Tidal flats and beaches as bioreactors

The Wadden Sea (UNESCO World Heritage), one of the largest tidal flat systems of the world, stretches along the southern and eastern coast of the North Sea.  The tidal flat area is located between the coastline and a chain of barrier islands, which are separated by tidal inlets. Twice a day, at high tide, the tidal flat area is covered by seawater and falls dry again during ebb tide, when seawater flows back through tidal channels and inlets between the barrier islands, which connect the tidal flat area with the open North Sea.

During flood, dissolved and particulate organic matter, e.g. the remains of marine algae, are incorporated into the tidal flat sediment by seawater infiltration or are deposited on the sediment surface. Furthermore, algae growth on the sediment surface is promoted by regular light exposure at low tide. Even higher organisms like sandworms have adapted to these extreme conditions (changing insolation, temperature, regular flooding and sediment reworking).

Microorganisms degrade organic matter within the sediment. This process releases inorganic nutrients (C, N, P) to the pore water. Additionally, opal dissolution from diatom frustules increases pore water silica concentrations. Microorganisms consume oxygen to degrade organic matter. When oxygen is already depleted, nitrate (NO3-), manganese- and iron oxides and finally sulfate (SO42-) can be used to oxidize organic matter. If these oxidants are exhausted, small organic molecules like acetate or formic acid can be decomposed via methanogenesis, releasing methane (CH4) and carbon dioxide (CO2). Our research demonstrates that large amounts of organic matter are degraded through these reaction pathways. Hence, tidal flat areas are efficient bioreactors.

Pore water drains from the tidal flats due to the hydraulic gradient that builds up when seawater level falls below pore water level during ebb tide. Due to discharge of nutrient-enriched pore water, we detect higher seawater nutrient concentrations during ebb tide at a time series station located in the tidal inlet between Langeoog and Spiekeroog Island. These nutrients (C, N, P, Si) and trace metals, which serve as micronutrients (manganese and iron), may in turn trigger primary production (phytoplankton growth) in coastal surface seawaters.

Seawater circulation through barrier island beaches results in a similar process. However, especially beaches facing the open North Sea are generally characterized by coarser grained sediments because they are subject to high wave and current energy. Coarse sediments exhibit a higher permeability than fine grained sediments, which results in faster pore water flow. Tide- and wave-induced pore water circulation therefore frequently supplies fresh organic matter and oxygen to the sediment and alternative oxidants (Mn-, Fe-oxides, SO42-) are only needed to a limited extent. Our research group further deals with characterizing the fresh groundwater, originating from the barrier island’s freshwater lenses, that discharges to the adjacent seawater.

We aim at evaluating the contribution of beaches and tidal flats, respectively, to the bioreactor-function of coastal sediments. Moreover, steep redox gradients alter redox sensitive metal concentrations (e.g. uranium, rhenium, molybdenum and vanadium) in the pore waters. Consequently, another research topic is to evaluate the role of tidal flats and beaches concerning redox sensitive metal cycling.

 

Selected publications:

Reckhardt, A, Beck, M, Seidel, M, Riedel, T, Wehrmann, A., Bartholomä, A, Schnetger, B, Dittmar, T & Brumsack, H-J (2015). Carbon, nutrient and trace metal cycling in sandy sediments: A comparison of high-energy beaches and backbarrier tidal flats. Estuarine, Coastal and Shelf Science, 1-14.

Beck, M & Brumsack, H-J (2012). Biogeochemical cycles in sediment and water column of the Wadden Sea: The example Spiekeroog Island in a regional context. Ocean & Coastal Management, 102-113.

Riedel, T, Lettmann, K, Beck, M & Brumsack, H-J (2010). Tidal variations in groundwater storage and associated discharge from an intertidal coastal aquifer. Journal of Geophysical Research, 1-10.

Beck, M, Dellwig, o, Liebezeit, G, Schnetger, B & Brumsack, H-J (2008a). Spatial and seasonal variations of sulphate, dissolved organic carbon, and nutrients in deep pore waters of intertidal flat sediments. Estuarine, Coastal and Shelf Science, 307-316. 

Beck, M, Dellwig, O, Schnetger, B & Brumsack, H-J (2008b). Cycling of trace metals (Mn, Fe, Mo, U, V, Cr) in deep pore waters of intertidal flat sediments. Geochimica et Cosmochimica Acta, 2822-2840.