High Seasonal Variability in Sediment Carbon Stocks of Cold‐Temperate Seagrass Meadows

Martin Dahl,Diana Deyanova,Maria E Asplund,Martin Gullström,Diana Perry,João N Franco,Alan Koliji,Mats Björk,Eduardo Infantes

doi:10.1029/2019jg005430

Abstract

AbstractSeagrass meadows have a high ability to capture and store atmospheric CO2 in the plant biomass and underlying sediment and thereby function as efficient carbon sinks. The seagrass Zostera marina is a common species in the temperate Northern Hemisphere, a region with strong seasonal variations in climate. How seasonality affects carbon storage capacity in seagrass meadows is largely unknown, and therefore, in this study, we aimed to assess variations in sedimentary total organic carbon (TOC) content over a 1‐year cycle in seagrass meadows on the Swedish west coast. The TOC was measured in two Z. marina sites, one wave exposed and one sheltered, and at two depths (1.5 and 4 m) within each site, every second month from August 2015 to June 2016. We found a strong seasonal variation in carbon density, with a peak in early summer (June), and that the TOC was negatively correlated to the net community production of the meadows, presumably related to organic matter degradation. There was seasonal variation in TOC content at all sediment sections, indicating that the carbon content down to 30 cm is unstable on a seasonal scale and therefore likely not a long‐term carbon sink. The yearly mean carbon stocks were substantially higher in the sheltered meadow (3,965 and 3,465 g m−2) compared to the exposed one (2,712 and 1,054 g m−2) with similar seasonal variation. Due to the large intra‐annual variability in TOC content, seasonal variation should be considered in carbon stock assessments and management for cold‐temperate seagrass meadows.

Highlights

Seagrass meadows have the ability to accrete thick sediment layers and efficiently accumulate belowground carbon deposits (Fourqurean et al, 2012; Mcleod et al, 2011), but not all seagrass areas can be considered carbon sinks (Belshe et al, 2018; Dahl, 2017; Enríquez et al, 2019; Morse et al, 1987)
Seasonal Variation in Sedimentary total organic carbon (TOC) Content When comparing mean TOC densities (g cm−3), based on pooled sediment core depths (0 to 37.5 cm from the sediment surface) over the year, we found an increase from winter to summer with June having significantly higher levels compared to all other months and a TOC density in October that was higher than in August (Tukey's honestly significant difference, p < 0.05; Figure 2)
This study highlights the within‐year dynamics of sedimentary carbon content and the influence of seasonality on carbon stock levels in cold‐temperate seagrass meadows

Summary

Introduction

Seagrass meadows have the ability to accrete thick sediment layers and efficiently accumulate belowground carbon deposits (Fourqurean et al, 2012; Mcleod et al, 2011), but not all seagrass areas can be considered carbon sinks (Belshe et al, 2018; Dahl, 2017; Enríquez et al, 2019; Morse et al, 1987). There is, a large variation in carbon storage efficiency among Z. marina habitats (Dahl, 2017; Green et al, 2018; Kindeberg et al, 2018; Röhr et al, 2016, 2018), which is mainly related to the location of the meadow, where water depth, salinity, and exposure to hydrodynamic forces are regulating the sedimentary carbon accumulation (Dahl, Deyanova, Gütschow, et al, 2016; Prentice et al, 2019; Röhr et al, 2018). In the cold‐temperate zone of the Swedish west coast, there is a strong seasonality with water temperatures ranging from icy conditions during winter to mild summers with up to 25 °C in the water

Objectives

Methods

Results

Discussion

Conclusion