salt marshes, kelp beds, and coral and bivalve reefs provide valuable ecosystem services such as sediment stabilization (Fonseca 1996, Wells and Ravilious 2006, Gedan et al 2011, provision of nursery habitat to nekton (Beck et al 2001, Heck et al 2003, Minello et al 2003), sequestration of carbon and nitrogen (Breaux et al 1995, Herbert 1999, Mcleod et al 2011, Smyth et al 2013, Fodrie et al 2017) and water filtration (Ermgassen et al 2012). Centuries of anthropogenic impacts on coastal and estuarine ecosystems have resulting in these systems being among the most degraded worldwide , Worm et al 2006).…”