1. Influence of Suspended Particle Properties on Optical Properties and Resultant Water Clarity along a Partially-Mixed Estuary, York River, Virginia, USA.
Kelsey A. Fall1, Carl T. Friedrichs1, Grace M. Cartwright1, and David G. Bowers2
1Virginia Institute of Marine Science
2School of Ocean Sciences, Bangor University

2. Motivation: Water clarity a major water quality issue in the Chesapeake Bay and its tidal Tributaries. Despite decreases in sediment input water clarity has continued to deteriorate (especially in the Lower (i.e., Southern) Chesapeake Bay).
good
poor
Percent of Secchi Depths which passed USA Environmental Protection Agency (EPA )water clarity thresholds (Williams et al., 2010)
Chesapeake Bay Main stem Management Segmentation
(modified from Tango and Batiuk, 2013)
Upper Bay  
Lower Bay
This project is motivated by the trends in water clarity in the Chesapeake Bay and its tributaries. Here is figure from showing trends in percent of secchi disks depth that were deep enough to meet the EPA’s threshold for the upper and lower over the last 20 some years. Over this time strong efforts and funds were put towards reducing sediment input, but water clarity is continuing to deteriorate. This is especially obvious in the lower bay.
EPA Water Clarity Thresholds: Minimum level of light penetration required to support the survival, growth and continued propagation of SAV (EPA Executive Summary, 2003).
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3. Motivation: Water clarity is an issue in the York River Estuary (Reay, 2009; Daur et al., 2005). Despite decreases in sediment input water clarity has continued to deteriorate (especially in the Lower York).
Yearly Average Normalized ZSD (m)
*Normalized by the Max ZSD observed at each station
A. Upper York: p-value=0.52
Study Site: York River Estuary, VA, U.S.A
CHESEAPEAKE
BAY
VIMS
Latitude
Longitude
West Point
ETM
RET 4.3
LE4.1
LE4.2
LE4.3
Upper York
Lower York
Normalized Secchi Disk Depth, ZSD (m)
B. Lower York: p-value = 0.02
*Partially mixed, muddy, microtidal estuary located adjacent to the lower Chesapeake Bay.
*Data from Bay Monitoring Program/VECOS
2/14

4. So the big question is: What is driving this decline in water clarity?
Gallegos et al., 2011: Combined visual (secchi disk) and direct (irradiance meters) methods of measuring light and define a dimensionless coefficient ZSDKd.
Annual means of ZSDKd for the Chesapeake Bay.
ZSDKd
ZSD: Secchi Disk Depth
Kd: Diffuse Light Attenuation Coefficient
ZSDKd: Ratio of Scattering to absorption
Every year, even in years with low flow, when fewer nutrients and less sediment wash into the estuary, resource managers watch measurements of water clarity worsen.
dimensionless coefficient" — a number that would stand for the product of these different units taken together. It was a way to compare the apples of the Secchi disk with the oranges of the radiometers.
Scattering quickly confuses the human eye-so secchi disks depth extremely influenced by scattering (particles)
Gallegos 2011 compiled historical monitoring data from the Chesapeake Bay and noticed a long term decreasing trend in ZSDKd with no increase in remote sensing reflectance (so no significant change in particulate backscattering). This trend suggests the something about the water’s composition is changing. He then ran various model simulations found that the most likely cause of this trend is an increase in smaller, more organic rich particles.
Typical estuarine particles are not single solid particles, but clusters of inorganic and organic particles and water, called flocs. Floc are challenging to observe in-situ, so their influence on the optical properties of the system have yet to be completely defined.
(Gallegos et al.,2011).
Gallegos et al. (2011) suggests the most likely explanation for theses trends is an increase in scattering due to an increase in the concentration of continually suspended, smaller, more organic rich particles.
3/14

5. Video of Suspended Particles (Flocs) in the York
Objective: Investigate the controls on suspended estuarine particle properties (size, concentration, composition) and evaluate the influence of these particles on light attenuation.
This study: Collected near surface observations at different stations ( ) along the York from
Study Site: York River Estuary, VA, U.S.A
Video of Suspended Particles (Flocs) in the York
West Point
ETM
Latitude
3 mm
YORK
RIVER
CHESEAPEAKE
BAY
VIMS
Longitude
4 mm
4/14

