Northward shift indicators
Over the last five decades there has been a progressive increase in the presence of warm-water/sub-tropical species into the more temperate areas of the North-East Atlantic and a decline of colder-water species. This trend seems to be accelerating over the last five years. The mass biogeographical movements are related to changes in sea surface temperature. A particularly interesting feature over the last five years is the decline in subarctic species to the south-east of Iceland and their movement to the north and west (see figure below).
Above: Biogeographical changes in plankton assemblages spanning five decades. Warm-water plankton (e.g. warm-temperate species) are moving north and cold-water plankton (e.g. subarctic species) are moving out of the North Sea.
Read more: Beaugrand, G., Reid, P.C., Ibanez, F., Lindley, J.A.
and Edwards, M., 2002. Reorganization of North Atlantic marine
copepod biodiversity and climate. Science,
296:1692-1694.
A useful indicator of the warming trend in the North Sea (a
northward shift indicator) is the percent ratio of the
cold-temperate Calanus finmarchicus and the warm-temperate
Calanus helgolandicus copepod species. Although these
species are very similar they do occupy distinct thermal niches.
The thermal boundary for the arctic-boreal distributed copepod
Calanus finmarchicus in the North-East Atlantic lies
between the ~10-11°C isotherm and is a useful indicator of major
biogeographical provinces. Calanus helgolandicus usually
has a northern distributional boundary of 14°C and has a population
optimum lying between 10-20°C; these two species can therefore
overlap in their distributions. When these two species co-occur
there is a tendency for high abundances of C. finmarchicus
earlier in the year and C. helgolandicus later in the
year. There is clear evidence of thermal niche differentiation
between these two species as well as successional partitioning in
the North Sea, probably related to cooler temperatures earlier in
the year and warmer temperatures later in the year.
The percentage ratio between C. helgolandicus and C.
finmarchicus in 2008 was again dominated by C.
helgolandicus, a trend that has evidently been accelerating
over the last decade. While C. helgolandicus is becoming
more abundant in the North Sea the overall Calanus biomass
has declined by 70% since the 1960s. This huge reduction in biomass
has had important consequences for other marine wildlife in the
North Sea including fish larvae.
Above: A simple ratio between a warm-water copepod species (Calanus helgolandicus) and a cold-water species (Calanus finmarchicus) per month from 1958-2008. Red values indicate a dominance of the warm-water species and blue values the dominance of the cold-water species. (0= total C. finmarchicus dominance, 1=total C. helgolandicus dominance)
