October 3, 2007
Changes in the deep
Abyssal plains—vast expanses of flat seafloor 3,000 meters to 6,000 meters deep—make up almost half of the earth’s surface. Crawling across these muddy plains are a variety of deep-sea animals, including sea cucumbers, brittle stars, and sea urchins. Because the physical environment of the deep sea doesn’t change very much over time, it’s easy to assume that these communities of deep sea animals don’t change much either. However, a recent paper by MBARI Postdoctoral Fellow Henry Ruhl shows that the number and sizes of deep-sea animals can change dramatically from year to year.
To study long-term changes in the deep sea, Ruhl analyzed over 35,000 deep-sea photographs taken between 1989 and 2005. These photos were taken on the seafloor at “Station M,” about 220 kilometers (140 miles) west of Point Conception, California (see map). Station M is located on the edge of the abyssal plain, about 4,100 meters (2.5 miles) deep. However, it is close enough to land that it is affected by the California Current and seasonal “blooms” of microscopic algae that are carried offshore from the coast.
MBARI Senior Scientist Ken Smith and other oceanographers have been studying this region for almost two decades, trying to understand (among other things) how animals on the deep seafloor get enough food to eat. Many animals of the abyssal plain eat bits of debris that drift down from the sunlit surface waters far above. Thus, changes in the surface waters can impact deep-sea animals. For example, Smith’s previous research showed that El Niño events, which affect drifting plants and animals at the sea surface, can also affect animals in the deep sea.
Between 1989 and 2005, Smith and his coworkers made about 50 trips out to Station M. Each time they took hundreds of photos of the seafloor by towing a “camera sled” over the ocean bottom. Sometimes they dragged a net behind the camera sled to capture some of the photographed animals for further study when the sled was hauled back up to the surface.
As part of his PhD research, Ruhl counted and estimated the sizes of all the animals in the thousands of photographs taken by this camera sled. Then he looked at how these animals changed in abundance and size over time.
Sea cucumbers were the most common animals visible in these photographs. In addition to studying eight different species of sea cucumbers, Ruhl also counted sea urchins and brittle stars.
What Ruhl found surprised him—some types of sea cucumbers became 100 or even 1,000 times more abundant for a few years, then became less abundant again. Others followed an opposite pattern. One species of sea cucumber (Elpidia miutissima) dominated the seafloor community for over a decade, then virtually disappeared.
When he looked at changes in the sizes of seafloor animals over time, Ruhl found that they were inversely related to the animals’ abundance. Thus, in many cases, when a particular species became more abundant, smaller individuals of that species became more common. Ruhl suggests that new generations of small, young animals may have been augmenting or replacing older, larger animals. If verified, this would be some of the first evidence of long-term reproductive changes in the deep sea.
Ruhl speculates that such pulses of new, smaller animals could have been triggered by changes in the amount or quality of food drifting down from surface waters. Such events might have favored deep-sea animals that were able to take advantage of the additional food.
In the ten years following one such pulse of small sea cucumbers in 1990, the average sizes of the animals gradually became larger, presumably as the 1990 “class” began to grow up. From these data, Ruhl was able to estimate that these animals may grow at up to 25-60 millimeters (1-2 inches) per year.
Ruhl and Smith’s research complements similar studies that have been performed in the Porcupine Abyssal Plain in the Northeastern Atlantic. However, Ruhl points out that much remains to be learned. “We just don’t know much about these animals—how long they live, how often they reproduce—even basic information like that.”
As improbable as it may seem, Smith and Ruhl’s work may also help scientists better understand long-term climate change. Microscopic algae in the oceans absorb vast amounts of carbon dioxide and thus affect concentrations of carbon dioxide in the atmosphere. When the remnants of these algae sink into the deep sea, carbon from their bodies eventually ends up in seafloor sediment.
As this sediment accumulates on the seafloor, the carbon from the algae may be buried by various processes, including the activities of abyssal animals. In this case, the carbon may be locked away from the earth’s atmosphere for millions of years. On the other hand, if the carbon is consumed and excreted by deep-sea animals or stirred up into the water column, it may be redissolved or resuspended in the the seawater. The balance between these two processes influences how much of the carbon ends up back in the atmosphere. Since abyssal plains cover so much of the Earth’s surface, knowing how abyssal animals affect carbon cycling is important for researchers developing long-term computer models of the Earth’s climate.
Having discovered that changes in populations of deep-sea invertebrates coincide with changes in their body size, Ruhl hopes to try and better understand what drives these changes. He is particularly interested in finding out how subtle changes in the amount or type of food can lead to dramatic changes in animal communities. One way of doing this is to look at the relationships between abyssal animal communities and near-surface conditions. For example, Ruhl plans to make use of the extensive oceanographic data that have been collected by the National Science Foundation’s Long Term Ecological Research (LTER) program. The LTER program supports ecological research over long time periods in a variety of terrestrial and marine ecosystems, including the California Current Ecosystem.
Smith and Ruhl hope to continue their oceanographic cruises out to Station M over the next few years. Their research shows, once again, that collecting data over years and even decades is essential if we are to understand life in the abyss, not to mention long-term climate change.
H. A. Ruhl, Abundance and size distribution dynamics of abyssal epibenthic megafauna in the Northeast Pacific. Ecology, 88:5 (2007).
For additional information or images relating to this article, please contact: Kim Fulton-Bennett