Observations of particle density and scattering in the Tamar Estuary
Introduction
Sediments are a dynamic constituent of the ocean, shelf seas and estuaries. The vertical and spatial distribution, size and composition of the suspended matter and bed sediments are constantly changing in response to high and low frequency processes. While tides produce regular patterns of resuspension, deposition and advection, wind events give rise to random resuspension events. Changes in the seasons are also reflected in the sediments. Lower wind speeds reduce random resuspension, while the evolution of thermal stratification due to increased solar heating produces clearer surface waters. Biological activity, enhanced during the summer months introduces a diversity of phytoplankton which in turn modifies the inorganic sediments, enhancing flocculation, leading to the formation of larger particles, increasing settling and reducing the total suspended sediment load during the summer months (Jones et al., 1998).
Increased turbidity has been shown to have a negative effect on marine flora and fauna, with plankton photosynthesis directly limited by increased suspended particulate matter (SPM) concentrations (Tett, 1990). Much of the microbial activity in the water column also occurs on the surface of particulate matter, especially fine particles, as they offer a large surface area per unit mass (Hoppe, 1984). These fine particles are also optically important as they are responsible for the greatest proportion of scattering of light which in turn determines the transit of light through the water column and so productivity.
Using the optical properties of the surface waters, satellite images have been used to show the spatial distribution of SPM and chlorophyll in the oceans (Weeks and Simpson, 1991, Bowers et al., 1998, Bowers et al., 2002). While there are several methods and instrumentation widely used for the determination of optical properties such as the scattering coefficient (b), the absorption coefficient (a) and attenuation (c), the determination of the compositional properties of sediments are much more challenging. In addition to the particle size, the density (ρ) and refractive index (n) of particles make a large contribution to the observed variation in scattering (Bowers et al., 2009). The apparent density (ρa) can be estimated from gravimetric measurements of mass concentration and volume concentration, using an instrument such as a LISST (Mikkelsen and Pejrup, 2001). The apparent (or effective density) is the ratio of dry weight to wet volume of particles and in this way is different from the actual density of the particles.
In this paper we aim to improve our understanding of the factors which control our ability to predict particle size from optical scattering. This link between the specific scattering coefficient (b*) and the particle size has the potential to enable us to map particle size from space using remote sensing reflectance. We seek to build on the findings of Bowers et al. (2009) who observed that almost 64% of the variance in the mineral specific scattering coefficient could be explained by changes in particle density (mass concentration/volume concentration), while only 15% of the variance was down to changes in the particle diameter.
In order to improve our ability to predict particle size from space, we further investigate the relationship between scattering and particle size and density. While Bowers et al. (2009) considered the bulk density, in this paper we will propose a relationship to relate the density of individual particle sizes to the density of an entire particle size spectra. We aim to evaluate the way in which particle size varies with density and gain a better understanding of the other factors which influence particle size.
Section snippets
Regional setting
The Tamar Estuary is situated on the southwest coast of England forming a boundary between the counties of Devon and Cornwall (Fig. 1). Composing of the Tamar river and two tidal sub-estuaries, the main channel enters the sea at Plymouth Sound some 31 km from the limit of tidal influence upstream. The Tamar has a mean tidal range of approximately 3.5 m and a mean annual discharge of 27 m3 s– 1 (Tattersall et al., 2003). Suspended sediment concentrations reach a peak in a turbidity maximum which
Materials and methods
Particle size, sediment load, optical and holographic camera data were collected during an observational program in the Tamar Estuary and Plymouth Sound in June 2008. The Plymouth University research vessel ‘Catfish’ was used to take measurements over a 12 h period. Data was obtained from a longitudinal transect of the Tamar Estuary heading out into Plymouth Sound. Sampling took place on 17th June beginning at Station 1 (Fig. 1(b)) at 6.00 am GMT. Conditions were calm and sunny for the duration
Particle density
The apparent density (ρa), which is also known as the effective or excess density is defined as the dry weight of sediment (mg l– 1), divided by the volume concentration (μl l– 1) measured in-situ. An estimate of the apparent density of the surface suspended particulates has been made from the gravimetric sediment samples and the LISST particle volume data. While the average apparent density for the whole transect was calculated to be 331 kg m– 3, the apparent density varies along the transect
Density
In this paper we have explored the factors affecting floc density and, like other researchers (Al Ani et al., 1991, Curran et al., 2007) have found a strong negative relationship between the apparent density and particle size. A relationship has also been found between apparent density and the concentration of chlorophyll-a, with an increase chlorophyll-a concentration producing a decrease in the apparent density. Combining the effect of changes in both particle size and chlorophyll-a
Conclusions
Results described in this paper will help to advance our understanding of the relationships between scattering, particle size, floc density and chlorophyll. We have shown that by applying a simple particle density model (Eq. (9)) to our data in order to calculate values for the primary particle size, density and the fractal dimension of the flocs, we can explain 99% of the variance in the observed apparent density. This indicates that the form of the equation used is an appropriate
Acknowledgements
The authors are grateful to NERC and DSTL for the funding which enabled this work to take place. Our thanks also go to the Captains of the Plymouth University vessels, Catfish and Aquatay, for their support during our Tamar Estuary fieldwork.
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