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Monitoring
the long term response of plankton to global change
The CPR
time series provides a unique opportunity to assess
the impact of the increase in global temperature
over the last century and the effect of this on
the base of the marine food web. ‘Phytoplankton
colour’ is a visual index of chlorophyll derived
from the intensity of the green colouration of
the CPR filtering silk. An increasing trend in
‘phytoplankton colour’ is evident in the North
Sea and the central north-east Atlantic between
520N and 580N, with evidence for a step wise increase
after the mid-1980s (Reid et al, 1998). The pattern
of increase in these areas generally showed two
phytoplankton peaks from 1948 –1988, corresponding
to a spring and autumn phytoplankton bloom. Thereafter,
the two peaks have merged, to show a continuous
bloom from March to almost December each year.
The response is not uniform throughout the North
Atlantic, showing a general decrease in ‘phytoplankton
colour’ since the mid-1980s. At this time this
area tended to experience colder surface temperatures.
These changes may be a response by marine vegetation
to climate forcing. If so they are likely to have
important implications for CO2 fluxes and productivity
of the North Atlantic.
The Effects
of Climate Change
The Continuous Plankton
Recorder survey has sampled regularly in the northwest
Atlantic since the early 1960’s. Over the last
decade there has been an increase in the abundance
of a number of arctic boreal plankton species
(see graph opposite), notably Calanus hypoboreus,
Calanus glacialis and Certatium arcticum,
and a southerly shift of the copepod C. hypoboreus
in this area. In 1998 the species was recorded
at its furthest position south in the survey,
390N, off the Georges Bank. These biogeographic
changes are thought to be a response to the variations
in the production of Labrador Sea Water (LSW),
and the strength of the Labrador Current. LSW
is formed predominately in the winter period when
cold north-eastly winds blow across the Labrador
sea, cooling the surface waters. In this area,
freshwater influx (mainly from glacial sources)
exceeds evaporation, therefore lowering the salinity
of the LSW. By these cooling and freshening processes,
LSW is formed seasonally, sinks to intermediate
depths and flows out of the Labrador Sea, in a
south east direction into the North Sea.
The hydrography of the north west Atlantic is
not fully understood, but it is hypothesised that
the protracted positive phase of the North Atlantic
Oscillation is affecting the production of LSW,
by causing colder temperatures and increased wind
speeds over the Labrador Sea. Results from the
CPR survey suggest that the increase in the arctic/
cold boreal species in the area is a response
to a change in LSW production in the late 1990s.
The southerly penetration of the cold, intermediate
water acts as a transport for Calanus hypoboreus,
which then surface in the Gulf Stream (in an area
typified by sub-tropical plankton species in the
CPR survey).
Monitoring non indigneous
species
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The
introduction of non-indigenous marine plankton
species can have considerable ecological
and economic impact on regional seas. Efforts
to monitor invasive marine species are at
best fragmented, as they are typically only
noticed when the species reaches nuisance
status, so there are few case histories
of the spatial and temporal expansion. In
recent years there were a number of species
recorded in the CPR survey that were outside
their expected range. These included Penilia
avirostris (a cladoceran), Stomatopoda
(mantis shrimp) and Clausocalanus
sp. These three species are more commonly
found in the warmer waters of the Bay of
Biscay and the Mediterranean Sea. Their
appearance in the North Sea may indicate
a significant environmental change.
The CPR survey has been used to trace the
expansion and subsequent persistence of
the large diatom, Coscinodiscus wailesii,
which was first recorded in the English
Channel in 1977. C. wailesii was originally
known only from the Pacific coast of North
America, and in Chinese and Japanese waters
of the North Pacific. It is presumed that
it made its way from its native seas via
ballast water of international ships. Since
its first appearance it has become a significant
member of the diatom community and is now
well established in the continental shelf
seas of NW Europe. In the southern North
Sea it may now reach such high abundance
that can dominate the phytoplankton biomass.
The ecological importance of such invasive
species can therefore have potential ecosystem
effects by out-competing native species
for resources and space, reducing biodiversity,
and effecting the exploitation of native
herbivores (Edwards et al, 2001). |
Plankton
- The FACTS
FACT: Plankton
blooms in the summer and spring. Sometimes the
blooms are so large they can be photographed by
satellites in space (see below left).
FACT: The word plankton comes
from the Greek word ‘planktos’ which means ‘to
wander’.
FACT: Some plankton release
a chemical called Dimethylsulphide, which is used
in the formation of clouds
FACT: The weight of all the
plankton in the oceans is greater than all the
dolphins, fish and whales
FACT: The ‘White Cliffs of Dover’
are made up of millions and millions of fossilized
remains of plankton and other sea creatures (see
below right).
Try
our ‘Human Effects’
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