Spatial and Temporal Patterns of Pore Water Chemistry in the Inter-Tidal Zone of a High Energy Beach

Hannelore Waska,Claudia Ehlert ,Katharina Pahnke ,Daniela Meier ,Heike Simon ,Nele Grünenbaum ,Janis Ahrens ,Janek Greskowiak ,Hans‐Jürgen Brumsack ,Mélanie Beck ,Oliver Zielinski ,Kai Schwalfenberg ,Julius Degenhardt ,Bernhard Schnetger ,Philipp Böning ,Hannah K Marchant ,Moritz Holtappels ,Bert Engelen ,Verona Vandieken ,Soeren Ahmerkamp ,Gudrun Massmann ,Thorsten Dittmar

doi:10.3389/fmars.2019.00154

Abstract

Submarine groundwater discharge (SGD) is a ubiquitous source of meteoric fresh groundwater and recirculating seawater to the coastal ocean. Due to the hidden distribution of SGD, as well as the hydraulic- and stratigraphy-driven spatial and temporal heterogeneities, one of the biggest challenges to date is the correct assessment of SGD-driven constituent fluxes. Here, we present results from a 3-dimensional seasonal sampling campaign of a shallow subterranean estuary in a high-energy, meso-tidal beach, Spiekeroog Island, Northern Germany. We determined beach topography and analyzed physico-chemical and biogeochemical parameters such as salinity, temperature, dissolved oxygen, Fe(II) and dissolved organic matter fluorescence (FDOM). Overall, the highest gradients in pore water chemistry were found in the cross-shore direction. In particular, a strong physico-chemical differentiation between the tidal high water and low water line was found and reflected relatively stable in- and exfiltrating conditions in these areas. Contrastingly, in between, the pore water compositions in the existing foreshore ridge and runnel system were very heterogeneous on a spatial and temporal scale. The reasons for this observation may be the strong morphological changes that occur throughout the entire year, which affect the exact locations and heights of the ridge and runnel structures and associated flow paths. Further, seasonal changes in temperature and inland hydraulic head, and the associated effect on microbial mediated redox reactions likely overprint these patterns. In the long-shore direction the pore water chemistry varied less than the along the cross-shore direction. Variation in long-shore direction was probably occurring due to topography changes of the ridge-runnel structure and a physical heterogeneity of the sediment, which produced non-uniform groundwater flow conditions. We conclude that on meso-tidal high energy beaches, the rapidly changing beach morphology produces zones with different approximations to steady-state conditions. Therefore, we suggest that zone-specific endmember sampling is the optimal strategy to reduce uncertainties of SGD-driven constituent fluxes.

Highlights

Submarine groundwater discharge (SGD) is a substantial input of dissolved nutrients, trace elements, as well as organic and inorganic carbon to the global ocean, which may even exceed the worlds’ rivers as a source of dissolved inorganic nitrogen, phosphorous, and silicon (DIN, DIP, and dissolved silicate (DSi), Cho et al, 2018)
The coefficients of variation for beach heights were calculated to be 21, 15, and 24% in October, March, and August, respectively. Both beach gradient and beach topography heterogeneity increased in the three campaigns in the order March < October < August
Low-salinity (

Summary

Introduction

Submarine groundwater discharge (SGD) is a substantial input of dissolved nutrients, trace elements, as well as organic and inorganic carbon to the global ocean, which may even exceed the worlds’ rivers as a source of dissolved inorganic nitrogen, phosphorous, and silicon (DIN, DIP, and DSi, Cho et al, 2018). SGD is any and all advective flux across the sediment-water interface toward the ocean, regardless of its origin or driving force. It commonly encompasses the advective flow of terrestrial, fresh groundwater, as well as recirculating seawater, through porous geological structures in the coastal zone (Burnett et al, 2003). The underground mixing zone between fresh groundwater and recirculating seawater, from which SGD originates, was termed the “subterranean estuary” (STE), as an analog to surface estuaries (Moore, 1999). STEs generally have longer water residence times ranging from hours to years, limited oxygen supply in deeper layers, no exposure to sunlight, and more intensive interactions between the water and the solid phase (i.e., porous rocks or sandy sediments). STEs are highly active bioreactors (Anschutz et al, 2009; Seidel et al, 2015), characterized by a salinity gradient and a complex redox zonation (McAllister et al, 2015)

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Conclusion