The drivers of biogeochemistry in beach ecosystems: A cross-shore transect from the dunes to the low-water line

Abstract

This study addresses key processes in high-energy beach systems using an interdisciplinary approach. We assess spatial variations in subsurface pore water residence times, salinity, organic matter (OM) availability, and redox conditions and their effects on nutrient cycles as well as on microbial community patterns and microphytobenthos growth. At the study site on Spiekeroog Island, southern North Sea, beach hydrology is characterized by the classical zonation with an upper saline plume (USP), a saltwater wedge, and a freshwater discharge tube in between. Sediment and pore water samples were taken along a cross-shore transect from the dunes to the low-water line reaching sediment depths down to 5 m below sediment surface. Spatial variations in pore water residence time, salinity, and organic matter availability lead to steep redox and nutrient gradients. Vertical and horizontal differences in the microbial community indicate the influence of these gradients and salinity on the community structure. shoreline. We propose that this nutrient efflux triggers growth of microphytobenthos on sediment surfaces of the discharge zone. A first comparison of nutrient discharge rates of the beach site with a nearby sandy backbarrier tidal flat margin indicates that the beach system might be of less importance in supplying recycled nutrients to nearshore waters than the backbarrier tidal flat area.

Seasonality of Organic Matter Degradation Regulates Nutrient and Metal Net Fluxes in a High Energy Sandy Beach

“…The western part of the island is underlain by a freshwater lens (Figure b), which is confined by clay layer at about 40 m below NHN (standard elevation zero) and exhibits water ages up to 51 years (Röper et al, ; Seibert et al, ). Due to varying flow paths, fresh groundwater of various ages is delivered to the intertidal subsurface (Beck et al, ).…”

“…The present study focuses on redox constituents in sediments influenced by mesotidal conditions at a high energy beach on Spiekeroog Island, Germany, located in the southern North Sea. Previous work at this site has indicated that the whole span of organic matter degradation pathways occurs, from aerobic respiration to sulfate reduction (Beck et al, ; Reckhardt et al, ; Waska et al, ).…”

“…Further, intense Fe and Mn cycling has been observed in intertidal sediments (Beck et al, ; McAllister et al, ; O'Connor et al, ; Reckhardt et al, ; Roy et al, ). Pore waters can be enriched in these dissolved metals by orders of magnitude compared to seawater and thus form a potential source for coastal waters.…”

“…So far, the impact of pore water advection on biogeochemistry and pore water constituent effluxes from permeable sediments has mostly been studied in microtidal sheltered beaches (Beck et al, ; Gonneea & Charette, ; Liu et al, ; O'Connor et al, ; Santos et al, ), tidal flat areas (Beck et al, ; Billerbeck et al, ; Gao et al, ; Huettel & Rusch, ; Marchant et al, ; Riedel et al, ), or subtidal sediments (Ahmerkamp et al, ; Marchant et al, ; Shum & Sundby, ), which are exposed to a lower wave energy level than high energy beaches. In contrast, only few studies have been conducted at wave exposed mesotidal to macrotidal sites like the French Aquitanian coast (Anschutz et al, ; Charbonnier et al, ; Charbonnier et al, ) or the beaches of Spiekeroog Island, Germany (Beck et al, ; Reckhardt et al, ; Seidel et al, ; Waska et al, ). In contrast to sheltered environments, chemical gradients in exposed sandy environments usually stretch over a larger depth range (McLachlan & Turner, ; Reckhardt et al, ).…”

Hydrologic Shifts Create Complex Transient Distributions of Particulate Organic Carbon and Biogeochemical Responses in Beach Aquifers

“…Detailed molecular characterization of dissolved organic matter on samples from the Northern Germany Wadden Sea suggested the release of labile marine DOC and other nutrients from buried algal and microbial biomass, which were preferentially consumed relative to DOC of terrestrial origin (Seidel et al, ). Dissolved organic matter composition of intertidal sands also showed significant correlation to the supplied marine and terrestrial organic matter, while organic matter composition in tidal flats were decoupled from seasonality and more related to early diagenetic processes (Beck et al, ; Seidel et al, ). These studies collectively indicate that organic matter dynamics within sediments are influenced by both hydrologic transport and diagenesis and call for further work that comprehensively integrate the roles of particulate and dissolved carbon dynamics in beach systems.…”