Biomass depletion in the big picture

Debbie MacKenzie debimack at
Mon May 28 12:07:12 EDT 2001

Hi Bob,

Thanks for your reply.

At 05:34 PM 5/25/01 -0500, you wrote:
>1.  Fossil fuel emissions:
>"Since 1751 over 270 billion tons of carbon have been released to the
>atmosphere from the consumption of fossil fuels and cement production.
Half of
>these emissions have occurred since the mid 1970s. The 1997 estimate for
>CO2 emissions, 6601 million metric tons of carbon, is
>the highest fossil-fuel emission estimate ever."

Yes, very big numbers, which may or may not be accurate. Regardless, they
are not particularly useful as long as the other half of the terrestrial
carbon equation remains unknown. The capacity of terrestrial systems to act
as carbon sinks is just starting to be realized.
And the very fact of the "missing sink" - approx. 30% of the carbon
"airborne fraction" going "missing" - this reflects the crudeness of our
understanding and probably our calculations. All of this obviously has a
huge margin of error.

Also, why was not "the highest fossil-fuel emission estimate ever"
accompanied by the "highest jump in global CO2 ever?"  (And why did not
"half of the CO2 rise" also occur since the mid 1970s, if that's the time
period during which "half of these emissions" occurred? These observations
weaken the direct cause and effect that is commonly believed: "rising
emissions = rising CO2 levels")

>2.  Global fishery production is cited by McGinn (1998) in Worldwatch
Paper 142
>as rising from 20 million tons in 1950 to about 120 million tons in the mid
>1990s.  This is in tons of wet weight biomass, which is typically on the
>of 1% carbon.  Even with a generous estimate of 5% C/wet weight, annual
>removal from the sea is <0.1% of the annual fossil fuel input to the

120 million tons - that's including aquaculture production - for the farm
fish that are fed fish meal are you counting the same wild fish twice? One
when you caught him and then again after he was incorporated into the flesh
of the farm fish? Regardless, annual wild fishery yields rose for a long
time but stabilized in the vicinity of 90 million tons about a decade ago.
And the average trophic level of what makes up the 90 million tons is
dropping...contrary to the expectations of "conventional wisdom" which hold
that as the trophic level drops in the system, the overall biomass at those
levels should increase significantly. (Some thinking has it increasing by a
factor of 10 for each trophic level dropped.) Why has the yield not
increased as the trophic level has dropped? It's because one key ingredient
for building fish is in short supply - fixed Nitrogen.

It's not clear to me why you would compare the carbon content of fishery
removals with that in fossil fuel emissions. Carbon does not appear to be
in short supply. It's the link between the carbon and nitrogen cycles that
is most important in assessing the effect of fishing on CO2.

And a simple calculation of tonnage is unlikely to tell the tale. As you
know, fixed nitrogen is (most times) the limiting nutrient in marine
ecosystems. (Actually another scientist did the math for me one time - dry
weight of nitrogen removed by fisheries is only a small fraction of the
nitrogen "put back" by humans via nutrient-enriched terrestrial runoff.
However, the sea knows how to get rid of that - sedimentation,
denitrification...and therefore very little becomes incorporated into the
living web, since it's "given back" in inappropriate form, amount and
location. Stunted growth of fish in an "overnourished" ocean presents a
bizarre paradox, IMO.) 

Nitrogen is the limiting nutrient factor in marine food webs, therefore the
availability of nitrogen determines the strength of the biological pump.
The "biological pump" contains two sections, each of which relies on the
presence of nitrogen, but in slightly different ways. The "organic pump"
delivers carbon to the deep water by sinking organic particles, and
nitrogen is a necessary part of their makeup. Therefore, it's via the
"limiting nutrient" route that nitrogen affects the strength of the organic

The other part of the biological pump, however, the "carbonate pump," may
be the more significant side, since besides consigning carbon to the deep
carbonate pool, it sequesters it in sediment, sand, limestone, skeletons of
coral reefs, seashells, etc. Nitrogen functions as a "catalyst" rather than
a key participating element in the carbonate pump. It allows the reaction
to proceed without being consumed by the reaction itself.

Visualize a scenario: 

phytoplankton uses ammonia from seawater as the critical N source for
production/carbon fixation -> a shell-forming marine organism consumes the
phytoplankton, incorporating 10% of the nitrogen into its flesh and
excreting 90% back into the seawater in a form usable by the phytoplankton
-> a small fish consumes the shell-former, the fish also keeps 10% and
excretes 90% of the N (in two short steps, 99% of the N has therefore been
returned to the phytoplankton), the fish excretes the carbonate shell since
it's nutrient content is too low and its indigestible (it's mineral, ends
up making sand)...

This cycle goes round and round, efficiently recycling the N but constantly
shunting more C into long-term storage in mineral and deep sea carbonate
pools. Building the shells uses only minute amounts of N, but N is the
"catalyst" for shell formation since the living shell-building organisms
will not exist without it. No molluscs and corals -> no N -> no
molluscs and corals... Therefore, although N is not a catalyst in the
chemical sense for the carbonate pump, it is so in the functional sense. 

So how could you calculate the effect on the carbon cycle of removing one
mole of N from the marine ecosystem? From the "biological pump" point of
view you've not only removed a building block, but an essential catalyst as
well. (The math will be very tough, a far cry from a linear relationship...)

>3.  If one assumes that most of the biomass extraction is at least two
steps up
>the food chain from the primary producers, the "factor of 10 per trophic
>rule of thumb suggests that fisheries deplete total marine biomass by no more
>than 1%.  This is probably a significant overestimate.

As suggested above, that rule of thumb seems not to be working in the real
world. "When theory conflicts with reality, reality always wins" - no?

>4.  Human acceleration of nutrient cycles has led to major eutrophication in
>many coastal areas (which are disproportionately important to the total
>productivity) -- this is production of EXCESS marine biomass at the most
>and quantitatively dominant level.

Now this is a dangerous myth. EXCESS phytoplankton in polluted estuaries
maybe, but this does not translate into EXCESS marine biomass. Ask any
fisherman...or any fish. We've made some very crude adjustments to what was
once a finely balanced system...Polluting the water does not produce fish,
it produces what you said, "major eutrophication." That means that the
waterway is now functioning as a septic system, accelerated sedimentation
and denitrification are the main things going on there. 

>5.  A review of the carbon cycle literature shows that the biggest scientific
>challenge is the identity of a "missing" carbon sink.  If fishery depletion
>were actually making an unrecognized contribution to the atmospheric CO2,
>would be a missing source, not a sink.

You've got it!

>I hope that counterarguments will be put forward quantitatively, in terms of
>the extensive literature on global carbon inventories and dynamics.

OK, sure, so do I. BTW, did you read my article: , or did you just react to
the abstract that I posted?

Debbie MacKenzie

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