Magazine article Oceanus

Marine Snow and Fecal Pellets: The Spring Rain of Food to the Abyss

Magazine article Oceanus

Marine Snow and Fecal Pellets: The Spring Rain of Food to the Abyss

Article excerpt

Until about 130 years ago, scholars believed that no life could exist in the deep ocean. The abyss was simply too dark and cold to sustain life. The discovery of many animals living in the abyssal environment by Sir Charles Wyville Thompson during HMS Challengers 18721876 circumnavigation stunned the late 19th century scientific community far more than we can now imagine. Major questions immediately emerged: How do deep sea animals obtain food so far from the ocean's surface where plants, the base of the ecosystem, grow? Do they all just wait until a whale corpse is occasionally delivered to the abyss? These questions were only answered fairly recently.

Twentieth century progress in oceanography resulted in further confusion. Microscopic particles suspended in the water column seemed so small and light that it was believed they should take hundreds, perhaps thousands, of years to settle through the water column. And what happens to labile (unstable) matter, particularly organic particles from dead, broken plankton cells? Scientists could not understand why coccoliths, the delicately architectured calcite shells of phytoplankton, only several micrometers in size, were preserved on the deep ocean floor just beneath the area where they were produced. Why were they not carried far from their source by currents, and how could they even exist there when chemistry clearly indicates they should be dissolved during their several-century trip to the bottom?

A WHOI experiment in the deep Sargasso Sea two decades ago shed light on this century-old question. Shipboard observation of the first successfully recovered sediment trap samples from 5 kilometers deep revealed that particles originating in the euphotic (light) zone aggregate: The fine, light particles do not settle individually but are repackaged into larger particles that settle to the deep sea at a much greater speed. Among the Sargasso Sea aggregates, we found an abundance of fecal pellets from upper ocean zooplankton. Viewing one of the fecal pellets under an electron microscope, I was fascinated to find it full of perfectly preserved coccoliths and undigested, many-celled organelles. Some oil droplets were obviously much lighter than seawater!

Because filter-feeding zooplankton are concerned only with the size of their food and graze phytoplankton almost indiscriminately, indigestible coccoliths and diatom frustules are concentrated in their fecal pellets. Many zooplankton fecal pellets are covered with a thin coating material. Although individual particles sink very slowly or are even buoyant, when they are bundled into a tight package and ballasted with particles of calcite-one of the densest materials produced in the ocean-they sink as rapidly as 100 to 200 meters a day. …

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