Scientific approach

Background

COSA project was initiated primarily to gain a detailed understanding of biogeochemical processes taking place in sandy ecosystems. To this end COSA brings together scientists from several disciplines (biology, geochemistry, hydrodynamics) with expertise in a range of sedimentary processes. Here we present a brief background and overview of the processes being studied within the project.

To date, number of detailed studies on sandy sediment biogeochemistry is relatively small; the vast majority of studies of benthic processes have taken place in cohesive, muddy sediments.

A key feature of sandy sediments is their permeability which means that water can flow through the interstices. The deflection of bottom water currents (due to tides, wind, or waves) by bottom topography (ripples, animal mounds, stones etc) creates horizontal pressure gradients at the sediment surface, which leads to advective flow of water through the sediment (Savant et al. 1987, Thibodeaux & Boyle 1987).

The resulting bottom water inflow and pore water release was demonstrated in flume experiments by staining pore waters with rhodamine dye (animation, 12,5 MB). To date, in-situ measurements of porewater advection rates are rare, but flushing rates of 5-590 L m^-2 d^-1 have been estimated (Precht & Huettel 2003, 2004) and references therein).

The consequences of advective processes on sedimentary processes may be profound. Sandy sediments, thus, may be unique environments with strong differences to the more widely studied cohesive sediments (Huettel et al. 2003).

One of the most striking features of sands is that these sediments can efficiently trap particles from the overlying water column, thus leading to a high delivery rate of labile organic matter to the sediment (Huettel & Rusch 2000). Advective flushing of the sediment also has important consequences for oxygen dynamics, which has been graphically illustrated by (Precht et al. 2004) (See: http://aslo.org/lo/toc/vol_49/issue_3/0693.pdf) using planar oxygen optodes.

Firstly, it results in a greater oxygen transport into the sediment, possibly affecting the extent and rate of organic matter breakdown in marine sediments (Kristensen et al. 1995, Hulthe et al. 1998, Dauwe et al. 2001).

Secondly it introduces O₂ into the sediment on rapidly changing spatial scales. This results in a rapid alternation of oxic and anoxic conditions, which is known to increase sediment metabolic activity (Aller & Aller 1998) and also potentially leading to a tight coupling between obligate aerobic and anaerobic processes such as nitrification and denitrification.

For more information contact Markus Huettel.

The project

While much progress has been achieved in our basic understanding of sandy sediments filtration, there have been no integrated field studies of sandy sediments. COSA utilises state of the art technologies and methods to further our understanding of sandy sediments. Key aspects of the project include:

• Benthic exchange and process studies
  Central to the study of benthic exchange processes (parameters being measured include O₂, CO₂, N₂, and dissolved nutrients) is the use of benthic chambers designed for permeable sediments as described by (Huettel & Gust 1992) and (Janssen et al in Press L+O) (See: http://aslo.org/lo/toc/vol_50/issue_3/). These chambers have been calibrated to produce a defined pressure gradient over the sediment on a scale mimicking that likely to occur in situ and thus reproducing advective flushing rates likely to be seen in the field.

  
  

  A schematic diagram of the benthic chamber used in COSA, illustrating the pressure gradient induced over the sediment (red line) and the ensuing advective flow of water induced through permeable sediments. This flow mimics the flow encountered in natural situations
  
  
  Other biogeochemical processes being measured include diatom response to light, anaerobic ammonium oxidation (anammox), sulfate reduction rates, Fe and Mn distribution, potential re-oxidation rates of reduced compounds. For more information contact Perran Cook.
• Integrated sediment topography and porewater flow measurements
  The advective flushing of porewater is dependent upon the sediment topography, currents and sediment permeability. In this project we are employing a new instrument (Lance a Lot.), which can simultaneously measure sediment topography and porewater flows, to further our understanding of the rates and controlling factors in-situ.
  For more information contact Felix Janssen.

