Shutting Down the Oceans
Act II: Abrupt Plankton Shifts
Global warming has seriously disrupted plankton growth and growth cycles,
putting the entire marine food web at risk. Dr.
Mae-Wan Ho
A fully
referenced version of this article is posted on ISIS members’ website. Membership
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The importance of plankton watching
The Sir Alister
Hardy Foundation for Ocean Science (SAHFOS) in Plymouth UK has been collecting data on plankton from
the North Atlantic
and the North Sea since 1931 [1]. The survey is named after
an instrument, the Continuous Plankton Recorder (CPR), invented by Sir Alister
Hardy, a brilliant marine biologist, perhaps better known to the public for
his theory that humans evolved in the sea. He may still be right about human
evolution, but it is his work on zooplankton and his foresight in establishing
the CPR survey that should receive the highest accolade. The CRP survey is
the longest running, large-scale marine biological survey in the world.
The CRP is towed by merchant ships on a number of designated shipping
routes at a depth of about 10 m, collecting plankton on silk filters en route
that are counted and analysed in the laboratory afterwards to generate a database,
which now contains details on 400 species of plankton found in over 170 000
samples taken since 1946. This database has provided excellent material for
researchers studying climate change and the impact of climate change on marine
organisms.
SAHFOS researchers Anthony Richardson and Martin Edwards, and marine
ecologist David Shoeman at the University of Port Elizabeth in South Africa,
are among the scientists analysing this data most recently who have discovered
abrupt changes in the plankton populations
in the northeast Atlantic within the past decades that may have radically disrupted the food web, including
commercial fisheries.
Phytoplankton primary production affected by climate change
Richardson and Shoeman [2] looked at more than a hundred
taxonomic groups including phytoplankton diatoms and dinoflagellates, herbivorous
zooplankton copepods that feed on phytoplankton, and carnivorous plankton
such as arrow worms and amphipods that feed on the herbivores.
They found that changes in the abundance of phytoplankton correlated strongly
with sea surface temperature, tending to be more abundant when cooler regions
in the North Sea warmed, possibly because warming cold waters encouraged metabolism
and growth, but decreased in abundance when warm regions in the North East Atlantic
became even warmer, possibly because warm water blocks the upwelling of nutrient-rich
deep water (Oceans
and global warming, this series).
Richardson and Shoeman also found that while neither herbivore- nor carnivore-zooplankton abundance correlated
with temperature, close correlations existed between the abundance of phytoplankton
and herbivorous zooplankton, and between the abundance of herbivorous zooplankplanton
and carnivorous zooplankton. This was evidence that the plankton food web
is controlled “bottom-up” by the primary producers, and not “top-down”
by the predators; and the effects of climate change on the primary producers
could propagate up to fisheries.
The bottom-up control of the marine food web differs fundamentally
from the terrestrial food web, and depends on the ability of phytoplankton
to sequester carbon over the shortest timescales of any primary producers
on earth. This gives phytoplankton a unique position not only in the marine
food web, but also in the entire biosphere and the carbon cycle.
In terrestrial ecosystems, soil algae and cyanobacteria are important
for maintaining soil fertility, controlling pollution and improving soil structure
for plant growth [3], but are not directly at the bottom of the food web.
In the oceans, phytoplankton effectively funnels carbon into the biosphere
to create an enormous standing carbon stock, and species that synthesize calcium
carbonate shells also transport large amounts of carbon dioxide as they die
and sink to the seabed where the shells remain as sediment for thousands of
years. Fossil fuels are believed to have originated from plankton sediment that
has been buried deep underground over millions of years (Oceans
and global warming, this series). Thus, phytoplankton is the most important
active carbon sink on earth, and without them, life in its current form is impossible.
Mismatch between plankton seasonal peaks
Transfer of
carbon into the living biomass in the oceans depends on phytoplankton being
consumed by larger organisms with longer life spans, which are in turn consumed
by still larger organisms with even longer life spans and so on up to fish,
seabirds, seals and whales. Efficient upward
transfer of biomass depends on coupling between the peak seasons for consumer
and the consumed, but that too appears to have become disturbed by global
warming.
Richardson and Edwards [4] followed the shifts in the timing of seasonal
plankton blooms (population peaks) over the past decades. Each species has
an annual cycle of maximum and minimum, and zooplankton herbivores (feeding
on the phytoplankton) and carnivores (feeding in turn on the herbivores) have
evolved to exploit the phytoplankton bloom so as to transfer biomass effectively
up the food web.
