Blue Carbon Stocks and Cross-Habitat Subsidies

Richard Hugh Bulmer,Jenny R Hillman,Fabrice Stephenson,Michael Townsend,Carolyn J Lundquist,Hannah F E Jones,Luitgard Schwendenmann

doi:10.3389/fmars.2020.00380

Abstract

Blue carbon ecosystems (including saltmarsh, mangrove, seagrass meadows, and other soft sediment habitats) play a valuable role in aquatic carbon dynamics and contribute significantly to global climate change mitigation. However, these habitats are undergoing rapid and accelerating shifts in extent due to climate change and anthropogenic impactsstressors. Here, we demonstrate that blue carbon stocks vary across habitats and that cross-habitat subsidies of carbon contribute significantly to blue carbon stocks. Using a case study estuary from New Zealand, organic carbon stocks in above ground biomass and sediment to 100 cm varied significantly between habitat types, from saltmarsh (90 t ha-1), to mangrove (46 t ha-1), to seagrass (27 t ha-1) and unvegetated habitats (26 t ha-1). Despite being typically overlooked in blue carbon literature, unvegetated habitats contained the majority of estuarine carbon stocks when adjusted for their large extent within the estuary (occupying 68.4% of the estuarine area and containing 57% of carbon stocks). When carbon stocks were further refined based on δ13C and δ15N mixing model results, coastal vegetation (saltmarsh, mangrove, seagrass) was found to provide important cross-habitat subsidies exchanges of carbon throughout the estuary, including contributing an estimated 41% of organic carbon within unvegetated sediments, and 51% of the total carbon stock throughout the estuary (yet occupying only 31.6% of the estuary). Given the connected nature of blue carbon ecosystems these findings illustrate the importance of considering the contribution and cross habitat subsidies of both vegetated and unvegetated habitats to blue carbon stocks in estuaries. This provides critical context when assessing the impact of shifts in habitat distributions due to impacts from climate change and anthropogenic stressors.

Highlights

Increasing greenhouse gas emissions and associated impacts on global warming (IPCC, 2013) have led to an urgent need to identify and protect ecosystems with high carbon stock capacity (Canadell and Raupach, 2008)
The combined organic carbon (OC) stocks within above ground biomass and sediment to 100 cm were highest in saltmarsh (90 t ha−1), followed by mangrove (46 t ha−1), seagrass (27 t ha−1), and unvegetated habitats (26 t ha−1) (Figure 2)
Our study demonstrates that blue carbon stocks vary among estuarine habitats and that cross-habitat subsidies of carbon contribute significantly to estuarine carbon dynamics

Summary

Introduction

Increasing greenhouse gas emissions and associated impacts on global warming (IPCC, 2013) have led to an urgent need to identify and protect ecosystems with high carbon stock capacity (Canadell and Raupach, 2008). Despite the value of blue carbon ecosystems, they are undergoing rapid and accelerating shifts in extent due anthropogenic stressors (such as resource extraction, dredging, reclamation, Blue Carbon Stocks and Subsidies declamation, catchment management influencing downstream inputs of sediments, nutrients, and other pollutants) and climate change (Waycott et al, 2009; Polidoro et al, 2010; Deegan et al, 2012) These shifts in habitat extent have potentially negative consequences for ecological and environmental health (Donato et al, 2011; McLeod et al, 2011; Fourqurean et al, 2012; Pendleton et al, 2012; Bauer et al, 2013). This is important as organic matter from one habitat is not contained within the habitat of origin, and instead may contribute to multiple habitats (Canuel and Hardison, 2016; Chen et al, 2017)

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