Abstract
The Celtic Sea is a productive temperate sea located on the Northwest European Shelf. It is an important pathway for the delivery of land-derived material to the North Atlantic Ocean, including dissolved organic matter (DOM). The aim of this study was to determine the seasonal and spatial variability in the magnitude, source and composition of DOM at three sites representing on shelf, central shelf and shelf edge regions in the Celtic Sea, using observations collected during the UK Shelf Sea Biogeochemistry (SSB) research programme (November 2014 – August 2015). The concentration of dissolved organic carbon (DOC) alongside DOM absorbance and fluorescence indices were measured and fluorescence Excitation and Emission Matrices (EEMs) combined with Parallel Factor Analysis (PARAFAC) were used to assess DOM composition and lability. The PARAFAC model identified four unique fluorescent components for autumn (November 2014), winter (March 2015), spring (April 2015) and summer (July 2015) consisting of two humic-like components attributed to terrestrial (C1) and marine sources (C2), and two protein components identified as tyrosine-like (C3) and tryptophan-like (C4) attributed to in situ production. DOC varied seasonally and there were strong cross shelf trends. The protein components (C3 and C4) exhibited large seasonal and within season variability particularly during productive periods. In contrast, there were persistent cross shelf gradients in the CDOM absorption coefficient at 305 nm (a305), the UV specific absorbance at 280 nm (SUVA280), the humification index (HIX), and the humic-like fluorescent components (C1 and C2), which were higher in the on shelf region and decreased towards the shelf edge. The humic-like components and the slope ratio (SR) were significantly correlated with salinity throughout all seasons, indicating a strong influence of terrestrially-derived organic matter in the Celtic Sea, with potentially up to 35% of DOC in the central shelf during winter originating from terrestrial inputs. Results from this study illustrate the importance of monitoring DOM quantitatively and qualitatively for a better understanding of the supply, production, cycling and export of this dynamic organic carbon pool in shelf seas.
Highlights
Dissolved organic matter (DOM) is the largest pool of organic material in the ocean, storing up to fifty times more carbon (C) than that stored in the particulate pool (POC 18 ± 5 × 1015 g C (Eglinton and Repeta, 2006)
This study extends the application of Excitation and Emission Matrices (EEMs) and Parallel Factor Analysis (PARAFAC) modelling, a technique commonly employed across a wide range of aquatic and marine environments (Stedmon et al, 2003, Yamashita and Tanoue, 2003, Yamashita et al, 2011) to the seasonally stratified, temperate Celtic Sea region in the Northwest European Shelf
This study demonstrated the strength of a multi-dimensional approach incorporating DOM optical properties alongside measurements of dissolved organic carbon (DOC) in delineating the complexities of the DOM pool in shelf seas
Summary
Dissolved organic matter (DOM) is the largest pool of organic material in the ocean, storing up to fifty times more carbon (C) than that stored in the particulate pool (POC 18 ± 5 × 1015 g C (Eglinton and Repeta, 2006). The amount of dissolved organic carbon (DOC) in the ocean (685 × 1015 g C) is comparable to the amount of carbon as CO2 in the atmosphere (Hansell and Carlson, 1998, Hansell et al, 2009)). DOM is produced autochthonously by plankton in the surface ocean during primary and secondary production (Hansell and Carlson, 2001, Hansell et al, 2009), with substantial amounts being released or exuded by phytoplankton (Hygum et al, 1997, Jiao et al, 2010). Viral cell lysis (Suttle, 2005, Suttle, 2007) and bacteria (Jiao et al, 2010) cause DOM release from particulate organic matter (POM). Syntheses of past and recent global estimates show that rivers, as part of the landocean continuum, contribute around 0.25 × 1015 g C yr−1 to the global ocean as DOC (Hedges et al, 1997, Cai, 2011, Raymond and Spencer, 2015)
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