Since 1987, however, the cycles of different
species have shifted out of synch, leading to a mismatch between the different
tiers of plankton food web all considered ‘producers’ of the oceans.
The phytoplankton diatoms and dinoflagellates are primary producers, the copepods,
herbivores that feed on phytoplankton are secondary producers, non-copepod
holozooplankton include both secondary producers and tertiary producers that
feed on secondary producers, and meroplankton consist of fish larvae that
are both secondary and tertiary producers.
The researchers found substantial shifts in the seasonal population
peaks over the past few decades. In particular, the meroplankton peaks have
shifted forward, echinoderms most of all by 47 days. In contrast, diatom peaks
in spring and autumn have collectively remained unchanged, albeit with considerable
variation among the species. Non-copepod holozooplankton show a wide range
of responses, with some classes also occurring substantially earlier. The
majority of dinoflagellates among the phytoplankton also occur earlier by
24 to 27 days.
For groups that peak when the water column is mixed or in a transitional
state, there is considerable variability; whereas groups associated with low
turbulent conditions have virtually all advanced in their seasonal peaks.
The bottom-dwelling larvae of the meroplankton also showed larger shifts forward
than their zooplankton prey.
During summer, meroplankton has moved forward collectively by 27
d, dinoflagellates by 23 days, copepods and non-copepod holozooplankton both
by 10 days over the 45-year period. Diatoms as a group showed the largest
variations with particular groups occurring both earlier and later during
the spring and autumn peaks
In warm waters therefore, phytoplankton bloom occurred three weeks
earlier, but the herbivores peaked only 10 days earlier. Thus, there could
be less herbivores to feed the next level of predators, the fish larvae and
carnivorous zooplankton.
“These effects at the base of the food web are so dramatic that they’re
bound to have an effect on the whole North Atlantic ecology,” Edwards said
[5].
Phytoplankton becomes unstable as surface waters warm
That’s not all. Jef Huisman of the University
of Amsterdam, The Netherlands, and colleagues used a computer model to find
out what happens to the phytoplankton when the surface waters of the oceans
get warmer, and large parts of the oceans become stratified, as climate models
predict; in other words, there is less vertical mixing as the lighter warm
water sits on top of the dense cold water beneath.
Huisman and colleagues found
that reduced vertical mixing caused major disturbances to phytoplankton growth,
undermining the efficient sequestering of carbon dioxide and export of carbon
to the deeper layers and the ocean floor. This could result in catastrophic
positive feedback on global warming as carbon dioxide in the atmosphere increases,
causing further increase to the sea surface temperature, more stratification
of the water column, and further disturbance to phytoplankton growth.
Phytoplankton grows best at a
depth of about 100 m, forming deep chlorophyll maxima (DCMs) in the oceans
that reflect a compromise between light supplied from above and nutrients
supplied from below. Scientists assumed these DCMs are stable features of
the oceans.
However, Huisman
and colleagues showed that reduced vertical mixing generate oscillations and
chaos in phytoplankton biomass that destroy the DCMs. The two critical processes
involved in the fluctuations are the sinking of dead plankton that removes
nutrients from the surface layers where there is sufficient light for growth
and the upward flux of nutrients required for new phytoplankton growth. The
DCMs remain stable so long as the two processes are in balance. When vertical
mixing is reduced beyond a critical threshold, nutrients in the top layers
are depleted faster than they can be replaced, oscillations and chaos set
in and the DCMs break up.
The model reproduces
many real features. It predicts that DCMs form at a depth of about 100m, and
span a similar range as in clear ocean waters (about 50 m). As consistent
with observations, the model predicts that nutrients are depleted to near-zero
levels above the DCM while the nutrient concentration increases linearly with
depth below the DCM. Detailed time-series measurements from the subtropical
North Pacific confirm the prediction of a vertical zoning of species, with
different collections of species dominating at different depths, and seasonal
patterns in chlorophyll and nutrient concentrations in the DCMs. The time
series measurements also show additional fluctuations superimposed on the
seasonal cycles, as predicted by the model. Also as predicted by the model,
the time series measurements show that species with relatively high sinking
velocities show larger variability than species with low sinking velocities.
These results are devastating enough, but they also converge with other studies
which show that warming itself will directly undermine phytoplankton growth,
quite apart from the stratification of oceans and the reduced supply of nutrients
(Shutting down the oceans part III, global
warming and plankton; snuffing out the green fuse, this series)
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