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

• Published in: Journal of geophysical research. Oceans Vol. 126; no. 12
• Main Authors: Turner, Jessica S., Friedrichs, Carl T., Friedrichs, Marjorie A. M.
• Format: Journal Article
• Language: English
• Published: 01.12.2021
• Subjects: Chesapeake Bay; estuaries; MODIS‐Aqua; remote sensing oceanography; remote sensing reflectance; water clarity
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2021JC017959

While ecosystem health is improving in many estuaries worldwide following nutrient reductions, inconsistent trends in water clarity often remain. The Chesapeake Bay, a eutrophic estuary with a highly populated watershed, is a crucial testbed for these concerns. Improved efforts are needed to understand why some measurements of downstream estuarine water clarity appear to be uncorrelated with watershed management actions, and multiple metrics of clarity are needed to address this issue. To complement in situ measurements, satellite remote sensing provides an additional tool with which to assess long‐term change in water clarity. In this study, remote sensing reflectance (Rrs) from the Moderate Resolution Imaging Spectroradiometer on satellite Aqua was evaluated from 2003 to 2020 at multiple wavelengths for surface waters of the Chesapeake Bay. Trends show an overall long‐term decrease in Rrs in the upper estuary for all wavelengths, yet an increase in Rrs in the lower estuary for green wavelengths. Trends in band ratios show long‐term decreasing red‐to‐green and red‐to‐blue ratios, yet long‐term increasing green‐to‐blue ratios. Seasonally, trends in band ratios were relatively consistent throughout the year and along‐estuary, whereas single band reflectance trends varied seasonally and along‐estuary. In the lower Bay, Septembers showed the strongest decreasing trends in red reflectance, while early spring and summer had the most pronounced increasing trends in green reflectance. These trends suggest that the system has experienced a long‐term reduction in suspended solids concentration and light attenuation without a systematic reduction in chlorophyll‐a concentration. Plain Language Summary In many estuaries around the world, waters have become less clear over time because of nutrient and sediment pollution. Watershed management efforts on land have reduced the delivery of nutrient and sediment from specific regions in recent decades. Through these efforts, some estuaries are showing improvements such as more seagrass and higher oxygen levels; however, the water does not always appear clearer. The Chesapeake Bay is one such place where not all measurements of water clarity show improvement, even though the ecosystem is recovering. Satellites provide snapshots of surface water color, covering large areas with frequent repeats in time. From 2003 to 2020, we analyzed long‐term trends in surface water reflectance, or the brightness of the light coming from water at specific colors of visible light. We also analyzed the change over time in ratios between reflectance at different colors, or reflectance ratios. Reflectance ratios are often used to estimate common water quality variables, such as light attenuation, chlorophyll pigment concentration, and particle concentration. Our results show that there has been a decrease in red‐to‐green and red‐to‐blue reflectance ratios, suggesting improving water clarity over time in most of the Bay. However, increasing green‐to‐blue reflectance ratios suggest increasing chlorophyll concentration. Key Points Since 2003, MODIS‐Aqua red‐to‐green and red‐to‐blue band ratios decreased, suggesting improving water clarity from the early 2000s to 2020 Green reflectance and green‐to‐blue band ratios increased in the lower Bay, suggesting larger contribution of phytoplankton since 2003 Trends in ratios were more seasonally and spatially (upper vs. lower Bay) consistent than were trends in absolute reflectances