• In-situ microsensors
  In-situ microprofiles and planar optode images proved valuable tools for investigations of solute distributions in cohesive sediments which substantially improved our understanding of these systems, eg (Glud et al. 2003, Wenzhöfer & Glud 2004). However, microsensors and planar optodes have rarely been utilised in sandy sediments. In COSA we are employing both methods to measure the temporal and spatial dynamics of O₂ and H₂S in situ.
  For more information contact Frank Wenzhöfer.
• Pulse chase experiments and inverse analysis of trophic structures
  'Pulse chase' experiments using stable isotopes of C and N (added in the form of H¹³CO₃- and ¹⁵NO₃- provide a powerful means of elucidating the flows of these elements through benthic food webs (Herman et al. 2000, Middelburg et al. 2000). In COSA we are utilising this approach in combination with inverse modelling techniques to gain new insight into the carbon and nitrogen flows in these systems.
  For more information contact Jack Middelburg.
• Biogeochemical modelling
  Biogeochemical modelling of processes in cohesive sediments is now a widely used approach to help understand these systems eg (Soetaert et al. 2001, Wijsman et al. 2002). Biogeochemical modelling in permeable sediment has however not been undertaken to date. In COSA, existing diagenesis models have been extended to two dimensions. Porewater advection has been included as the major transport mechanisms in the upper layers. Program codes are freely available on the website http://www.nioo.knaw.nl/homepages/meysman/media.htm
  For more information contact Jack Middelburg.
• Time series measurements of key parameters
  At each of the two study sites a time series of measurements of key physical and biological variables are underway. The measured variables include sediment permeability, porosity, porewater nutrient profiles, benthic nutrient exchange, benthic primary production, sediment chlorophyll, water column chlorophyll, and primary production.
  For more information on the Sylt site contact Justus van Beusekom and for the Hel site Lech Kotwicki.
• Sediment flora and fauna abundance, diversity and function
  Of key importance to the processes studied above is the diversity and abundance of benthic flora and fauna at each site. Detailed analysis of species inventories and abundances at each site are being conducted concurrently with the process measurement studies. Furthermore, experiments are conducted to elucidate the relative importance of bioturbation and bioirrigation in respect to porewater transport.
  For more information contact Lech Kotwicki.

References

Dauwe B, Middelburg JJ, Herman PMJ (2001) Effect of oxygen on the degradability of organic matter in subtidal and intertidal sediments of the North Sea area. Mar Ecol-Prog Ser 215:13-22

Glud RN, Gundersen JK, Røy H, Jørgensen BB (2003) Seasonal dynamics of benthic O₂ uptake in a semienclosed bay: Importance of diffusion and faunal activity. Limnol Oceanogr 48:1265-1276

Herman PMJ, Middelburg JJ, Widdows J, Lucas CH, Heip CHR (2000) Stable isotopes as trophic tracers: combining field sampling and manipulative labelling of food resources for macrobenthos. Mar Ecol-Prog Ser 204:79-92

Huettel M, Gust G (1992) Solute release mechanisms from confined sediment cores in stirred benthic chambers and flume flows. Marine Ecology Progress Series 82:187-197

Huettel M, Roy H, Precht E, Ehrenhauss S (2003) Hydrodynamical impact on biogeochemical processes in aquatic sediments. Hydrobiologia 494:231-236

Huettel M, Rusch A (2000) Transport and degradation of phytoplankton in permeable sediment. Limnol Oceanogr 45:534-549

Hulthe G, Hulth S, Hall POJ (1998) Effect of oxygen on degradation rate of refractory and labile organic matter in continental margin sediments. Geochim Cosmochim Acta 62:1319-1328

Kristensen E, Ahmed SI, Devol AH (1995) Aerobic and anaerobic decomposition of organic matter in marine sediment: Which is fastest? Limnol Oceanogr 40:1430-1437

Middelburg JJ, Barranguet C, Boschker HTS, Herman PMJ, Moens T, Heip CHR (2000) The fate of intertidal microphytobenthos carbon: an in situ ¹³C-labeling study. Limnol Oceanogr 45:1224-1234

Precht E, Franke U, Polerecky L, Huettel M (2004) Oxygen dynamics in permeable sediments with wave-driven porewater exchange. Limnol Oceanogr 49:693-705

Precht E, Huettel M (2003) Advective pore-water exchange driven by surface gravity waves and its ecological implications. Limnol Oceanogr 48:1674-1684

Precht E, Huettel M (2004) Rapid wave driven porewater exchange in a permeable coastal sediment. J Sea Res 51:93-107

Savant SA, Reible DD, Thibodeaux LJ (1987) Convective transport within stable river sediments. Water Resour Res 23:1763-1768

Soetaert K, Herman PMJ, Middelburg JJ, Heip C, Smith CL, Tett P, Wild-Allen K (2001) Numerical modelling of the shelf break ecosystem: reproducing benthic and pelagic measurements. Deep-Sea Res Part II-Top Stud Oceanogr 48:3141-3177

Thibodeaux LJ, Boyle JD (1987) Bedform-generated convective transport in bottom sediment. Nature 325:341-343

Wenzhöfer F, Glud RN (2004) Small-scale spatial and temporal variability in coastal benthic O₂ dynamics: Effects of faunal activity. Limnol Oceanogr 49:1471-1481

Wijsman JWM, Herman PMJ, Middelburg JJ, Soetaert K (2002) A model for early diagenetic processes in sediments of the continental shelf of the Black Sea. Estuar Coast Shelf Sci 54:403-421