Visualisation of the Sediment Water Interface (SWI)

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1         Literature review

  1. Why O2 plays a critical role in the environment

One of the most important factors controlling water quality and environmental conditions is O2 (Hondzo et al., 2005). Many systems such as drinking water reservoirs, aquatic ecosystems, fisheries and oceans have been adversely impacted by depleted levels of O2. The condition of water bodies in various areas of the world are deteriorating, the situation still not fully understood. It has been emphasized that water is, and will further become, the most precarious resource dictating human and ecosystem health (Gleick, 2003 and NRC, 2004).

  • The global oxygen biogeochemical cycle – draw and print into the perspective with marine sediment

The global O2 biogeochemical cycle incorporates of three main zones; atmosphere biosphere and lithosphere. O2 moves through these zones through processes such as photolysis, weathering as well as respiration/photosynthesis and decay (atmosphere – biosphere); weathering or burial occurs between the biosphere and lithosphere.

  • Processes leading to O2 depletion

Monitoring of delicate ecosystems starts with understanding the mechanisms which lead to the depletion of O2 in the environment. This knowledge is critical in preventing harmful consequences such as hypoxia within systems.

  • How algal blooms influence the O2 concentration in the water column

Turbulent mixing processes are known as the main transporter of atmospheric O2 from surface to bottom waters. This physical method of turbulence and mixing to transport O2 can be reduced by stratification, leading to a reduction of O2 transfer to bottom waters. Biogeochemical factors leading to O2 depletion are algal blooms which form due to excess amount of nutrient. Anthropogenic activities such as sewage, agriculture and urban runoff can lead to this excess levels of nutrients within waters (Scalo et al., 2012).  O2 deplete due to higher levels of nutrients occur to the creation of algal blooms, as the algal die and descend to the bottom waters and sediment. As the dead algae reaches the sea bed bacteria decomposition occurs in turn absorbing the dissolved O2, creating an oxygen sink within sediment (Scalo et al., 2012).

Seasonality impacting these biogeochemical processes as it has been well established that light and nutrient availability dictates seasonal patterns in pelagic primary production.  Olesen and Lundsgaard, (1995) show distinct autumn and spring blooms characterise many temperate coastal environments which allow for vertical O2 transport. Studies have shown benthic response and simulated sedimentation of plankton blooms is characterised by an increase in benthic O2 uptake with a decline in background levels dropping within several days to weeks (Hansen and Blackburn, 1992).

C:\Users\vr294\AppData\Local\Microsoft\Windows\INetCacheContent.Word\Capturedd.png2. Visualisation of the sediment water interface (SWI)

  • Components of sediment water interface

The sediment water interface is a boundary layer set between the water column and sediment. The form of this interface is affected by biological (bioturbation) and physical (tides, currents) processes.  Figure 1 depicts the SWI which incorporates the bottom boundary layer and the diffusive boundary layer, these specific features are further discussed.

Figure 1, Illustrating the diffusive boundary layer above the sediment and demonstrating the O2 movement.

  • Understanding the structure SWI

The bottom boundary layer (BBL) (Figure 1) is of great importance to the biology, chemistry, physics and geology of water bodies. It is comprised of elements of the sediment and water columns which are directly affected in the division of their characteristics and processes by the occurrence of the sediment surface interface. The benthos is the primary site for dissipation of waves, currents and other turbulent energies (Boudreau and Jorgensen, 2001). These turbulent energies allow for the exchange of heat, particles and solutes linking the water column and sediment.  The water column which exists near the bed allows for relatively strong gradients in physicochemical properties such as permeability and solubility (Boudreau and Jorgensen, 2001).

  • Drivers effecting the BBL and DBL

The wall effect has been used to describe BBL in large-scale hydrodynamics; the BBL has unique additional characteristics such as internal friction in water (Varsakelis and Papalexandris, 2012). Therefore, the diffusive boundary layer (DBL) is known as a water film (only millimetres thick) (Figure 1 and 4) overlying the sediment surface in which eddy diffusion is exceeded by only molecular diffusion (Boudreau and Jorgensen 2001).

Within porous sediment, hydrodynamic forces infiltrate into the sediment, the combination of sediment surface topography and water flow allows for lateral pressure which drives advection flow through the upper decimetres. The EC will provide flux estimates based on a larger footprint whereas the microprofile data will provide a single-point-based flux analysis. (have been asked by one examiner to delete and one to comment on what flux is so I created this paragraph below)

  • What is O2 flux at the SWI

The flux (J) relates to the state of constant change, when determining the total flux the direction, gradient and size of surface it passes through is required.

