data region with the strongest inverse relation of Calanus stock to the NAO. The
sequence goes:
(^)
(^) Each effect works in the opposite direction on the abundance of C. helgolandicus,
the species found in warmer waters to the south and east. High NAO corresponds with
more northward transport of not only C. helgolandicus, but the whole eastern
boundary current plankton community from off northern Africa and from the
Mediterranean shifts northward, raising diversity and reducing mean body size. The
shift from one regime to the other is likely to have knock-on effects up the food chain.
Calanus finmarchicus stocks peak in spring when cod (Gadus morhua) spawn and
larvae depend upon its nauplii for food, and the juveniles upon the later copepodites.
Calanus helgolandicus peaks in mid- to late summer, too late to nourish cod
reproduction. Euphausiids and the small copepod Pseudocalanus are also important in
larval–juvenile cod nutrition. By considerably obscure arithmetic, Beaugrand and
Kirby (2009) generated from North Sea CPR data an index of availability from March
to September of planktonic cod food and compared it to recruitment of “age 1” cod
(Fig. 16.21). While the diversity of zooplankton, particularly copepod species, went
up after about 1983, the likely food for juvenile cod went down and so did cod
recruitment. The pulses of North Sea cod recruitment centered at 1963 and 1978 are
termed the “Gadoid outbreak”, which possibly ended due to the shift in plankton
abundance, species, and seasonal timing.
Fig. 16.21 Long-term changes in North Sea zooplankton (1958–2007, upper series at
the left), compared to rate of cod recruitment at age 1 lagged one year (1963–2007).
(^) (After Beaugrand and Kirby 2010.)
Along the coasts of Labrador, Nova Scotia, and New England there are actually