A storm in a shelf sea: Variation in phosphorus distribution and organic matter stoichiometry

Clare E. Davis,Claire Mahaffey,George A. Wolff,Jonathan Sharples

doi:10.1002/2014gl061949

Clare E. Davis, Claire Mahaffey + Show 2 more

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https://doi.org/10.1002/2014gl061949

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Abstract

AbstractOrganic matter (OM) plays an important role in productive shelf seas and their contribution to global carbon (C) and nutrient cycles. We investigated the impact of storm mixing on OM dynamics in the seasonally stratified Celtic Sea. After the storm, OM production was decoupled from consumption in the euphotic layer. Over the 15 day study, dissolved OM (DOM) became phosphorus (P) rich relative to C, whereas particulate OM (POM) became P‐deplete relative to C. Upward diapycnal phosphate fluxes were accompanied by reciprocal downward mixing of dissolved organic P (DOP) and particulate P (PPhos). Transfer of DOP and PPhos below the thermocline accounts for 22% and 26%, respectively, of the upward phosphate flux. Given the changes in stoichiometry of POM and DOM after the storm, the form in which OM is transferred below the thermocline has important implications for the efficiency of elemental transfer, impacting C cycling and storage in the ocean.

Highlights

Shelf seas represent less than 10% by area of the global ocean, yet are responsible for 15 to 30% of its primary production and up to 50% of the export production [Wollast, 1998]
This is reflected in the chl a fluorescence distributions, which show entrainment and mixing through the euphotic layer during the earlier period of strong mixing and later show that the subsurface chlorophyll maxima (SCM) developed associated with the base of the thermocline (Figure 2c)
During the sampling period the average dissolved inorganic nitrogen (DIN):dissolved inorganic phosphate (DIP) ratio was ~13:1 in the bottom mixed layer, which was lower than the Redfield ratio of 16:1, [Redfield et al, 1963] implying that productivity in this region is limited by the availability of N

Summary

Introduction

Shelf seas represent less than 10% by area of the global ocean, yet are responsible for 15 to 30% of its primary production and up to 50% of the export production [Wollast, 1998]. The annual cycle of primary production in temperate shelf seas is dominated by a relatively short spring bloom and weaker but more sustained subsurface chlorophyll maxima (SCM) that develops within the base of the seasonal thermocline during summer stratification. These draw down approximately equal amounts of carbon (C) from the atmosphere [Hickman et al, 2012]. The SCM is sustained by a diapycnal nutrient flux across the base of the thermocline, which is sensitive to tidal- [Sharples et al, 2007] and wind- [Williams et al, 2013b] driven mixing processes. Recent field studies have highlighted the importance of storms or wind events in enhancing these diapycnal nutrient fluxes [e.g., Tweddle et al, 2013] and sustaining summertime productivity [Williams et al, 2013b]

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