Beads Fall Into Burrows. Can You Dig It?
While we are busying ourselves aboveground, marching around, measuring things, and generally living our terrestrial lives, there is a whole lot of activity going on beneath our feet: an underworld bristling with burrowers, both on land and at sea. This world is fascinating, and we also have a hard time truly comprehending it, or seeing what goes on. Burrowing also has an incredibly important role in ecology.
The term “bioturbator” refers to any animal that moves sediment.
In soft sediments, bioturbators either dig through sediment (like crabs or shrimp) or eat through it, tunneling by eating sediment and detritus and excreting whatever they don’t digest (these are deposit feeders, like sea cucumbers and clams). Bioturbators move water and sediment through their burrows. This can mix the sediment, and expose old, anaerobic material to the surface waters, oxygenating what they dig up and whatever material lines their burrow.
When you want to aerate your garden, you add earthworms. The same thing can happen in the sea: if sediments are anoxic, burrowers can aerate the sediments by digging in them. One of the barriers to this at my study site is the dense root fibers that can block the paths of these burrowers. However, if they do dig more in some areas, those areas are likely to have different oxygen concentrations. Oxygen concentrations will affect how many and what kind of infauna are present. It may also be important in predicting how fast the mangrove roots break down after removal: more oxygenated sediments means breakdown by macrofauna instead of just bacteria, which means faster decomposition.
Therefore, it’s important that we know how active burrowers are at all the sites where we’re studying infauna. Alas, the murky depths of a fishpond’s invasive mangrove sediments cannot be replicated in the lab, and we want to know what’s happening in situ in the field.
One tool that we can use to at least measure how much bioturbation is occurring is a tracer experiment. People use dyes or physical markers for this– I’m using tiny glass beads, so small that each bead is the size of a grain of mud. At the beginning of our predator exclusion experiment in the mangrove, we sprinkled beads in some of the cages. We took sediment cores, just like we did for infauna, but these cores were sectioned at 1 cm intervals. The idea is that when you count the number of beads in each section, you can see how far down they’ve traveled. At the beginning of the experiment, most if not all of the beads should be at the surface, where you just scattered them. If there is a lot of bioturbation going on over time, beads will fall into the burrows and get transported lower in the sediment column, so they’ll show up in deeper sections when you return to your site a few months later. So we take an initial core, a final core, and count how many beads fell how far.
“But how can you count beads that are each the size of a grain of mud?” I am glad to say that instead of picking the beads out from a muddy mess, we can digest away the sediments with acid, and combust samples to remove shells and other organic material. What remains is beads only (and a few pesky sponge spicules)– they can then be counted by weight, or by counting subsamples. What we end up with is a depth profile of the beads in the sediment column. Different depth profiles mean different things. One with lots of beads in just one core section may mean that sediment has come in and layered on top of the beads. If, however, the beads are distributed low and unevenly in the sediment, this may indicate that the beads have fallen into tunnels created by burrowers.
If I may say so myself, this is a simple and elegant way to measure a process that we can’t observe firsthand. In collaboration with Craig Smith‘s lab in the Oceanography department, we’ve already taken our initial cores, and final cores will be collected in September.
Smith, C.R., & Kukert, H. (1996). Macrobenthic Community Structure, Secondary Production, and Rates of Bioturbation and Sedimentation at the Kane’ohe Bay Lagoon Floor Pacific Science, 50 (2), 211-229