Organic Matter Composition of Biomineral Flocs and Its Influence on Suspended Particulate Matter Dynamics Along a Nearshore to Offshore Transect

Michael Fettweis,Rolf Riethmüller,Markus Schartau,Byung Joon Lee,Dimitry Van Der Zande,Koen Parmentier,Nathan Terseleer,Xavier Desmit

doi:10.1029/2021jg006332

Abstract

AbstractThe seasonal variation in concentration of transparent exopolymer particles (TEPs), particulate organic carbon (POC) and particulate organic nitrogen (PON) were investigated together with floc size and the concentration of suspended particulate matter (SPM) along the cross‐shore gradient, from the high turbid nearshore toward the low‐turbid offshore waters in the Southern Bight of the North Sea. Our data demonstrate that biophysical flocculation cannot be explained by these heterogeneous parameters, but requires a distinction between a more reactive labile (“fresh”) and a less reactive refractory (“mineral‐associated”) fraction. Based on all data, we separated the labile and mineral‐associated POC, PON, and TEP using a semi‐empirical model approach. The model's estimates of fresh and mineral‐associated organic matter (OM) show that great parts of the POC, PON, and TEP are associated with suspended minerals, which are present in the water column throughout the year, whereas the occurrence of fresh TEP, POC, and PON is restricted to spring and summer months. In spite of a constantly high abundance of total TEP throughout the entire year, it is its fresh fraction that promotes the formation of larger and faster sinking biomineral flocs, thereby contributing to reducing the SPM concentration in the water column over spring and summer. Our results show that the different components of the SPM, such as minerals, extracellular OM and living organisms, form an integrated dynamic system with direct interactions and feedback controls.

Highlights

The particulate organic matter (POM) is, together with particulate inorganic matter (PIM), the main component of the suspended particulate matter (SPM) in marine ecosystems
In the turbid nearshore areas where SPM concentration often exceeds 100 mg/l the OM fraction in association with the mineral particles is dominant, masking the smaller signal of fresh OM produced by primary production
In the offshore stations the fresh OM components prevail and explain most of the variability seen in the SPM concentration

Summary

Introduction

The particulate organic matter (POM) is, together with particulate inorganic matter (PIM), the main component of the suspended particulate matter (SPM) in marine ecosystems. The flux and export of mineral and organic particles depends on flocculation processes, which influence the particles' composition and settling velocity. Flocculation combines organic and mineral particles into biomineral flocs J. Lee et al, 2019; Shen et al, 2018; Skinnebach et al, 2020; Verney et al, 2009), which overcompensates the resulting decrease in overall density and yields higher settling velocities compared to most of their constituents (e.g., Bach et al, 2019; Engel & Schartau, 1999; Fall et al, 2021; Manning & Bass, 2006; Manning et al, 2011; Neumann et al, 2019). Mostly shallow environments, floc break-up dominates during periods of high currents and FETTWEIS ET AL

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