Abstract
AbstractWhile 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.
Highlights
Studying long-term change in water clarity in estuaries is an integral part of assessing improvements from historically polluted conditions
We aim to answer the questions: How have Chesapeake Bay Rrs(λ) and band ratios evolved over the last two decades? What do common and/or distinct trends found among groups of bands and ratios suggest in the context of long-term change with regards to water clarity? Answering these questions will allow us to better understand how major contributions to Chesapeake Bay water clarity, such as band ratios commonly associated with total suspended solids (TSS), Kd, and Chl-a, have changed over the last two decades
Green band Rrs (531–555 nm) showed a smaller magnitude seasonal cycle in the upper Bay in terms of percent change, with maximum in January through April, yet a relatively dynamic seasonal cycle in lower Bay, with larger values in January and October and lower values through the summer months May to August (Figures 3d–3f )
Summary
Studying long-term change in water clarity in estuaries is an integral part of assessing improvements from historically polluted conditions. Some of the world's estuaries have experienced widespread eutrophication and degraded water clarity, such as the Chesapeake Bay, the Baltic Sea, and the Wadden Sea (Cloern, 2001; Dupont & Aksnes, 2013; Kemp et al, 2005; van Beusekom, 2005). Other estuaries have recovered from past degradation and experienced improved water clarity conditions in recent decades, such as Tampa Bay, the San Francisco Bay, Danish estuaries and coastal waters, and the Black Sea (Boesch, 2019; Cloern & Jassby, 2012; Greening et al, 2014; Sherwood et al, 2016).
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