Deep-sea ecosystems affected by climate change

The vast muddy expanses of the abyssal plains occupy about 60 percent of the Earth's surface and are important in global carbon cycling. Based on long-term studies of two such areas, a new paper in the Proceedings of the National Academy of Sciences (PNAS) shows that animal communities on the abyssal seafloor are affected in a variety of ways by climate change.

Historically, many people, including marine scientists, have considered the abyssal plains, more than 2,000 meters below the sea surface, to be relatively isolated and stable ecosystems. However, according to Ken Smith, a marine ecologist at the Monterey Bay Aquarium Research Institute (MBARI) and lead author of the recent PNAS article, changes in the Earth's climate can cause unexpectedly large changes in deep-sea ecosystems. Based on 18 years of studies, Smith and his coauthors show that such ecosystem changes occur over short time scales of weeks to months, as well as over longer periods of years to decades.

The recent paper covers two time-series studies—one at "Station M," about 220 kilometers off the Central California coast, and a second on the Porcupine Abyssal Plain, several hundred kilometers southwest of Ireland. The flat, muddy seafloor at these sites lies between 4,000 and 5,000 meters beneath the ocean surface.

In this cold, dark environment, very little food is available. What food there is takes the form of bits of organic debris drifting down from the sunlit surface waters, thousands of meters above. During its long descent, this organic matter may be eaten, excreted, and decomposed, drastically reducing its nutritive value. It is estimated that less than five percent of the organic matter produced at the surface reaches the abyssal plains.

Research by Smith and his coauthors has shown that the amount of food reaching the deep sea varies dramatically over time. For example, at the Porcupine Abyssal Plain, the amount of organic material sinking from above can vary by almost an order of magnitude from one year to another.

Such variations in food supply have several causes. On a seasonal basis, algal blooms near the sea surface send pulses of organic material to the deep seafloor. Other factors may also come into play, including how much of the algae is eaten by marine animals, and how the material is moved by ocean currents.