From the crabs on the
seabed to the seagulls in the sky.
|Angular crab larvae|
I explained in my previous article why there wouldn't be any polar bears on the ice
without any plankton at the sea surface. In this blog I want to
begin to tell you how studying the plankton has helped to understand
climate-induced changes in the North Sea, a productive marine
ecosystem that once provided 5% of the total global seafood harvest.
Here, a recent warming of this shallow sea has altered the ecosystem
from the animals that live on the seabed to the the seagulls in the
sky above, and these changes were first noticed in the plankton.
Analysis of long-term
plankton samples collected in the North Sea by the Continuous
Plankton Recorder Survey (http://www.sahfos.ac.uk) revealed that the
number of decapod larvae, and particularly the larvae of swimming
crabs, had increased in abundance since the mid 1980s. This increase
appears to be related to temperature. Following warm winters there
are more decapod larvae in the plankton than following cold winters,
and as the North Sea has warmed since the mid 1980s the number of
decapod larvae has increased. (As you can see from the correlation
between sea temperature and decapod larvae in the graph below).
|Swimming crab (Polybius henslowii)|
A simple bioenergetic model produced by German scientists showed that
the 22,000 individual lesser black-backed gulls in the most important breeding colony in
the south-eastern North Sea consumed approximately 35 million
swimming crabs annually (i.e. 1590 swimming crabs per individual
gull) during the breeding period (http://www.sciencedirect.com/science/article/pii/S0272771413000243).
While the increase in
swimming crabs may be good news for lesser black-backed gulls, the
general increase in decapods in the North Sea suggested from plankton
analysis may not be such good news for their prey species, such as
bivalves and flatfish, which have both shown a decline in abundance
as the number of decapod larve in the plankton have increased. The
decapod megalopa larva is a voracious predator in the plankton and
the newly settled decapods prey upon young bivalves and flatfish
recruits on the seabed.
So, I hope that this
small story shows how by studying the plankton, in this case the
abundance of decapod larvae, we can not only detect early signs of
change, but perhaps also understand how changes in the marine
ecosystem may extend from the seabed to the skies above.
Dr Richard Kirby is a British plankton expert, scientist, author and speaker. Follow Richard @planktonpundit on Twitter. You can see more images of plankton and learn more about them in Dr Richard Kirby’s book “Ocean Drifters, a secret world beneath the waves” available on Amazon and as an iBook.