The influence of mussel restoration on coastal carbon cycling

Abstract

Increasing responsiveness to anthropogenic climate change and the loss of global shellfish ecosystems has heightened interest in the carbon storage and sequestration potential of bivalve-dominated systems. While coastal ecosystems are dynamic zones of carbon transformation and change, current uncertainties and notable heterogeneity in the benthic environment make it difficult to ascertain the climate change mitigation capacity of ongoing coastal restoration projects aimed at revitalizing benthic bivalve populations. In this study we sought to distinguish between direct and indirect effects of subtidal green-lipped mussels (Perna canaliculus) on carbon cycling, and combined published literature with in-situ experiments from restored beds to create a carbon budget for New Zealand's shellfish restoration efforts. A direct summation of biogenic calcification, community respiration, and sediment processes suggests a moderate carbon efflux (+100.1 to 179.6 g C m^-2 year^-1 ) occurs as a result of recent restoration efforts, largely reflective of the heterotrophic nature of bivalves. However, an examination of indirect effects of restoration on benthic community metabolism and sediment dynamics suggests that beds achieve greater carbon fixation rates and support enhanced carbon burial compared to nearby sediments devoid of mussels. We discuss limitations to our first-order approximation and postulate how the significance of mussel restoration to carbon-related outcomes likely increases over longer timescales. Coastal restoration is often conducted to support the provisioning of many ecosystem services, and we propose here that shellfish restoration not be used as a single measure to offset carbon dioxide emissions, but rather used in tandem with other initiatives to recover a bundle of valued ecosystem services.

Keywords: benthic fluxes; benthic-pelagic coupling; bivalves; blue carbon; carbon; carbon budget; restoration; soft-sediments; subtidal.

FIGURE 1

Location of mussel restoration sites in Mahurangi Harbour and Kawau Bay, New Zealand. Site labels: MM, Mahurangi Mid; MBN, Martins Bay North; MBS, Martins Bay South; MR, Motuora; MK, Motoketekete; NB, Ngaio Bay; NLB, New Lagoon Bay; LB, Lagoon Bay; OT, Otarawao Bay; PP, Pukapuka.

FIGURE 2

Infographic illustrating carbon cycling and transformations in restoration sites comprised of adult, green‐lipped mussels (size 95–115 mm). Negative values (−) are indicative of processes that promote carbon storage, drawdown, or fixation while positive values (+) denote carbon effluxes. Information is conceptually divided into three parts: (a) calcium carbonate production, (b) ecosystem metabolism, and (c) sediment consumption/storage. Note that part (c) has no associated total net flux, as biodeposit remineralization processes are mechanistically linked to experimental methods utilized in part (b) of the budget.

FIGURE 3

Results of benthic chamber experiments in nine restored mussel beds, comparing average benthic oxygen consumption inside and outside (>5 m away) of mussel beds, under light and dark conditions (grey and black bars, respectively). For visual reference, white bars show the calculated net oxygen flux (difference between light and dark chambers). Diagonal bars inside beds represent sediment oxygen demand after removing the direct effect of mussel respiration on measured O₂ fluxes from Hillman et al. (2021) (oxygen demand of 15 mussels determined using regression equation in Figure S3).

FIGURE 4

Calculated carbon degradation rates at various sediment depths using rapid organic matter assay (ROMA) plates in (a). subtidal restored mussel beds, and (b). control sediments devoid of mussels (~5 m away from restoration sites). Site labels: MM, Mahu Mid; MK, Motoketekete; NB, Ngaio Bay; OT, Otarawao Bay; PP, Pukapuka.

FIGURE 5

Total carbon from sediment cores, separated by depth at mussel restoration sites (solid lines) and nearby sediments (>5 m away) devoid of mussels (dashed lines). Site labels: MR, Motuora; NLB, New Lagoon Bay; LB, Lagoon Bay; PP, Pukapuka.