Food web dynamics are integral to reef ecosystems

Miami HeraldJanuary 30, 2014 

Yellowhead jawfish live in small groups on the sandy or rubble bottom near coral reefs.

TIM GROLLIMUND

Lately (since the reef shark encounter in August) we have been diving Logan's Run a lot. We have a route that takes us out to the spot we frequently see the shark, all the way up past the nice elkhorn coral stand near Hole in the Wall, back through the Winch Hole, then out to the ledge for a straight shot back to the boat.  

It usually takes about 80 minutes for this route, and it's a healthy kick, covering a lot of area.

One of my favorite spots is not the winch itself, but the flat rubble area around the winch. I stop there to watch the yellowhead jawfish do their dance. It's one of those little entertaining tidbits along the route. Somebody is always doing maintenance and moving pebbles around their cubbyhole.

I frequently wonder what else happens on the bottom of the reef. I have not spent a lot of time reading about bottoms (of the reef, I mean), so I needed some help. I was fortunate to have an informative interview with Dr. Amy Hirons of Nova Southeastern University. One of her areas of focus is food web dynamics. So now I have another pile of scientific papers to read. It's ok, since there was no football this past weekend. She made a few slides for me with some definitions and diagrams, which you can see here: http://timgimages.com/PDF/Hirons_FoodWeb.pdf

As you look at her slides, it is important to note that a food web is much more complex than a food chain. Food webs are tracing the flow of energy as it is consumed from the lower, microscopic level all the way up to apex predators.

A note of clarification needs to be injected here. In the Keys, when I see the term "artificial reef," I think big, as in retired vessel big (Spiegel Grove, Duane, Bibb, Vandenberg).  Her studies concentrate on much smaller sizes, like patch reefs and reef ball groupings, not the gargantuans in the Keys.

Dr. Hirons has a project going with Broward County to compare food webs among limestone boulder artificial reefs, natural reefs and the adjacent sand bottom. Her study will assemble observations on food web variations among these habitat types, using stable isotope analysis. These can be used to differentiate marine food webs by identifying the sources of organic matter at the base of each web, and tracing those isotope values up through the subsequent trophic levels.

According to Dr. Hirons, "Little is known about the role artificial reefs have in the trophic dynamics of reef fish. If our management objective is to help by biologically enhancing selected fishery stocks, then understanding the role of organisms recruited to artificial reefs and the surrounding infaunal organisms in the food web is of critical importance."

She wants to find out how effective artificial reefs are, relative to natural reefs and existing sand bottom, for the enhancement of fisheries for key species like hogfish, mutton snapper, red grouper and spiny lobster. It's a very complicated equation since local conditions, water movements and sedimentation may change frequently. Some species forage on the sandy bottom, and placing an artificial reef there may alter their foraging behavior. That could have implications for both commercial and recreational fishers, as well as for placement of future structures.

To get a grip on this food web thing, I had to go back to the basics. I found a few studies on food web theory and structure that helped me understand these relationships. In a study led by Jennifer Dunne (http://timgimages.com/PDF/NetworkStructure_Dunne.pdf), they examined marine food web characteristics relative to estuarine, fresh water and terrestrial food webs. All the webs have similar structural and ordering characteristics that can be compared across food web types.

They point out that most food web properties are scale-dependent, changing as biodiversity and complexity change. They constructed a model using species richness and connectance to simulate the potential effects (type and magnitude) of species loss in triggering cascading effects on other species. "For marine ecosystems, historically subject to intense fisheries pressure and subsequent collapse, more detailed knowledge of the complex network of trophic relationships that encompass species of economic interest will be important for guiding more sustainable policy."

Perhaps the most relevant takeaway from the Dunne study was the finding "that effects from perturbations, such as overfishing, can be transmitted more widely throughout marine ecosystems than previously appreciated." To me that means that extractive efforts that are deemed as acceptable practices may be more intrusive and detrimental to the ecosystem than presently calculated. How do we learn to tweak the sustainability equation so commercial and recreational fishers have robust populations to catch?

Amy told me they are looking at samples from the pelicans in the Keys to learn more about what is going on. Starving pelicans, and recently, starving pilot whales in the Everglades, indicate to me that something more fundamental is happening, and the implications have a temporal component I do not understand. And as you may recall, a couple years ago the spotted eagle rays that were beached up the west coast of Florida were also starved.

How is it all connected? How long does it take for these connections to break down? Something to do with food, all webbed together, starting at the microscopic, sandy bottom level? Hopefully Amy and her colleagues can begin to shed some light on these complex relationships.

Tim Grollimund is a freelance photographer and PADI divemaster based in Key Largo.  He can be reached at tim@timgimages.com.  Tim is a member of the Florida Keys National Marine Sanctuary Advisory Council.  Opinions expressed by Tim are not the official views of the FKNMS.

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