ADVANCES
16 Scientific American, March 2022 Graphic by Amanda MontañezSource: “The Global Ocean Size Spectrum from Bacteria to Whales,” by Ian A. Hatton et al., inScience Advances,Vol. 7; November 10, 2021Heterotrophic
bacteriaBody Mass Order
of Magnitude
(grams)Major GroupsPhytoplankton10 -14–10-10 -13–10-10 -12–10-10 -11–10-10 -10–10-10 -9–10-10 -8–10-10 -7–10-10 -6–10-10 -5–10-10 -4–0.105 –10^6106 –10^7107 –10^8108 –10^90.001–0.0.01–0.0 .1 – 11–10–100–1,1,000–10,10,000–10^5Zooplankton Fish MammalsSmallest
organismsLargest
organismsCircle sizes show total
biomass in gigatonsFor organisms with a
body mass of less than
10 grams, human
activities have not
affected total biomass
since 1850For most organisms with
a body mass of more
than 10 grams, fishing
and other human
activities have
significantly decreased
total biomass since 1850Outlines show estimated
biomass before 1850Filled circles
show current
biomass1.5 Gt 0.02 GtBIODIVERSITYWeight of Life
Humans may have altered the
mass distribution of sea lifeWeigh all the creatures that roam the sea,
and a striking balance emerges. Researchers
have found that the total mass of this life,
when grouped in size classes, roughly follows
a regular mathematical distribution—
although humans may have disrupted part
of the pattern.
For a study in Science Advances, research-
ers combined satellite images, water samples,
commercial fishing catch data and computer
simulations to estimate the combined weight
of all the organisms that move through the
open ocean. Next they visualized biomass dis-
tribution by placing species on a size spec-
trum segmented into 23 weight classes. To
accommodate the vast size differences, the
researchers divided classes using a mathe-
matical function called a logarithm: the aver-
age weight of organisms in one class differed
by a factor of 10 from adjacent classes. One
class contained organisms weighing between
0.01 and 0.1 gram, another between 0.1 gram
and 1 gram, and so on.
The scientists found that most of the
weight classes contain roughly one gigaton
of life each. “This could be one of the largest-
scale regularities among life on Earth,” says
the study’s lead author Ian Hatton, a biologist
at the Max Planck Institute for Mathematics
in the Sciences in Leipzig, Germany.
Exceptions included a few classes contain-
ing bacteria, which were overweight because
of microbes’ domination of deep waters,
and classes containing animals bigger than
10 grams, which had disproportionately little
mass. Wondering if humans had contributed
to this divergence, Hatton’s team used previ-
ously published computer simulations and
animal population estimates to reconstruct
the ocean size spectrum of the 1850s, before
modern industrial fishing. The researchers
found that the combined weight of organisms
above 10 grams, including whales and many
fishes, has decreased by 60 percent since then.
Overfishing is a well-known problem, but
this work helps illuminate its extent, says
Andrea Bryndum-Buchholz, a marine ecolo-
gist at Memorial University in Newfoundland,
who was not involved in the study. “It visual-
izes how we’ve actually changed the ocean
fundamentally,” she says. — Nikk Ogasa