The Secret Oceanic Life of a Tree
Each day a tree dies. Its carcass may lay there on the forest floor, ravaged by fungus, insects, and plants as they grow from its remains. But, perhaps the tree meets a watery grave–gently floating down river ultimately finding itself bobbing in the ebb and flow of the tides. Eventually this tree becomes waterlogged and begins sinking.
At a critical depth and pressure the ocean squeezes the last bit of terrestrial air out of the wood replacing it with brine. So begins the story with a tree sinking into the deep. Wood falls range from small fragments to 2000+ pound behemoths. In the deep, devoid of light and plants, a wood carcass brings a rare commodity—food. The amount of carbon a tree or piece of wood brings to the seafloor can be anywhere from ten to a thousand times that regularly encountered on the seafloor. From the carcass comes nourishment to sustain deep-sea life.
Ocean Creatures Eat Snacks From Land
What’s surprising given the rarity and uncertainty of that wooden treat along the deep-sea floor, is that wood falls possess a fauna wholly specialized for living and consuming them. The digestion of a fibrous and solid food source requires talent, a hearty gut, and some bacterial help–traits not many species possess. These organisms finish the tree off, tearing it apart from inside and out. Bivalves of the genus Xylophaga (measuring less than an inch) use a ridged shell to bore into the wood, ingesting the wood fragments. On their gills they host an endosymbiotic bacteria that can digest their woody snack. But, something extraordinary also happens on wood falls, special bacteria anaerobically breakdown the wood. One of the by-products of this, sulfide, can be used by chemosynthetic bacteria, similar those at hydrothermal vents. These bacteria provide a tasty snack for animals at wood fall. Scientists know little of which species gain nutrition through a pure wood versus the sulfur route. Nor do we know which species are preying upon others. Using the chemistry of the animals, by looking at stable isotopes of carbon, nitrogen, and sulfur, we can begin to reveal this.
Understanding Food Webs with Stable Isotopes
Carbon, nitrogen and sulfur are all important elements obtained from eating other stuff. The old adage “you are what you eat” is actually true to some extent. Each animal has a slightly different signature of stable isotope ratios that reflect the signature of preferred food items. In nature, 1.1% of all carbon isotopes are the stable Carbon-13. However, plants and phytoplankton take up Carbon-13 differentially; Carbon-13 serves as sort of a chemical thumbprint of the food source. Terrestrial plants possess higher values than phytoplankton which in turn are higher than seagrass. The nitrogen isotopes, specifically Nitrogen-15, how high up the food chain an animal is. As organism eat each other, Nitrogen-15 is transferred to the predator. Thus, top predators have the highest Nitrogen-15 values. Looking at the Sulfur-34 concentrations can tell us whether an animal is munching on the sulfidic bacteria or predator of something that does.
Chunking Wood Into the Ocean: The Experiment!
In 2006, Jim Barry (Monterey Bay Aquarium Research, MBARI) and I (with the generous support of MBARI) chunked 36 logs overboard to begin to examine wood fall communities. Chunked may be a strong verb for sending them down on a benthic elevator. Once on the bottom, a remotely operated vehicle dispersed them over a 1600 square foot area now affectionately referred to as Deadwood. In 2011 and 2013, we retrieved these wood falls. On the surface, I and others picked through the once solid but now bore-riddled and crumbling logs for Xylophaga, limpets, worms, snails, and other wee beasties. As we picked through the rotting wood carcasses, my level of excitement was only matched by the sulfidic, rotten egg, stench of decomposition.
Wood represents a significant source of carbon input into the deep oceans and ties oceans and land together in the carbon cycle. Wood has been found frequently on the deep-sea floor several miles deep and several miles from land. During major storms that drive erosion and move debris down river, the movement of wood into the oceans can be sizable. During the Typhoon Morakot in 2009, the total amount wood weight carried to the oceans was estimated to be the equivalent weight of ~850,000 school buses. On the deep-sea floor, a single wood fall can enrich the carbon of the sediment by >25% even after several years. This represents a major pathway of natural carbon sequestration that has received little attention. We know very little of the fate of this wood based carbon after it arrives to the seafloor.
But this forest to deep ocean carbon pathway may be changing owing due to deforestation and increasing river discharge. From 2000 to 2005, 7.3 million hectares of forest per year were lost. In most of insular Southeast Asia, the forest cover decrease exceeds 50% and is well over 80% in New Guinea. Across Mexico, only 27% of the original forest cover remained intact by 1990. In addition, global warming is driving greater transport of atmospheric moisture in certain areas leading to increasing river flows. From 1936-1999 annual discharge of water in the largest Eurasian rivers into the Arctic Ocean increased by 7%.
We plan to send samples to the Washington State Stable Isotope Core Facility for analysis of the stable isotopes. Each isotopic analyses for carbon:nitrogen costs $8.20 and for sulfur $16 per sample. At 5 samples for each of the 30 species equals $3, 630. An additional $370 is required for the shipping of the samples to Washington state. Funds beyond the $4,000 will allows us to deploy more wood falls!