Seasonal organic matter dynamics in a temperate shelf sea

Clare E Davis,Sabena Blackbird,George Wolff,Malcolm Woodward,Claire Mahaffey

doi:10.1016/j.pocean.2018.02.021

Clare E Davis, Sabena Blackbird + Show 3 more

Open Access

https://doi.org/10.1016/j.pocean.2018.02.021

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Abstract

Organic matter (OM) plays an important role in productive shelf seas and their contribution to global carbon (C) and nutrient cycles. We investigated dissolved and particulate OM (DOM and POM, respectively) dynamics over a seasonal cycle in the Celtic Sea. The quantity of OC was largest during the spring bloom and lowest in autumn. DOM was always C rich relative to the POM pool and the Redfield ratio (106C:16N:P). There was clear decoupling between C, N and P and the response of OM composition to different seasons and nutrient statuses of the microbial community. The C:P stoichiometry was much more variable than the C:N stoichiometry, which was near constant. Downward OC fluxes were dominated by POM during bloom events and DOM during the stratified summer. In terms of partitioning, 92–96% of OC was in the DOM pool throughout sampling, which given its high C:N (12.4–17) suggests it was an efficient vehicle for potential off-shelf export of C during winter mixing.

Highlights

Organic matter (OM) is a broad term applied to a large group of carbon containing molecules that are produced throughout the food web
We found decoupling of the dissolved and particulate organic pools as well as deviations in OM stoichiometry at key times of the year, results will be presented focussing on these features while addressing the following specific research questions: (a) how does the distribution and magnitude of organic C, N and P pools vary across a seasonal cycle in the Celtic Sea? (b) How does variable elemental stoichiometry in the Celtic Sea influence the continental shelf pump (CSP)? (c) How does the role of sinking Particulate organic matter (POM) compare to that of Dissolved organic matter (DOM) in the CSP?
Chl a concentrations were evenly dispersed throughout the surface mixed layer (SML) in autumn (∼1 mg m−3), and were elevated relative to those observed during the summer cruise (Fig. 2) indicative of an autumn bloom

Summary

Introduction

Organic matter (OM) is a broad term applied to a large group of carbon containing molecules that are produced throughout the food web. It is operationally defined as either dissolved (DOM, i.e. that passing through a 0.7 μm filter) or particulate (POM, i.e. that retained on the filter). OM constitutes the largest bioactive C reservoir on Earth (Kepkay, 1994) This reservoir is dominated by the DOM pool, which has a typical C:N:P ratio of 300:25:1 and is an efficient potential export vehicle of organic C from surface water to the deep ocean (Hopkinson et al, 1997; Hopkinson and Vallino, 2005). Marine OM dynamics have a strong influence on both global nutrient cycles and atmospheric carbon dioxide

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