Ecological Engineering on the West Florida Shelf Edge

Posted on July 26, 2010 by Chris Koenig, Research Associate, Florida State University

Mini biodiversity hotspots pockmark the seafloor of the shelf edge of northwest Florida, appearing like oases in an otherwise featureless sand bottom. These hotspots are rock-filled depressions approximately 15 feet in diameter and 6 feet deep. The rocks are covered with sponges, corals, anemones, crustaceans, and algae, and the depressions are teeming with small fishes in a wide variety of colors and forms. What created these oases? How are they maintained? How old are they? Those were some of the questions my colleagues and I asked ourselves almost 10 years ago when we first discovered them in our initial surveys of Steamboat Lumps Marine Reserve.

Photo of Johnson Sea-Link.

The importance of keeping water out of the JSL’s sphere cannot be overstated, whether the vehicle is on the deck or diving 3,000 feet below the surface. Frank Lombardo holds a large golf umbrella over the hatch while scientist Chris Koenig and pilot Phil Santos disembark.

teamboat Lumps Reserve is located about 100 miles west of Tampa. It was established by the Gulf of Mexico Fishery Management Council and the National Marine Fisheries Service in 2000, along with Madison Swanson Reserve, which is about 80 miles to the northwest of Steamboat. The reserves were established to protect spawning groupers, which are abundant on the West Florida shelf edge, but highly vulnerable to fishing pressure. U.S. Geological Survey geologist, Kathy Scanlon, provided the initial sonar-generated maps of the seafloor of both reserves, allowing us to locate a variety of habitats in each of the 100-square mile protected areas. At that time, we saw clusters of bright spots which reminded Kathy of gas seeps, common in the Gulf of Mexico. But these spots were different from gas seeps in one important respect—they were uniform in size whereas gas seeps are typically variable in size.

We got our chance to explore these sonar-generated bright spots in 2001 using a submersible called Deepworker, as part of the NOAA-sponsored Sustainable Seas Expedition. That is when we discovered the oases, but it wasn’t until this year, 2010, that we understood these formations well enough to publish the results of our studies. What we found was that red grouper created these oases-like habitats by digging the sand with their mouths, thereby exposing rocks imbedded in the sand and creating reef habitat for themselves and for many other reef fish in the region. Not only did they dig the depressions or pits, but they maintained them by sweeping sediment off rocks with their tails and carrying off dead shells and crustacean exuviae. This constant clearing of sediment and debris off rocks apparently provides an ideal settlement surface for invertebrates, and the exposed rocky reef provides cover for small fishes. Because the grouper construct habitat used by many other species, their activities are called “ecological engineering”. They are termed “keystone species” because without them the oasis-like habitat would be lost along with habitat for myriad other species. Our scientific report on this subject can be downloaded from The Open Fish Science Journal.

Photo of red grouper.

The red grouper shares its den with smaller fish species. Chris Koenig of Florida State University is interested in determining the reason for the burrows and the relationship between the grouper and the many “tenant” species that take up residence with the large landlord.

In our Florida Shelf Edge Exploration (FLoSEE) Expedition, we revisited the red grouper excavations in Steamboat Lumps Reserve with the Johnson Sea Link II (JSL II) manned submersible to gain further insight into the structure and function of these natural engineering projects and to study the possible impacts of the Deepwater Horizon oil spill on them. At this point, the protected grouper habitat in Steamboat Lumps looks healthy and oil-free. There are numerous species of fishes and invertebrates associated with the grouper excavations. Dominant fishes include bank seabass, bank butterflyfish, tomtates, yellowtail reeffish, cardinalfish, juvenile vermilion snapper, wrassebasses, and greenband wrasses. Within each pit, we saw a cleaning station – distinct sites where several species of shrimp solicit cleaning services involving the removal of external parasites from fishes. The cleaner shrimp are associated with anemones and likely provide their services to the red grouper exclusively.

Our concern is that the red grouper pits will accumulate dense oil and sediment particles and that the oil contaminants will be taken up through the gills of the fish as they transfer sediment with their mouths. So, on this cruise we have collected sediment and fish samples from the pits as a before-contamination sample and will sample again if the oil eventually contaminates the West Florida Shelf Edge. These data will allow us to estimate unseen damage resulting from the oil spill to the grouper engineers, their habitat dependents, and the important fisheries that depend on them.