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

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). 1/14

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

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

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 2013-2015 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

Coastal Hydrodynamics and Sediment Dynamics (CHSD) Water Column Profiler Pump sampler: Total suspended solids (TSS) and organic content LISST-100X: Particle Sizes ~2.5-500 μm Beam attenuation (beam c) from optical transmission Particle Imaging Camera System (PICS): Particle Sizes ~30-1000 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 2-500 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

Characterization of Particle Size (Area and Volume) with the LISST and PICS LISST (2.5-500 μm) uses laser diffraction to determine VC in size class i (VCi) then PICS (30-1000 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

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

*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

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

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

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

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

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 0.0269 ° 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

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 0.0269 ° 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

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 0.0269 ° 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

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 (400-700 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

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 (400-700 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

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

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

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

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 (400-700 nm), so it’s a bit more complicated. *Different Symbols/colors represent different cruises 12/14

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=1.67 - 0.0667*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

*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 = 1.67 - 0.0667*Salinity Kdp=Kd-KdDISS *Still see influence due to absorption by particles *Different Symbols/colors represent different cruises 13/14

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

Conclusions Observations found small, organic particles suggested by Gallegos et al., 2011. 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

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