Critters in the Mud
18 October 2010
Kevin Briggs
One of the measureable effects of oil on the environment in the Gulf of Mexico is the health of the organisms living on the sea floor. I am a benthic ecologist/sedimentologist trying to assess what is living in the mud at the zone that is directly impacted by dispersed oil that falls to the bottom: the sediment-water interface. Although there are several size classes of benthic (read: sea bottom) organisms that are affected by exposure to oil, from bacteria to megafauna, I am specifically looking at the macrofauna. Macrofauna are defined as those animals retained on a 0.5-mm screen when sieved out of the sediment. Because some of the depths at which we are sampling are off the continental shelf and into the deep-water realm of the Gulf where the critters tend to be smaller, I am using a 0.3-mm screen.
The health of the benthic community can be assessed in a number of measureable ways. The most obvious question to answer is: how many critters are living there? Beyond that, useful information such as size (biomass), where (depth) in the sediment the critters are living, the numbers of different critter species (diversity), and what mode of feeding behavior they use are important to assess the state of the benthic community. To get to the critters we have to sieve them from the mud. After slicing cores collected with the multicorer (see photo in Gallery) at 0-1, 1-2, and 2-6 cm sections, the mud is placed in sieves to allow the mud and other material to pass through the 0.3-mm screen and exposing what macrofauna were present at those depths.
Sieving is partly a science and partly an art. The objective is to extract the critters with the minimum of damage to them. Because many critters are soft-bodied and somewhat fragile, such as polychaete worms, the process should be gentle. Several methods of sieving have evolved. The first is the Up-and-Down: raising and lowering the sieve to allow the seawater to pass through. This is the staple of the siever" gentle enough to preserve the softest critters, but time-consuming. This method is augmented by the Twist: swiveling the sieve clockwise and counterclockwise. Not to be confused with the Twist is the Swirl: rotating the sieve without twisting. Both of these latter techniques are used to speed up the separation of mud from the critters. The bane of the siever is "mud balls". These little spheres of recalcitrant mud tend to resist dissolving into small enough particles to pass through the screen and are the source of much angst (and time spent sieving) among the fraternity of sievers.
Most of the critters are only visible upon later examination with a microscope. So, all material left on the sieves is fixed in formalin that contains a vital stain (Rose Bengal) and then brought back to the laboratory. There, after preservation in alcohol, the critters left on the screens will be sorted from the non-living material, counted, weighed, and identified as to species and feeding type (surface deposit feeder, head-down deposit feeder, filter feeder, predator, etc). Nevertheless, we have seen some critters that don't require a microscope to see: polychaete worms, a peanut worm (sipunculids), a brittle star (ophiuroid), amphipods (flea-like crustaceans), a shrimp, and a siliceous sponge.
The sediments in the cores collected at the deeper stations are mostly similar in appearance. There is a fluffy, brown, oxidized layer that is usually 5 cm thick. Sometimes there is a thin, very dark brown layer at the base of the brown oxidized layer. Below that the sediment changes to a dark gray and has more rigidity. Farther down the core (40 to 50 cm) the sediment changes to a lighter gray. This appearance matches what I have seen in deep-water hemipelagic sediments from the Caribbean Sea. There are sand-sized carbonate fossils of foraminiferans and pteropods embedded in the terrigenous sediments. What is different from the Caribbean hemipelagic sediments is the almost "chaotic" nature of the sediment fabric of this oxidized layer. Instead of being homogeneous with some burrows interspersed in the fabric, the sediment seems composed of individual clasts" perhaps a result of avalanching of continental slope sediments as turbidites from canyons feeding into the basin in which we are coring. Further analysis of sediment fabric, physical properties, and the macrofauna back in the laboratory will give us a better idea of the nature of these sediments.
see photos in Photo Gallery