October 11, 2012
Fish Poop May Play Critical Role In Oceans’ Carbon Cycle
Alan McStravick for redOrbit.com - Your Universe Online
Professor Deborah Steinberg of the Virginia Institute of Marine Science (VIMS) has dedicated her professional life to investigating crustaceans and their role in the “biological pump,” which is the process by which marine life transports carbon dioxide from the atmosphere and ocean´s surface to the deep sea. This cycle removes the carbon to a depth where it contributes nothing to global warming.
In a new study published in Tuesday´s issue of Scientific Reports, professor Steinberg partnered with Dr. Grace Saba of Rutgers University and retrained her focus from crustaceans to small forage fish in order to gain an understanding of their role in this carbon removal process.
The research pair collected their data off the coast of southern California on an exploratory expedition aboard the research vessel Point Sur. Building on Steinberg´s knowledge of copepods and other small, drifting marine animals, gleaned from two decades of research, the team wanted to explore whether forage fish like crustaceans played a discernible role in the biological pump through their consumption of photosynthetic surface algae and subsequent release “fecal pellets”.
With copepods, the ingested carbons from the algae are released as fecal pellets which sink quickly into the deep ocean. This is important because the algal cells are generally too small and too light to sink on their own.
“℠Fecal pellet´ is the scientific term for ℠poop,´” laughs Steinberg. “Previous studies in our lab and by other researchers show that zooplankton fecal pellets can sink at rates of hundreds to thousands of feet per day, providing an efficient means of moving carbon to depth. But there have been few studies of fecal pellets from fish, thus the impetus for our project.”
Adding to Steinberg´s comment, Saba says, “We collected fecal pellets produced by northern anchovies, a forage fish, in the Santa Barbara Channel off the coast of southern California.” The researchers found that sinking rates for the anchovies´ fecal pellets averaged around 2,500 feet per day. This figure was arrived at during experiments conducted in the shipboard lab.
At that rate, says Saba, “pellets produced at the surface would travel the 1,600 feet to the seafloor at our study site in less than a day."
Additionally, Saba and Steinberg determined the overall abundance of the pellets, finding approximately 6 per cubic meter of seawater. This finding, paired with measurements of the pellets´ carbon content — an average of 22 micrograms per pellet — allowed them to create a model that would show the effect that forage fish have on the biological pump.
“Twenty micrograms of carbon might not seem like much,” says Steinberg, “but when you multiply that by the high numbers of forage fish and fecal pellets that can occur within nutrient-rich coastal zones, the numbers can really add up.”
The team´s calculation of the total “downward flux” of carbon within the forage fish fecal pellets saw a maximum figure of 251 milligrams per square meter per day. This figure is equal to or greater than previously measured values of sinking organic matter collected through the use of suspended sediment traps.
“Our findings show that — given the right conditions — fish fecal pellets can transport significant amounts of repackaged surface material to depth, and do so relatively quickly,” says Saba.
Steinberg and Saba believe these conditions are likely to occur commonly on the Pacific coastlines of North and South America. This is because ocean currents abut and even cross over the continental shelves allowing for cold, nutrient-rich waters from depth to move into the sunlit surface zone.