Archive for water clarity

Want to improve the spatiotemporal coverage of coastal water clarity? This approach combines high-resolution satellite data with low-cost in situ methods

Posted by mmaheigan 
· Friday, December 1st, 2023 

To maintain marine ecosystem health and human well-being, it is important to understand coastal water quality changes. Water clarity is a key­ component of water quality, which can be measured in situ by tools such as Secchi disks or by satellites with high spatial and temporal coverage. Coastal environments pose unique challenges to remote sensing, sometimes resulting in inaccurate estimates of water clarity.

Figure caption: Maps of model-corrected Landsat-8 derived Secchi depths from monthly clear sky images (2019–2021).

In this study, we couple low-cost in situ methods (Secchi disk depths) with open-access, high-resolution satellite (Landsat-8 and Sentinel-2) data to improve estimates of water clarity in a shallow, turbid lagoon in Virginia, USA. Our model allows the retrieval of water clarity data across an entire water body and when field measurements are unavailable. This approach can be implemented in dynamic coastal water bodies with limited in situ measurements (e.g., as part of routine water quality monitoring). This can improve our understanding of water clarity changes and their drivers to better predict how water quality may change in the future. Improved water clarity predictions can lead to better coastal ecosystem management and human well-being.

Figure caption: Workflow for obtaining Secchi disk depth with l2gen in NASA SeaDAS, bio-optical algorithms, and empirical adjustments.

Authors
Sarah E. Lang (University of Rhode Island’s Graduate School of Oceanography)
Kelly M.A. Luis (Jet Propulsion Laboratory, California Institute of Technology)
Scott C. Doney (University of Virginia)
Olivia Cronin-Golomb (University of Virginia)
Max C.N. Castorani (University of Virginia)

 

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Counterintuitive effects of shoreline armoring on estuarine water clarity

Posted by mmaheigan 
· Wednesday, February 24th, 2021 

Around the world, human-altered shorelines change sediment inputs to estuaries and coastal waters, altering water clarity, especially in areas of dense human population. The Chesapeake Bay estuary is recovering from historically high nutrient and sediment inputs, but water clarity improvement has been ambiguous. Long-term trends show increasing water clarity in terms of deepening light attenuation depth, yet degrading clarity in terms of shallowing Secchi depth over time. High water clarity is needed to support seagrass meadows, which act as nursery habitats for commercially important fish species such as striped bass. How are these opposing water clarity trends possible?

In a recent paper published in Science of the Total Environment, researchers performed experiments with a coupled hydrodynamic-biogeochemical model to test a simulated Chesapeake Bay under realistic conditions, more shoreline erosion, and highly armored shorelines. Comparing the two extreme conditions (Figure 1), there was a striking difference between (a) an estuary experiencing more shoreline erosion and greater resuspension versus (b) a highly armored estuary with decreased resuspension. Reduced erosion yielded improved water clarity in terms of light attenuation depth, but a shallower Secchi depth (reduced visibility). In estuaries, reducing sediment inputs is often proposed as a strategy for improving water quality. This study shows that, under certain conditions in a productive estuary, reduced sediments can have unintended secondary effects on water clarity due to enhanced production of organic particles. This study also highlights the need to consider other sediment sources in addition to rivers, such as seabed resuspension and shoreline erosion, especially at times and locations of low river input.

Figure 1. Schematic of how shoreline armoring causes deepening light attenuation depth (navy) yet shallowing Secchi depth (green) during the spring growing season in the mid-bay central channel.

Authors:
Jessica S. Turner
Pierre St-Laurent
Marjorie A. M. Friedrichs
Carl T. Friedrichs
(all Virginia Institute of Marine Science)

 

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