The flux JO2 is quantified by a balance rate where O2 is supplied by the sediment at a rate at which O2 is consumed in the sediment. This JO2 consumption rate can be quantified and then be used to establish the extent of the sediment oxic zone (Boudreau and Jørgensen, 2001; Higashino et al., 2004).

In relation to direct measurement of scalar fluxes, the microprofiler technique is a suitable method. Fick’s law of diffusion supports the flux gradient method (Zielinski, 2006), which estimates the O2 flux over across a virtual plane.

Solutes diffuse from regions of high to low concentration. The flux of solute is defined as mass of solute that passes through a known area (Boudreau and Jorgensen, 2001). A plane in the solution which is perpendicular to the flow of the solute is used to determine the flux (Boudreau and Jorgensen, (Figure 2).

C:\Users\vr294\AppData\Local\Microsoft\Windows\INetCacheContent.Word\Capture44.png
Figure 2, This schematic demonstrates the movement of flux to be toward the right.
Equation 1 Mathematical illustration of this processes (Fick’s first law of diffusion) (Zielinski, 2006).

Where J is the flux; D is known as the diffusion coefficient and

2003). The result of an increase in the DBL thickness has implications for the sediment surface as the decrease in the O2 concentration occurs due to the decrease in O2 depth penetration. These interaction can also been seen vice versa, as energy increase in the environment and velocities increase, the DBL thickness reduces and O2 concentration increase within the sediment surface (Jørgensen & Des Marais 1990).

Other studies have focused on the O2 fluxes analyses on either the sediment or water side of the SWI (Berg et al., 2003).

This study will demonstrate the O2 flux within a well-studied area and add biogeochemical information which will link the water column data to the benthic sediment data (Berg et al., 2003).

  1. O2 movement and how it is measured in the environment

Mineralisation of organic material and regeneration (how??)of nutrients in coastal environments play a key role in aquatic sediments (Middelburg et al. 2008). The coastal food web has extensive contributions from sediments with high phototrophic biomass (Jahnke et al., 2000, Glud et al., 2009). The majority of research into benthic production and degradation of organic material have been conducted in the laboratory or in situ determination of benthic O2 exchange rates (Glud, 2008). These techniques include sediment enclosures or microprofile measurements, though these techniques are limited to soft sediments (Glud et al., 2010).

O2 transfer to the sediment and sediment O2 consumption processes can be physically limited by the function of sediment O2 uptake (Glud et al., 2009). O2 dynamics in aquatic systems can be characterized by measuring the sediment O2 uptake rate (JO2); this critical parameter can be quantified by resolving the vertical distribution of O2 at the sediment water interface or by measuring total O2 uptake by methods such as EC (Wetzel 2001; Glud 2008). Measurements of O2 fluxes via microprofiles (diffusive uptake) and EC (total uptake) will be a foundation of this project.  Microprofiles will allow for a single-point-based flux approach whereas the EC will provide a flux estimates over a large foot print. Sediment cores collected at the site will focus on bio-irrigation which looks at the benthic organisms flushing their burrows with the overlying water. These three methods complement each other in building a picture of the O2 movement within a benthic ecosystem.

Therefore, papers such as Bouldin, 1968, Jorgensen and Revsbech, 1985, and Boudreau, 2001 have indicated that O2 sediment uptake is influenced on both water and sediment side. Factors influencing the O2 transport to the sediment and subsequent O2 consumption can be recognized by changing rates of turbulence and O2 levels close to the benthic layers (Brand et al 2008; Lorke et al., 2003). Sediment –water fluxes are greatly influenced by internal currents; Glud et al., 2009 has confirmed the high dynamic structure of vertical distribution of O2 at SWI and sediment O2 consumption processes. O2 uptake can be influenced by ecological factors such as bio-irrigation and bioturbation, occurring the sediment side of the SWI, this is further discussed, were bio-irrigation will be investigated in our study.

  • Measuring O2 concentration in Laboratory  (how we are solving the problem)

Laboratory experiments have shown that permeable sediments that are flushed with pore-water increase the O2 respiration. Reimers et al., 2004, de Beer et al., 2005 and Cook et al., 2007 show that in situ O2 respiration must be highly variable and dependent on current and wave conditions. Nevertheless Berg et al., 2013 argues  flow rates and in situ pore-water exchange with their effect on biogeochemical cycling is currently not fully quantified. Even though laboratory studies are cost effective and environmental measures can be controlled it does not outweigh the value of in situ data as these field conditions are optimal for characterising fluxes.

