From toes to top-of-atmosphere: Fowler’s Sneaker Depth index of water clarity for the Chesapeake Bay

Abstract

Fowler's Sneaker Depth (FSD), analogous to the well known Secchi disk depth (Zsd), is a visually discerned citizen scientist metric used to assess water clarity in the Patuxent River estuary. In this study, a simple remote sensing algorithm was developed to derive FSD from space-borne spectroradiometric imagery. An empirical model was formed that estimates FSD from red-end remote sensing reflectances at 645 nm, Rrs(645). The model is based on a hyperbolic function relating water clarity to Rrs(645) that was established using radiative transfer modeling and fine tuned using in-water FSD measurements and coincident Rrs(645) data observed by NASA's Moderate Resolution Imaging Spectroradiometer aboard the Aqua spacecraft (MODISA). The resultant FSD algorithm was applied to Landsat-8 Operational Land Imager data to derive a short time-series for the Patuxent River estuary from January 2015 to June 2016. Satellite-derived FSD had an inverse, statistically significant relationship (p<0.005) with total suspended sediment concentration (TSS). Further, a distinct negative relationship between FSD and chlorophyll concentration was discerned during periods of high biomass (> 4 μg L^-1). The complex nature of water quality in the mid-to-upper Chesapeake Bay was captured using a MODISA-based FSD time series (2002-2016). This study demonstrates how a citizen scientist-conceived observation can be coupled with remote sensing. With further refinement and validation, the FSD may be a useful tool for delivering scientifically relevant results and for informing and engaging local stakeholders and policy makers.

Clarifying water clarity: A call to use metrics best suited to corresponding research and management goals in aquatic ecosystems

“…Z SD is also relevant for sighted animals and their trophic interactions, such as visual foraging efficiency of zooplankton and fish (Aksnes 2007;Aksnes et al 2004;Goździejewska and Kruk 2022) and interactions between predators and mesopredators (Benfield and Minello 1996;Baptist and Leopold 2010;Lunt and Smee 2014;Reustle and Smee 2020). Z SD also serves an important role in citizen science and community engagement (Crooke et al 2017;Pitarch 2020) and in maintaining especially long time-series (Jassby et al 2003;Opdal et al 2019).…”

Applications of GIS and remote sensing in public participation and stakeholder engagement for watershed management

“…Satellite imagery have been successfully applied to monitor inland and coastal waters for more than 50 years (Anding & Kauth, 1970;Seyhan et al, 1986;Ferrari et al, 1996;Handcock et al, 2006;Schaeffer et al, 2012). They have been used to monitor stakeholder driven indicators such as Fowler's Sneaker Depth index, and index analogous to Secchi disk depth, which is a citizen scientist metric used to assess water clarity in the Chesapeake Bay (Crooke et al, 2017). Commercially available data like Planetscope and Worldview provide spatial details on less than a meter resolution on a daily to even sub-daily scale.…”

Long‐Term Trends in Chesapeake Bay Remote Sensing Reflectance: Implications for Water Clarity

“…The red-band Rrs approach has historically been the most widely used satellite-derived index of water clarity for estuaries having similar turbidity conditions as the Chesapeake Bay (Crooke et al, 2017;Constantin et al, 2016;DeLuca et al, 2018;Hudson et al, 2017;Ondrusek et al, 2012;Stumpf & Pennock, 1989;Yunus et al, 2021). In moderately turbid estuaries like the Chesapeake Bay, red wavelengths (∼640-660 nm) are most commonly used to estimate suspended sediments or TSS concentrations due to the relatively lower influence of phytoplankton and CDOM in this red portion of the visible spectrum (Tzortziou et al, 2007).…”

“…Of the past approaches for estimating Chesapeake Bay water clarity from remote sensing, the most widely used method involves the MODIS high-resolution land band at 250 m resolution for the 645 nm wavelength (Aurin et al, 2013;Crooke et al, 2017;DeLuca et al, 2018;Hasan & Benninger, 2017;Ondrusek et al, 2012;Yunus et al, 2021). In the present study, we consider more wavelengths and band ratios in addition to this widely used MODIS red-band approach.…”

“…Satellite estimates have been established for many water quality variables using multiple algorithms for the Bay, and retrieval of those variables has been well-documented; however, these data are not often used to answer science questions about temporal trends in the mainstem Bay. Change over time has been heavily studied using in situ cruise observations, yet analysis of long-term change in satellite-derived clarity indices has been limited to red-to-green Chl-a (Le et al, 2013), briefly noted for Rrs(645)-based clarity estimates (Crooke et al, 2017), and analyzed for Rrs(555) in a specific mid-estuary region of the Bay (Gallegos et al, 2011). Past studies of long-term trends in satellite-derived water quality, though useful, only extended to the early 2010s, missing the recent years characterized by continued improvements in watershed management.…”