“Ewwww!”
“Yuck — it’s icky, squishy and smelly!”
“Cool. Can we take off our shoes and play in it?”
These are the typical comments I get the first time I take kids out to the estuarine mud flats for a lesson on the web of life. Scientists refer to it as the “intertidal benthic layer.” To the casual observer it may seem devoid of interest — even a little disgusting — but it’s one of the most productive habitats on the planet and the foundation of our entire food chain.
It is here that the detritus (dead organic matter) of forests and fields accumulates, attracting creatures numbering in the billions per square yard. Each year more than a million tons of sediment wash into the Tampa Bay estuary, mixing with salt water to create the perfect conditions for marshes, mudflats, mangrove swamps and tidal channels — the very cradles of life.
Life in the MUD
Mud flats (also known as tidal flats) are usually found next to salt marshes. They tend to occur along a delta where a river or creek meets the daily tides. As decaying organic detritus is swept into shallow areas, a thick oozy layer builds up when the sediments fall to the bottom. It fuels the growth of microscopic plants and animals. Oysters and clams and other filter feeders cling to nearby mangroves, enjoying the nutrient soup.
At high tide, you’ll find baby fish, shellfish spawn, snails and brittle stars hunting for food. When the tide goes out and the mudflat is exposed, armies of fiddler crabs emerge from their muddy burrows to forage and court. The males have one enlarged claw that they wave around to attract females and ward off the competition. They and their cousins, the ghost crabs and the mud shrimp, dig burrows up to three feet deep across the tidal flats. Clams and worms also excavate tunnels throughout the mud, bringing in oxygen for the microbial communities and allowing nutrients to cycle back into the water.
The mud flats also are important to herons, egrets and other wading birds. Predatory animals like raccoons make their rounds frequently looking for a salty snack. As the flat matures, pioneer plants including the herbaceous species like black needle rush (Juncus roemerianus), salt meadow cord grass (Spartina patens) and smooth cord grass (Spartina alterniflora) move in. As they capture more sediment and the land rises, red and black mangroves take root, culminating in what’s known as a climax community.
Over time, the plants trap additional sediments to strengthen and stabilize the formation, providing habitat and food for larger creatures, as well as protection against storm tides and erosion for the humans who live on nearby uplands.
MUD vs. Muck
Highlighting the importance of mud, the Environmental Protection Commission of Hillsborough County monitors dozens of benthic locations twice a year to track the health and vitality of the estuary. They analyze the type and quantity of tiny creatures that live in the mud. Of the more than 1400 benthic species, worms, mollusks and crustaceans make up the largest populations. Scientists use “indicator species” (see page 13) to determine how much oxygen is available in the mud — a key factor in measuring the health of the ecosystem.
Ed Sherwood, program scientist with the Tampa Bay Estuary Program, explains it this way: “In the areas where light penetrates and the water is moving, the dissolved oxygen levels stay high. Seagrasses can grow, oysters filter sediment, and a wide variety of species thrive. Where we get into trouble is in shallow water when the sediment gets stirred up or lack of movement causes the water column to separate into layers of different temperatures or saline levels. If the light gets blocked too long, an algae bloom may occur, using up all the dissolved oxygen, creating a condition called hypoxia when the sediment falls into deep holes that are immune to tidal influences. No light — no movement — no life.”
Robert Rorebeck, senior environmental technician at Sustany Foundation, recalls road construction projects where digging and dredging disturbed the mudflats around the edge of Tampa Bay, blocking off tidal flow to a nearby salt marsh. “The water quickly became stagnant and in a matter of days, it was covered with a green noxious scum. In the industry, this is no longer mud — it’s muck. When hypoxic muck conditions continue for long without tidal flushing, the nearby mangroves and grasses will eventually start to die.”
Deeper out, channels dredged for boating traffic also become dead zones as they fill with benthic mud. Too deep for sunlight to reach and separated from daily tidal flushing, they are almost lifeless. Repeated dredging must be done carefully because it can release decades of accumulated organic matter, as well as heavy metals and petroleum products into pristine environments, creating a whole new ecological challenge.
Bacteria to the Rescue
Are decades of muck building up in Tampa Bay? I started to wonder how much toxicity from human pollution could be hiding in these areas, so I called Dr. Ernst Peebles, the USF biological oceanographer who’s been involved in researching the impact of the Deepwater Horizon oil spill in the gulf. Are we creating our own “oil spill” with all the toxic wastes we pour down our drains?
