Controls on light attenuation and Secchi depth as a function of water column suspended particle properties and other water column constituents: Insights form the York River estuary, Virginia, USA

Improved water clarity is a key management goal given its aesthetic value and the limit poor clarity imposes on aquatic plant growth. Two useful measures of water clarity are (i) the rate at which total light decays with depth, known as the diffuse attenuation coefficient (Kd), and (ii) the depth at which a Secchi disk disappears from view (Zsd), which scales with the transparency of water as seen by the human eye. At lowest order, Zsd is often approximately proportional to 1/Kd, and management criteria for clarity attainment have in the past assumed that the product of Zsd and Kd is a regional constant. However, recent observations and theory has indicated that the product of Zsd and Kd can vary by a factor of two or more as a function of either particle organic content or the overall magnitude of total suspended solids (TSS). The availability of extensive, high quality, management-oriented monitoring data in York River estuary, Virginia, which is a tidal tributary of the Chesapeake Bay, makes it an ideal location to further explore the behavior of Kd and Zsd in response to observed water column constituents.

The Virginia Institute of Marine Science (VIMS), The Chesapeake Bay National Estuarine Research Reserve in Virginia (CBNERRVA), and the EPA Chesapeake Bay Program (CBP) have been monitoring Kd, Zsd, and related water column constituents at various locations along the York since the 1980s. The VIMS/CBNERRVA data set contains > 7500 simultaneous and co-located measurements of TSS, loss on ignition, chlorophyll, and Kd; and available EPA data in the York provide 1200 more analogous observations. Of these 9700 samples, 5900 also have corresponding measuremenst of light absorbance by colored dissolved organic matter, 1500 have Zsd, and at least 300 have turbidity. A recent National Science Foundation study in the York has provided 100 additional stations that also include beam attenuation, suspended particle cross-sectional area, and suspended particle density. The statistical analysis of the above data sets presented here will improve understanding of how particles and other water column constituents degrade estuarine water clarity in terms of both Kd and Zsd and help explain why disparities in Kd and Zsd trends in the Chesapeake Bay region and elsewhere may be occurring.