What the Mahogany Tide Means for the Bay

Dead fish_mahoganytide2020_RobBeachCBF_1171x593

Dead fish appeared in Valentine Creek near the Severn River in Maryland June 3. Tidal rivers in the mid-Chesapeake Bay have been experiencing a mahogany tide, which can deplete oxygen levels in the water when the bloom dies and decomposes.

Rob Beach / CBF Staff

We talk with CBF’s Maryland Senior Scientist Doug Myers to explain what's behind the rust-colored water in the mid-Bay's tidal rivers and what it means.

Over the last week, reports of rust-colored water and dead fish in the tidal rivers of the mid-Chesapeake Bay have appeared in the news. These are signs of a naturally-occurring phenomenon, called a mahogany tide, that is exacerbated by human sources of nutrient pollution that cause excessive levels of nitrogen and phosphorus to wash into the Bay and its tributary rivers.

We asked CBF's Maryland Senior Scientist Doug Myers to explain what a mahogany tide is, why it occurs, and what it means for the health of the Bay. 

Why are we seeing a mahogany tide in the Bay now?

It is a common occurrence in the spring as the water warms up. This year's bloom is especially widespread. It usually pops up in one or more tributaries at a time as the conditions for its rapid reproduction (salinity and temperature) occur. But it popped up in quite a few tributaries this year after a prolonged period of clear water and expanding seagrass beds.

Where is it in the Bay and how long might it last?

This year's tide has been seen from Back River to the Potomac River on the Western shore and the Chester and Wye rivers on the Eastern shore. The most severe outbreaks seem to be in the Severn River and Magothy River, where fish kills are occurring. It has already dissipated from tributaries with good tidal flushing, like the West and Rhode rivers, but will take longer to dissipate where residence time for the water is longer.

How do mahogany tides affect fish, wildlife, and humans? 

While not a direct threat to humans, fish and invertebrates caught in a mahogany tide are at first irritated by the large number of cells in the water. As the bloom persists, the decomposition of dying cells depresses the level of dissolved oxygen, killing fish. If the bloom lasts a long time, the shading effect can also kill underwater grasses, which could add to the decomposition problem locally.  

What does the mahogany tide mean about the health of the Bay?

Where mahogany tides happen, the affected tributary can see discolored water, dead fish, foam, and the smell of decomposition that could last for several weeks.  Eventually, the bloom will subside. The mainstem of the Bay and saltier areas to the South should not be affected.  Human sources of nutrient pollution cause blooms to be more severe and last longer than they would normally. It is therefore critical to continue and accelerate implementation of the Chesapeake Clean Water Blueprint to reduce pollution.

While no one likes to see a bloom, it is important to note that this year's bloom follows a prolonged period of clear water and expanding underwater grass beds. Efforts to reduce pollution in accordance with the Chesapeake Clean Water Blueprint—the historic federal-state plan to restore the Bay watershed—are working. Over time, underwater grasses are becoming more resilient and the Bay's dead zone—a seasonal area of low oxygen caused by algal blooms—is getting smaller. But as the mahogany tide reminds us, the recovery is fragile, and the road to finishing the job is steep, underscoring the urgent need to continue pushing forward with implementation of the Blueprint across the watershed.

Aerial view of Maryland's Severn River shows the river is rust-red.

A mahogany tide can be seen along the Severn River in June, 2020.

Rob Beach

Codi Kozacek-90x110

Codi Yeager

Senior Writer, CBF

cyeager@cbf.org

Issues in this Post

Dead Zones   Algal Blooms   Fisheries   CBF in Maryland  




DISCLAIMER

PLEASE READ OUR TERMS OF USE

The views and opinions expressed in the media, articles or comments on this site are those of the speakers or authors and do not necessarily reflect the views and opinions held by CBF and the inclusion of such information does not imply endorsement by CBF. CBF is not responsible for the contents of any linked Web, or any link contained in a linked Web site, or any changes or updates to such Web sites. The inclusion of any link or comment is provided only for information purposes. CBF reserves the right to edit or remove any comments and material posted to this website and to ban users from the site without notice. Partisan, pornographic or other inappropriate content, product or service promotion, foul language or bad behavior is expressly forbidden and will be removed.


Support the Chesapeake Bay Foundation

Your donation helps the Chesapeake Bay Foundation maintain our momentum toward a restored Bay, rivers, and streams for today and generations to come.

Donate Today

Stay Up-to-Date on Bay News

Want to stay up-to-date on all news and happenings in your region and across the Chesapeake watershed? Join our digital community.

Sign Up
x
This website uses cookies to tailor and enhance your online experience. By continuing, you are agreeing to our use of cookies. For more information, including details on how to disable cookies, please visit our Privacy Policy. Close