6. Coastal Hydrodynamics and Sediment Dynamics (CHSD) Water Column Profiler
Pump sampler: Total suspended solids (TSS) and organic content
LISST-100X: Particle Sizes ~ μm
Beam attenuation (beam c) from optical transmission
Particle Imaging Camera System (PICS): Particle Sizes ~ μm
Individual particle densities and settling velocities
Acoustic Doppler Velocimeter (ADV)
provide estimates of:
current velocities and mass concentration of suspended particle matter
Bottom: turbulence/bed stress, bulk settling velocity
CTD with a turbidity sensor
water clarity proxy
Radiometer or LI-COR light sensor
Vertical profile of diffuse light attenuation (Kd)
CHSD Profiler
ADVs
PICS
LISST
CTD
Pump
Sampler
Pump
Sampler
ADVs
PICs
[Added “total suspended solids”, shortened ADV entry, added “vertical profile”]
ADVs provide information about physical parameters as well as bulk particle characteristics
LISST and PICS provide particle size information. LISST gives particle distribution spectrums from microns. PICs provides particle sizes and settling velocities for particles 30-
Previously, water clarity information from a CTD with a turbidity sensor and for this project we will also use a Li-Cor Light sensor or Radiometer.
CTD
LISST
(Smith and Friedrichs, 2014, 2010; Cartwright et al., 2013; 2009; Fugate and Friedrichs, 2002).
5/14

7. Characterization of Particle Size (Area and Volume) with the LISST and PICS
LISST ( μm) uses laser diffraction to determine VC in size class i (VCi) then
PICS ( μm) uses particle imaging analysis to measure the major and minor axis lengths and calculates equivalent spherical diameter and projected area.
Example from surface water June 2013:
Particle Size μm
LISST
PICS
Volume Concentration Distribution [Vi]
VCi μL/L
Ai cm2/L
Area Concentration Distributions [Ai]
Particle Size μm
LISST
PICS
6/14

8. Characterization of Particle Size (Area and Volume) with the LISST and PICS
Define particle characteristics with new combined distributions ( D50A, AT, ρa ):
D50A : Median particle size based on area distribution.
AT: Total area per liter (AT=Σai)
ρa: Effective density (ρa=TSS/VCT, where VCT=Total Volume Concentration)
Example from surface water June 2013:
Volume Concentration Distribution [Vi]
Area Concentration Distributions [Ai]
Ai cm2/L
Particle Size μm
D50v≈74 μm
D50A ≈11 μm
AT≈206 cm2/L
Particle Size μm
VCi μL/L
7/14

9. *Most of the AT attributed to smaller particles
Characterization of Particle Size (Area and Volume) with the LISST and PICS
Define particle characteristics with new combined distributions ( D50A, AT, ρa ):
D50A : Median particle size based on area distribution.
AT: Total area per liter (AT=Σai)
ρa: Effective density (ρa=TSS/VCT, where VCT=Total Volume Concentration)
*Most of the AT attributed to smaller particles
*Quite a bit of difference between D50v and D50A
Example from surface water June 2013:
Volume Concentration Distribution [Vi]
Area Concentration Distributions [Ai]
Ai cm2/L
Particle Size μm
D50v≈74 μm
D50A ≈11 μm
AT≈206 cm2/L
Particle Size μm
VCi μL/L
7/14

10. Trends in Particle Size (in terms of D50A) in the Upper Water Column of the York
*Different Symbols/colors represent different cruises
A. TSS versus D50A
TSS (mg/L)
D50A (microns)
D50A increases as TSS increases
8/14

11. D50A increases as organic content decreases
Trends in Particle Size (in terms of D50A) in the Upper Water Column of the York
*Different Symbols/colors represent different cruises
A. TSS versus D50A
B. Organic Fraction versus D50A
TSS (mg/L)
Organic Fraction
D50A (microns)
D50A (microns)
D50A increases as TSS increases
D50A increases as organic content decreases
8/14

12. Trends in Particle Size (in terms of D50A) in the Upper Water Column of the York
*Different Symbols/colors represent different cruises
A. TSS versus D50A
B. Organic Fraction versus D50A
C. Effective Density versus D50A
TSS (mg/L)
Organic Fraction
ρa (kg/m2)
D50A (microns)
D50A (microns)
D50A (microns)
D50A increases as TSS increases
D50A increases as organic content decreases
D50A increases as ρa decreases
8/14

13. Generally, Smaller particles tend to be more organic and denser.
Trends in Particle Size (in terms of D50A) in the Upper Water Column of the York
*Different Symbols/colors represent different cruises
A. TSS versus D50A
B. Organic Fraction versus D50A
C. Effective Density versus D50A
TSS (mg/L)
Organic Fraction
ρa (kg/m2)
D50A (microns)
D50A (microns)
D50A (microns)
D50A increases as TSS increases
D50A increases as organic content decreases
D50A increases as ρa decreases
Generally, Smaller particles tend to be more organic and denser.
8/14

