Carbon accumulation and storage in a temperate coastal lagoon under the influence of recent climate change (Northwestern Adriatic Sea)

Abstract

Carbon captured and stored in sediments and soils from vegetated tidal wetlands (mangroves, saltmarshes, freshwater and brackish marshes), where the rates of organic carbon accumulation (OC) from multiple sources is high, constitutes an active fraction of the global carbon sink (Wang et al., 2021). However, a global inventory of this coastal ‘blue carbon’ remains a challenge, as observations of accumulation rates and stock in vegetated tidal wetlands are labor intensive, expensive, scarce, and unevenly distributed, with few sediment records even for relatively well-studied temperate areas in the Northern Hemisphere (Beaumont et al., 2014). Recent reviews (Duarte et al., 2005, Wilkinson et al., 2018) report a mean carbon accumulation rate of 151 g C m yr for saltmarshes (maximum 1720 g C m yr), 41.4 g C m yr for lagoons (maximum 340 g C m yr), and 62.9 g C m yr for coastal wetlands (maximum 335.8 g C m yr) exceeding the mean burial rate of estuaries and continental shelves (17–45 g C m yr

).

The accumulation of ‘blue carbon’ stored in soil and sediments within tidal wetlands, is sensitive to rapidly changing climate factors (e.g. temperature, rainfall, sea level rise, and inundation frequency), and non-climatic anthropogenic drivers (e.g. subsidence from groundwater extraction, reduction of sediment supply due to river damming, and land use change) (Pendleton et al., 2012, Macreadie et al., 2013, Arriola, 2017, Kelleway et al., 2017, Simpson et al., 2017, Ewers Lewis et al., 2018, Ruiz-Fernández et al., 2018, Cuellar-martinez et al., 2019, Macreadie and Saintilan, 2019, Negandhi et al., 2019, Rogers et al., 2019).

Fast rates of relative sea level rise (RSLR) and low sediment supply are the main drivers of vertical drowning in tidal wetlands (Mariotti and Carr, 2014, Fagherazzi et al., 2020). A global review suggests that between 60 and 91% of saltmarshes will be drawing under the IPPC predicted rates of sea-level rise (Crosby et al., 2016).

Carbon stable isotopic composition ( C) and C/N analysis have been used as tracers to distinguish between OC derived from autochthonous C3 and C4 saltmarsh vascular vegetation (i.e. coastal blue carbon; C 12‰ to −30‰, C/N 5.80 to 41.10; Khan et al., 2015b), and allochthonous sources including fluvial and marine particulate organic matter (POM) derived from freshwater or marine phytoplankton (C

12‰ to −30‰, C/N 5 to 9; Lamb et al., 2006), as well as past sea level indicators in coastal vegetated​ habitats in North West Europe (Wilson, 2017).

Data on the spatial and historical changes of OC sources and accumulation coupled with long-term time series of climatic factors are limited for vegetated tidal wetlands in the Mediterranean, which make it hard to assess the response of OC accumulation to relative sea-level rise (RSLR) in this region. Sea level observations from satellite altimetry showed an increase in absolute sea level of 2.6 ± 0.28 mm yr

across the Mediterranean Sea during the period 1993–2015, and low-lying coastal areas will be prone to marine flooding according to projections for the 21st century (Moatti and Thiébault, 2016).

To better understand the spatial and temporal changes in OC accumulation and sources and assess the influence of SST and RSLR, we measured the stable isotopic composition ( C) and accretion rates in sediment records, applied the MixSIAR model to estimate the OC sources, and analyzed Sea Surface Temperature (SST) and Sea Level (SL) climatic data sets in two different tidal wetland habitats: (i) an impacted habitat affected by strong landscape and anthropogenic alterations, and (ii) an undegraded saltmarsh habitat, within a coastal lagoon (Pialassa Baiona) located in the northwestern Adriatic Sea (Mediterranean Sea).