To date, benthic chambers have been the most common way to measure in situ benthic O2 fluxes. Wild et al., 2004 and Tengberg et al., 2005 describe experiments using traditional chamber incubations to determine the O2 fluxes for an enclosed sediment area of 0.25m2, calculated from concentration changes measured in the enclosed water through time (hours – days). Disadvantages shown by this technique are the invasive method that excludes natural flow as well as the water-bound constituents exchange (Cook et al. 2007; Berg and Huettel 2008; Glud 2008). This invasion and disregard for the natural flow can have severe implications to the understanding and assessment of the benthic carbon forms and transformations. Microsensor profile measurements can be used to estimate O2 flux and are considered a better link than chambers to point measurement, having some advantages such as not disturbing the porewater chemistry (Reimers et al 2012).  (They asked me to explain what porewater is??)

  • Measuring O2 concentration in situ

O2 depletion is largely governed by the amount of O2 taken up by the sediment waters with organic-rich sediment and/or by biological and chemical reactions (diagenetic and redox reactions) in the sediment (Bouldin 1968; Veenstra and Nolen 1991). O2 transfer to the sediment and sediment O2 consumption processes are physically limited by sediment O2 uptake (Boudreau and Jørgensen 2001). It is possible via the microprofiler technique to quantify the sediment O2 uptake flux by observing the vertical distribution of O2 at the SWI, this measurement allows for the characterisation of the O2 dynamics in the aquatic system (Wetzel 2001). Bryant et al., 2010 states it is therefore vital to explore both the water-side and sediment- side factors that control sediment O2 uptake in freshwater and marine systems (Bouldin 1968; Revsbech and Jørgensen 1985).

The EC technique enables O2 fluxes to be quantified in permeable sediments under field conditions; this is a characteristic which other flux approaches do not support (Berg et al., 2013). They showed that while explaining the permeable sands and the overlying water column with the EC technique was more expensive and technically challenging compared to traditional methods, considerable advantages were realised especially for the permeable sediment. The experiment allowed for the current direction to be identified without disturbing the natural light or flow conditions. This method incorporated in situ effects of pore-water flushing as well as allowing the footprint, which is the sediment surface area that contributes to the flux, to examine a large area (10-100m2) (Berg et al., 2007). This method integrated spatial heterogeneities found in benthic systems due to biological (e.g., bio-irrigation from benthic fauna and microbial respiration) and chemical (e.g., oxidation of reduced chemical species) processes in the sediment (Glud and Fenchel, 2000; Wenzhofer and Glud, 2004; Thouzeau et al., 2007). This project has and will continue to use the microprofiler and EC to characterise combined effects of these processes.

(reason explain why I am carrying out these factors.)

In summary, outcomes from this interdisciplinary research will provide important methodological, scientific and practical contributions to a variety of fields, including sediment-water flux analysis, aquatic science research and lake management; highlighting the significance of exercising multiple measure measurements to obtain average conditions at the SWI (Berg et al., 2013).

4.    Importance of self-sea ecosystems

(move to the beginning of lit review?? (what the problem is)

The continental shelf makes up only 7% of the world’s ocean and contains 15% of the ocean’s plants it is also of great economic importance to the UK (National Oceanography Centre, 2016). This region around the world is subject to oil and gas extraction, shipping, telecom, fisheries, aquaculture commerce as well as for aquaculture, raw material extraction and renewable energy (National Oceanography Centre, 2016). Coastal regions are continually under development from shoreline properties to coastal flood defence. Over 1.2 billion people rely on shelf sea protein production.  It is known the shelf sea waters have been regarded as a sink with discharges of contaminants (Huettel and Webster, 2000). This is of concern as ecology in these areas are vital for carbon cycling. Pollution and climate change may have a profound effect these areas (Huettel and Webster, 2000).

         Habitats which make up the self-sea ecosystems

Seagrass, mangroves, macro algae, reef systems and many other coastal habitats form critical marine ecosystems which are characterized by productivity and high diversity (Wilkinson, 2012; Harrison and Booth, 2007). It has been well understood that photosynthesis conducted by coastal vegetation converts carbon dioxide into biomass (Atwell et al. 1999). Due to these processes taking place carbon dioxide is removed from the atmosphere and is stored as organic material, creating a carbon store (Atwell et al., 1999).

It is thus vital to continually measure primary production to understand threatened as well as globally significant habitats as long a global anthropogenic climate change continues because of over fishing, intensive agricultural activities, and industrialization (Hughes et al., 2008).

A key component of overall system metabolism and nutrient cycling is benthic metabolism, as the majority of the seafloor lies within the photic zone (McGlathery et al., 2007). These benthic autotrophs include seagrasses as well as micro and macro algae. Metabolism of these primary producers and associate communities act as a vital carbon and nutrient sink in shallow coastal systems (Hume et al., 2011).