14. Characterization Light Attenuation in the Upper Water Column of the York
attenuation = amount of light lost through either absorption or scattering .
Beam Attenuation Coefficient ( beam c) from LISST (laser=670 nm)
Calculated from transmission, T and instrument path length, x (LISST x=0.05m):
Absorbed
Light
Scattered
Light
Initial
Intensity, Io
Final Intensity, IT
Light Source:
670 nm Laser
Detector with ° acceptance angle
Transmission is the ratio of the intensity of light reaching a receiver through a sample relative to the light intensity in pure water.
LISST beam c VERY sensitive to scattering
Assume CDOM is not absorbing at 670 nm x
9/14

15. Characterization Light Attenuation in the Upper Water Column of the York
attenuation = amount of light lost through either absorption or scattering .
Beam Attenuation Coefficient ( beam c) from LISST (670 nm)
Calculated from transmission, T and instrument path length, x (LISST x=0.05m):
Assumed loss
Scattered
Light
Absorbed
Light
Initial
Intensity, Io
Final Intensity, IT
Light Source:
670 nm Laser
Detector with ° acceptance angle
Transmission is the ratio of the intensity of light reaching a receiver through a sample relative to the light intensity in pure water.
LISST beam c VERY sensitive to scattering
Assume CDOM is not absorbing at 670 nm x
*LISST beam c EXTREMELY SENSITIVE to scattering
Small acceptance angle
9/14

16. Characterization Light Attenuation in the Upper Water Column of the York
attenuation = amount of light lost through either absorption or scattering .
Beam Attenuation Coefficient ( beam c) from LISST (670 nm)
Calculated from transmission, T and instrument path length, x (LISST x=0.05m):
Assumed loss
Scattered
Light
Absorbed
Light
Initial
Intensity, Io
Final Intensity, IT
Light Source:
670 nm Laser
Detector with ° acceptance angle
Transmission is the ratio of the intensity of light reaching a receiver through a sample relative to the light intensity in pure water.
LISST beam c VERY sensitive to scattering
Assume CDOM is not absorbing at 670 nm x
*LISST beam c EXTREMELY SENSITIVE to scattering
Small acceptance angle
The effect of water is removed during LISST calibration.
CDOM (and most) Absorption minimal at 670 nm
9/14

17. LICOR: Measures Scalar Ed
Characterization Light Attenuation in the Upper Water Column of the York
attenuation = amount of light lost through either absorption or scattering .
Vertical Diffuse Attenuation of PAR, Kd :
Change of downward irradiance (Ed) with depth (z) measured by either Radiometer or LICOR
Most popular parameter used to characterize systems the availability of photosynthetic useful radiant energy ( nm).
Example Ed from 10/26/15
LICOR: Measures Scalar Ed
(Almost angles)
TRIOS: Measures Ed
from 0-180°
Transmission is the ratio of the intensity of light reaching a receiver through a sample relative to the light intensity in pure water.
“Best single parameter by means to characterize systems by the availability of photosynthetic useful radiant energy.”
The linear regression coefficient (i.e. slope) of the logarithm of deck-corrected Ed with respect to depth yields Kd (Kirk, 1994).
10/14

18. LICOR: Measures Scalar Ed
Characterization Light Attenuation in the Upper Water Column of the York
attenuation = amount of light lost through either absorption or scattering .
Vertical Diffuse Attenuation of PAR, Kd :
Change of downward irradiance (Ed) with depth (z) measured by either Radiometer or LICOR
Most popular parameter used to characterize systems the availability of photosynthetic useful radiant energy ( nm).
Example Ed from 10/26/15
Kd captures absorbance (Water, CDOM, Particles), and scattering (Particles).
LICOR: Measures Scalar Ed
(Almost angles)
TRIOS: Measures Ed
from 0-180°
Transmission is the ratio of the intensity of light reaching a receiver through a sample relative to the light intensity in pure water.
“Best single parameter by means to characterize systems by the availability of photosynthetic useful radiant energy.”
The linear regression coefficient (i.e. slope) of the logarithm of deck-corrected Ed with respect to depth yields Kd (Kirk, 1994).
10/14

19. As TSS increases up estuary, both beam c and Kd increase.
Relationship between Attenuation and TSS in the Upper Water Column of the York
A. Beam c versus TSS
Kdm-1
TSS (mgL-1)
B. Kd versus TSS
beam c m-1
TSS (mgL-1)
As TSS increases up estuary, both beam c and Kd increase.
*Different Symbols/colors represent different cruises
11/14

