Monday, October 5, 2009

Sea Urchin POOP! The Importance of Strongylocentrotus feces!

(Images courtesy of Dr. Robert E. Scheibling!)
Why is Green Sea Urchin poop important??

Today's blog is based on data from a neat article produced by Leah Sauchyn and Robert Scheibling at Dalhousie University in Nova Scotia, Canada. Aquatic Biology 6: 99-108 (and its OPEN ACCESS so everyone can read it!).

First, the hero of our story...Strongylocentrotus droebachiensis- the Green Sea Urchin, which lives in the boreal North Atlantic, Arctic and North Pacific.

Like its purple and red sister species, S. droebachiensis is an ecologically important and ABUNDANT member of northern cold-water ecosystems. These urchins can voraciously devour kelp and can transform rich kelp beds to urchin barrens when densities are high. So, these urchins have an important impact on their surroundings..

Here's a recent issue of the Biological Bulletin (with photo by Rob Scheibling) that shows the crazy abundance of this species!
Green Sea Urchin+Kelp+Kelp+etc..=POOP!

Green sea urchins can consume kelp at astonishing rates, consuming 454 to 530 grams (dry weight) per day (along a linear meter). Some 20% of the kelp mass consumed is digested and egested as mucus-covered fecal pellets about 1 to 3 mm in diameter. That "translates" to 91 to 106 grams (dry weight) per day.

What does that mean? Your standard nut-type candy bar weighs about 65 grams or so, So they poop close to 1.5x the weight of your standard Power Bar!!

Now imagine HUNDEDS or THOUSANDS of them doing this. Now see below:

Sauchyn and Scheibling monitored how Green Sea Urchin feces changed over 4 depth points (6, 9, 12 and 15 meters) over a 19 day site on the Atlantic coast of Nova Scotia

They looked at:
  • fecal composition (what the poop is made from)
  • pellet size (how big the poop is)
  • settling velocity (how long it takes for poop to settle to the ground)
Just to be clear..individual "fecal pellets" aren't that big (as indicated above only a few mm)..but you've literally got THOUSANDS of animals each putting out a substantial pile of feces all the time... After you've got a bunch they start to form big piles like this one:
Images courtesy of Dr. Robert E. Scheibling
So What Happens to it?
    SO. You've got all this sea urchin poop that's floating around. What next? Well..obviously, it starts to break down. and that's what happens here.

    The mass, density and composition of the fecal pellets begins to come apart exponentially as time goes forward!

    This almost certainly means that all of the good "stuff" in feces begans to CYCLE back into the nutrient and food web segments of the ecosystem. Because corresponding to the breakdown there was also an increase in different components in "nature's nutrient recycling system":

  1. Organic carbon
  2. Nitrogen
  3. Lipid and available energy content
  4. Decrease in the Carbon to Nitrogen ratio. This one is important because it suggests that degraded sea urchin feces are an important food source for suspension feeding and deposit-feeding invertebrates (such as worms, some snails, etc.)
  5. Settling velocity also decreases over time.
So, bottom line: Sea urchin feces breaks down and cycles BACK into the ecosystem in a BIG WAY.
Image courtesy of Dr. Robert E. Scheibling

The above #5. "Feces settling velocity decreases over time" is an important one. Density for these fecal pellets decreases, so it seems logical for their ability to settle to ground to decline as well..

The authors said it best:
As the pellets degrade and become less dense, they are likley to be transported to even deeper, less productive waters.
(Images courtesy of Dr. Robert E. Scheibling!)

So, the poop (feces) from sea urchins contributes a significant portion of materials toward feeding microbial food webs and, in the bigger picture, to nutrient food webs in the marine realm.

An estimated 20 PERCENT of kelp biomass is consumed by sea urchins and enters into the recycling pathways in marine ecosystems.

Who knows WHERE all that stuff ends up???  Maybe here...

1 comment:

Kristen said...

As someone who has spent way too much time contemplating the impact of copepod feces on nutrient recycling, I very much enjoyed this post.