Surficial and deep pore water circulation governs spatial and temporal scales of nutrient recycling in intertidal sand flat sediment

Abstract

This study addresses organic matter decomposition in permeable sediment of a sloping intertidal sand flat (German Wadden Sea) affected by current-induced pore water exchange and pore fluid drainage. Seasonal and spatial scales of aerobic and anaerobic mineralization were investigated at 2 sites, one near the water line and one on the upper flat. Hydrodynamic forcing during inundation caused deeper oxygen penetration through flushing of the uppermost sediment layer. This flushing resulted in higher areal oxygen consumption rates and lower depth integrated sulfate reduction rates in the submerged flat compared to the rates measured during exposure. Mineralization rates in the top 15 cm of the sediment were similar between both study sites and ranged from 38 (winter) to 280 mmol C m -2 d -1 (summer), with sulfate reduction contributing 3 to 25% to total mineralization, depending on the season. At the upper flat, these seasonal differences were reflected in the pore water concentrations of nutrients, dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC). Near the low water line, however, pore water nutrient and DIC concentrations were independent of the season and up to 15 times higher compared to the values recorded in the upper flat. The differences in concentrations of metabolic products between the 2 sites resulted from a low tide drainage extending deep below the uppermost flushed layer and causing seepage of pore water near the low water line. Mineralization and nutrient release in these permeable intertidal sediments is affected by 2 circulation processes that work on distinctly different temporal and spatial scales: (1) rapid 'skin circulation' through the uppermost sediment layer during inundation that is characterized by short flow paths, low pore water residence time and immediate feedback to the ecosystem, and (2) slow 'body circulation' through deeper sediment layers during low tide that is characterized by long flow paths and pore water residence times, and acts as a buffered nutrient source to the ecosystem.

Effects of soil stratigraphy on pore-water flow in a creek-marsh system

“…The trace tracks the particle's movement in the marsh sediment from the release point (at a selected location on the marsh surface) to the exit while the travel time measures the duration of the movement (Robinson et al, 2007;Zimmerman, 1976). Particles were released uniformly across the marsh surface to the surface soil layer at a depth of 0.1 m. Thus, the results represent the behavior of both "skin circulation" near the sediment surface and "body circulation" that occurred underneath the surface layer as proposed by Billerbeck et al (2006).…”

“…6, these particles traveled initially a distance towards the main channel, but quickly moved upward and out across the sediment surface, undergoing only a relatively short (in time and distance) "skin circulation". Billerbeck et al (2006) suggested that "skin circulation" near the sediment surface is induced mainly by local topographic variations, at relatively small scales. However, such a circulation in our modeled marsh with a plane platform was instead induced by the seepage face flow, which occurred across the sediment surface during the falling tide (Xin et al, 2011).…”

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

“…The latter were shown to be of great importance for carbon, nutrient, and trace metal cycles in the large tidal flat area called Wadden Sea stretching from the Netherlands to Denmark, and the frequent water exchange between tidal flat areas and the North Sea transport carbon, nutrient, and trace metals to coastal waters of the North Sea (e.g. Beck et al, 2008;Billerbeck et al, 2006;Grunwald et al, 2010;Marchant et al, 2014;Moore et al, 2011;Riedel et al, 2011;Røy et al, 2008;Santos et al, 2015). Pore water discharge rates calculated for nearby sandy Wadden Sea sediments vary depending on the method applied and the sediment depth considered Moore et al, 2011;Riedel et al, 2010), among which the method most comparable to the model approach applied in our study resulted in lower flux rates (0.97 m −3 per tide and meter of shoreline, which equals~2 m −3 per day and meter of shoreline; Riedel et al, 2010) compared to our study.…”

A review on substances and processes relevant for optical remote sensing of extremely turbid marine areas, with a focus on the Wadden Sea

“…Lübben et al (2009) present proWles of CDOM Xuorescence for these pore waters, with 25 times higher values (2.5 Raman Xuorescence units ¡nm ) at samples at 5 m deep than at the bottom surface (where values were just over 0 Raman Xuorescence units ¡nm ). Due to these high concentrations in pore water, release during resuspension of sediment or seepage from sediment (Billerbeck et al 2006) possibly attributes signiWcantly to the CDOM concentration in the water column ).…”