20. So what about the particles?
Relationship between Attenuation and Area in the Upper Water Column of the York
A. Beam c versus AT
B. Kd versus AT
beam c m-1
Kd m-1
AT (cm2/L)
AT (cm2/L)
Attenuations are explained better by AT than TSS. Beam c less noisy than Kd due Influence of absorption (CDOM, Water, Particles)..
So what about the particles?
*Different Symbols/colors represent different cruises
12/14

21. Relationship between Attenuation and Area in the Upper Water Column of the York
A. Beam c versus AT
B. Kd versus AT
beam c m-1
Kd m-1
AT (cm2/L)
AT (cm2/L)
CDOM is unlikely to absorb at 670 nm, and effect of water is removed during LISST calibration so
cp ≈ beam c.
*Different Symbols/colors represent different cruises
12/14

22. Kd captures absorbance (400-700 nm), so it’s a bit more complicated.
Relationship between Attenuation and Area in the Upper Water Column of the York
A. Beam c versus AT
B. Kd versus AT
beam c m-1
Kd m-1
AT (cm2/L)
AT (cm2/L)
CDOM is unlikely to absorb at 670 nm, and effect of water is removed during LISST calibration so
cp ≈ beam c.
Kd captures absorbance ( nm), so it’s a bit more complicated.
*Different Symbols/colors represent different cruises
12/14

23. KdDISS varies as a function of salinity.
Relationship between Attenuation and Area in the Upper Water Column of the York
beam cp m-1
A. Beam c versus AT
AT (cm2/L)
B. Kd versus AT
Kd m-1
ployfit=least squares
Based on change in intercept due to salinity, best-fit equation for KdDISS for the York:
KdDISS= *Salinity
AT (cm2/L)
Find KdDISS (CDOM +water) using intercept between Kd vs AT with the following assumptions:
At AT=0, Kd= KdDISS
KdDISS varies as a function of salinity.
Freshwater is the main source of CDOM
Analysis showed shift in intercept as a function of salinity.
*Different Symbols/colors represent different cruises
12/14

24. *Still see influence due to absorption by particles
Relationship between Attenuation and Area in the Upper Water Column of the York
A. Beam c versus AT
B. Kdp versus AT
Kdp m-1
beam cp m-1
ployfit=least squares
AT (cm2/L)
AT (cm2/L)
cp ≈ beam c
KdDISS = *Salinity
Kdp=Kd-KdDISS
*Still see influence due to absorption by particles
*Different Symbols/colors represent different cruises
13/14

25. Cp not very sensitive to density while Kdp increases with density.
Controls on Attenuation Efficiency (Beam cp/AT and diffuse Kdp/AT): Influence of density (ρa)
If you take out the effect of area, what other properties influence beam cp and Kdp?
B. Kdp efficiency versus ρa
A. Beam cp efficiency versus ρa
cp /AT
Kdp /AT
Be sure to say why this matters!!!
conceptually this is consistent with the idea that as particles become more opaque they absorb more light.
ρa (kg/m3)
ρa (kg/m3)
Cp not very sensitive to density while Kdp increases with density.
Higher density particles absorb more light per area increasing Kdp/AT. But high and low density particles scatter the same amount of light per area, so cp/AT remains constant.
*Different Symbols/colors represent different cruises
14/15

26. Conclusions
Observations found small, organic particles suggested by Gallegos et al., Preliminary results from the York indicate importance of these small, more organic particles on total particle area concentration (AT).
Attenuations coefficients (beam c and Kd) are better explained by total particle area concentration (AT) than total suspended solids (TSS). C less noisy than Kd.
Beam attenuation due to particles (beam cp) was not sensitive to density but was strongly proportional to total particle area (AT). Beam cp (scattering) is controlled mostly by particle area (Bowers et al., 2011; Neukermans et al.,2012).
Particle Diffuse Attenuation Coefficient (KdP) increased more strongly with density than beam attenuation (beam cp). Kd PAR (absorption and scattering) is influenced by area and density (Bowers et al., 2011).
Future work will include (i) many more sampling cruises and (ii) Further analysis incorporating other proxies to strengthen estimates of Kdp (iii) Further analysis of spatial and temporal trends in floc properties and water clarity .
14/14

27. Acknowledgements Tim Gass Wayne Reisner Funding: Jarrell Smith
Jennifer Stanhope
Steve Synder
Erin Shields
Danielle Tarpley
Hunter Walker