Travis Graves: Mahogany tide

Have you noticed the large patches of soapy foam on the Neuse recently, or the long brown streaks snaking across the water on calm days? This isn’t pollution, it’s an algae bloom known as the mahogany tide.

The term algae bloom is all too often followed by the first stirrings of panic in anyone that spends time in, on, or around the water. This fear is not entirely unwarranted as stories of toxic blue-green algae and red tides have gotten significant press in the last few years.

You may recall the contamination of drinking water in Ohio this past fall, due to high levels of nutrients and pollution responsible for a toxic blue-green algae bloom that shut down water supplies to more than half a million people, or the harmful algae blooms that occur off of Gulf Coast of Florida nearly every summer referred to as “red tides” because they turn the coastal waters a deep red color.

Harmful algae blooms, or HAB’s can do more than just discolor the water, they can produce toxins that kill fish, make shellfish harmful to eat, and can even make the air difficult to breath. Fortunately these blooms are exceptions, and not the rule.

Algae blooms occur in all coastal states and the majority of them are not only harmless, but the microscopic plants they consist of are an intricate part of the ecosystem providing an abundant and important food source for copepods, tiny members of the crustacean family, and shellfish. Right now, the Lower Neuse is experiencing just this sort of bloom.

The algae bloom occurring in the Lower Neuse is referred to as a mahogany tide due to the brown color it causes in waterways, and is primarily a dinoflagellate called Prorocentrum minimum which is the most common bloom former in the Neuse and the Newport rivers during the late fall through early spring period, according to Nathan S. Hall, Ph.D., Research Associate at UNC-Chapel Hill Institute of Marine Sciences in Morehead City.

This mahogany tide comes to the Neuse River like clockwork every year due to a confluence of several different conditions. R Wayne Litaker, Research Fisheries Biologist, Center for Coastal Fisheries and Habitat Research, National Ocean Service, National Oceanic & Atmospheric Administration, explained to me that the reason the blooms occur this time of the year is because, even though growth conditions are not ideal (they can grow much faster at higher temperatures), this is the time of the year when the copepods which are the primary consumers of phytoplankton are at their lowest concentrations. This means that the cold wintertime temperatures provide conditions where they can escape predators and grow long enough to develop very high cell densities.

This is very fascinating at a microscopic level but the mahogany tide causes dramatic changes to the river that can be observed with the naked eye as well. I have been contacted by numerous concerned citizens reporting what appeared to be a massive discharge of raw sewage, or large amounts of soapy looking foam blowing onto shorelines. This is because our friend Prorocentrum minimum secretes an organic compound that turns the water brown, and in windy conditions gets whipped up into a nasty looking foam that coats anything on the water and looks suspiciously like someone’s washing machine has exploded. Rest assured though, this bloom is not toxic and has never been linked to any human health problems.

There is one aspect of the mahogany tide that concerns me, though, which is the question of where all of this organic matter goes after the bloom collapses, usually some time between January and March.

Studies have shown that it is a good source of food for shellfish and it is assumed that much of it is eaten or respired, but it is a concern that what is not eaten can accumulate in the sediments and contribute to low oxygen levels in the water later in the warm summer months as decaying matter uses up the oxygen.

This is of particular interest because in the Neuse we already have a serious issue with low oxygen levels due to excessive levels of nitrogen and phosphorus, mostly from industrial meat production, or CAFOs, and excessive chemical fertilizers being conveyed to our public waters by stormwater runoff. These nutrients promote the growth, and eventual decay of organic matter that robs the waters of precious oxygen and trigger the fish kills that we are all too familiar with.

Fortunately, in this case, we don’t have to raise the alarms. What we are seeing is a natural part of life in the Neuse River. Our annual mahogany tide is an interesting dichotomy, one would assume that the colder months would slow down activity below the surface of our waters, but in this case it is just the right set of circumstances to allow for one aspect of life to thrive. Another example of the fascinating, and ever-changing ecosystem of the Lower Neuse River.


Photo: A microscopic view of the dinoflagellate Prorocentrum minimum, the ‘D’ shaped cells, which cause the mahogany tide every winter. Contributed photo/UNC-Chapel HIll Institute of Marine Sciences

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