“Benthic mud, by its very nature, has a strong affinity for the toxins released from boats, autos and runoff,” he said. “Marinas and commercial boat traffic contribute the largest amount of oil and hydrocarbons but they are far more refined, and probably less toxic than the suite of chemicals dumped in the gulf during the BP accident.”
Despite the number of restoration projects conducted in Tampa Bay over the past 25 years, the US Environmental Protection Agency’s 2007 Coastal Condition report identified the bay’s overall benthic condition as somewhere between fair and poor. Along with hydrocarbon contaminants from petroleum products, heavy metals such as airborne mercury and even household cleaners or prescription drugs can increase water toxicity. When medications are disposed of down the drain, endocrine disruptors are released into the waterway and settle in the mud, ultimately interfering with the hormone systems of marine life. (Note: take your old meds to the Sherrif’s Office to be incinerated. Don’t dump them down the toilet!)
I asked Dr. Peebles if there was a hopeful note in all this. “Well, of course, we can improve things with prevention through increased public awareness. But in the meantime, we need to be thankful for bacteria — they eat just about anything. The microbes are the real heroes doing the work to clean up our mess.”
The Fragrance of MUD
And speaking of bacteria… Did you ever wonder what creates the occasional “Eau de la Bay” aroma along Tampa’s Bayshore Boulevard? Although raw sewage was discharged into the bay decades ago, it hasn’t happened in years — so it’s not that.
Arising in the Green Swamp, the Hillsborough River flows 59 miles before discharging tons of sediment into Hillsborough Bay just below downtown Tampa (in fact, that’s where the land came from to create Davis Islands). Unlike sand, thick mud sediments are not very porous and don’t allow water carrying life-giving oxygen to pass through. The shallow mud flats provide an ideal home for anaerobic bacteria living just below the surface. These bacteria thrive in an environment of low oxygen, high organic material and warm temperatures — all of which are present along the Bayshore on a warm summer’s day at low tide.
As these bacteria convert matter into energy, they produce hydrogen sulfide— the same gas that emanates from rotten eggs. The stench grows more powerful in the summer because gasses released in warmer air move faster and spread more quickly. It’s like cheese — when you put it out, it warms up, and it smells stronger. On a sultry day, mudflats can smell worse than aged Gorgonzola, but console yourself knowing that while these little stinkers are breaking bits of leaves, algae and wood into stinky gasses, they are serving an important ecological role.
It’s that fragrant chemical activity that releases nutrients into the water column where they’re available to other microscopic creatures at the base of the food web.
Play in the MUD
So let’s return to the kids who came to learn about the web of life. After a short bit of instruction and donning the proper footwear, I’ve armed them with benthic sieves and nets and sticks for poking things. And for one amazing afternoon they’ve forgotten about video games and Facebook. They’re actually engrossed in a scavenger hunt for squirmy little creatures. Who’d have thought a bunch of icky mud would provide so much pleasure?
There are a number of organizations and locations where you and your family can learn about benthic mud, and even get some of the heavenly goo between your toes. (See sidebar left.)
While you’re enjoying this new encounter with Mother Earth, take a moment to appreciate the vital role humble mud plays in recycling natural waste materials into new life. As so often is the case, we humans are the greatest threat to the process. Maybe we should all ponder this question a little more: When WE create waste materials and throw something away — exactly where is “away”?
Discover the marvels of mud yourself!
- Grab a net and see what you can find. While many benthic creatures are too tiny to see with your naked eye, lots are large enough to be interesting. Depending on the time of year, some of the creatures you may find in Tampa Bay include crabs, clams, scallops, starfish or sand dollars. (Steer clear of white sand beaches and look for the bayous and inlets that surround the bay, because more critters live in calm water than in surf.)
- Sign up for the Tampa Bay Watch Great Bay Scallop Search on August 18 or help build oyster beds. Visit www.tampabaywatch.org for times and places.
- Find an expert. Get a group together and hire a naturalist. They’re available through groups like Pinellas County Extension, Lettuce Lake Resource Center, Hillsborough Community College, American Littoral Society and local aquariums.
- Plant a salt marsh with the Southwest Florida Water Management District SWIM program. Marsh grasses stabilize sediment so other benthic organisms can grow. Visit www.watermatters.org.
- Participate in a clean-up. Tampa Bay Estuary Program (www.tbep.org) schedules regular “Give A Day for the Bay” events across the region and local Keep America Beautiful chapters offer ongoing opportunities. Or set up your own family fun day and see who can pick up the most trash in a set amount of time.
- Check out Bay Sounding‘s calendar of events online for more ideas!
Story and illustration by Sigrid Tidmore, originally published Spring 2012