  • Biogeochemical processes in permeable sediment

Much of the continental shelve is covered with high permeability sediment (Emery 1968) where high-energy is maintained (estuaries, rivers and streams) (Huettel and Webster 2000). Tight advective coupling between the water column and sand is due to the sediment surface topography and the open pore space of the sand (Wiberg et al., 1994). This coupling allows for a dynamic reaction scheme for biogeochemical processes to occur which more commonly are found in muddy cohesive sediments (Webb and Theodor 1968).

As bottom currents deflect due to the sediment topography, this produces ripples as well as forces water and water- bound constituents into the sand because of sediment depressions along with drawing the pore water from the sand at surface elevations (Webb and Theodor 1968). Huettel and Rusch, 2000, Pilditch and Miller, 2006, Huettel et al., 2007 express how compounds can be reduced within the sediment as well as the removal of the anoxic pore water by a supply of O2 and ‘nutrition’ (mineralization processes). This occurs in the form of dissolved organic matter deep into the sands and particulates (Berg et al., 2013).   Subsequently, biogeochemical processes such as aerobic respiration are tightly linked to bottom-current characteristics which can differ largely with tidal oscillation (Berg et al., 2013). Berg et al., 2013 has demonstrated this tight coupling in eddy flux measurements which were obtained in natural field conditions, thereby elucidating that within benthic O2 flux in permeable sediment, current flow is one of the governing controls. The EC technique enables the measuring of these eddy fluxes as O2 and turbulence is measured simultaneously.

The flux JO2 is quantified by a balance rate where O2 is supplied by the sediment at a rate at which O2 is consumed in the sediment. This JO2 consumption rate can be quantified and can then be used to establish the extent of the sediment oxic zone (Boudreau and Jørgensen, 2001; Higashino et al., 2004).

1.2      Ecological drivers of O2 transport to sediment

Bioturbation, bio-irrigation, molecular diffusion as well as current and/or wave advection are the main drivers for O2 transport across the sediment – water interface (Ziebis et al., 1996). I will be examining bio-irrigation rates within this project, methods to be found in section (xxx).

C:\Users\re346\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.IE5\N6FY16AT\Capture.PNG
Chipman et al., 2012 explains light and turbulence characteristics are connected to O2 flux, therefore it is important for a measurement to be taken to gage benthic flux as these parameters interfere with the light field or boundary layer flows. Sedimentary O2 availability and benthic oxidation processes may be largely influenced by photosynthesis and interfacial transport; both of these processes have considerable impact on the sediment O2 fluxes and consumption (Chipman et al., 2012).

Figure 4, A diagram showing the burrow with ventilation and bio-irrigation indicated by arrows. Arrows represent direction of ventilation (right) and bio-irrigation (left).

  • Bio-irrigation process

Benthic organisms flushing their burrows with the overlying water is a process called bio-irrigation. This is an important process in the biogeochemistry of the oceans due to the involvement of dissolved substances between the pore water and the overlying sediment. Bio-irrigation comprises of processes known as particle reworking and ventilation. These processes involve benthic macro-invertebrates which burrow feeding, defecating, burrowing and respiring in the sediment (Figure 4). Bioirrigating fauna and bioturbation has been suggested as a predominant driver of benthic- pelagic particle and solute exchange (Kristensen 1988, Aller 1998, Glud et al., 2003). Large amounts of oxidative transport are caused by bio-irrigation and subsequently have a large impact on biogeochemical cycling. Physical variables such as waves and currents induce pressure gradients over porous sand, resulting in advective pore-water flow into the permeable sediment (Huettel et al., 1998, D’Andrea et., 2002, De Beer et al., 2005). This process allows OM degradation and nutrient cycling due to the supply of O2 to the sediment regulating the distribution of microbial communities. Studies have shown when coastal ecosystems do not have bioirrigating organisms this results in sedimentary problems, such as clogging of organic rich fine sediments as well as reductions in sediment permeability (Volkenborn et al., 2007). A lack of bio-irrigation organisms can prevent deep O2 penetration into the sediment as well as creating an accumulation of reduced mineralization products in pore water (Volkenborn et al., 2007). As the shelf sea comprises mainly of sandy sediment an accurate evaluation of the biogeochemical influence of advective bio-irrigation is crucial for understanding of cycling in sediment.

C. Scalo, U. Piomelli and L. Boegman, “Large-eddy simulation of oxygen transfer to organic sediment beds.” J. Geophys. Res.-Oceans 117, C06005, pp. 1-17, 2012.

Scalo, C., Piomelli, U. and Boegman, L., 2012. High-Schmidt-number mass transport mechanisms from a turbulent flow to absorbing sediments. Physics of Fluids, 24(8), p.085103.

Lorrai, C., 2010. Estimating benthic boundary layer oxygen dynamics in lakes (Doctoral dissertation, ETH Zurich).

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