Synchronous Ecological Regime Shifts in

the Central Baltic and the North Sea

in late 1980s

Jürgen Alheit, Christian Möllmann, Jörg Dutz1, Georgs Kornilovs2,

Peter Loewe, Volker Mohrholz1 and Norbert Wasmund1

ICES J. mar. Sci. (in press)

Ecological Regime Shift – Definition

Changes in marine system function that are relatively abrupt, persistent, occurring at a large spatial scale, observed at different trophic levels and related to climate forcing

deYoung, Harris, Alheit, Beaugrand, Mantua, Shannon. 2004. Progr. Oceanogr. 60

North Sea Regime Shift after 1987

Reid, Holliday, Smyth. MEPS. 2001

Reid, Edwards. Senckenbergiana marit. 2001

Kröncke, Zeiss, Rensing. Senckenbergiana marit. 2001

Reid, Borges, Svendsen. Fish. Res. 2001

Edwards, Beaugrand, Reid, Rowden, Jones. MEPS. 2002

Beaugrand, Reid. Glob. Change Biol. 2003

Beaugrand. Progr. Oceanogr. 2004

Monthly mean contour plotsEdwards, Reid, Planque. 2001

Annual meansEdwards, Reid, Planque. 2001

Summer, winter and annualMeans, central North SeaReid, Holliday, Smyth 2001

Phytoplankton Colour

Abundance zooplankton

Reid, Borges, Svendsen 2001

SST central North Sea

Reid, Borges, Svendsen 2001

Decapod + fish larvae

Reid, Borges, Svendsen 2001

Rel. ratios C. helgolandicus and C. finmarchicus in black, reverse in grey Reid et al. 2003

NAO

SST

Zooplankton 30 taxa

Horse mackerel catches, NE Atl.

Modelled inflow

PhytoplanktonColour

Benthic biomassNorderney

Annual meansReid and Edwards 2001

Long-term changes, North SeaBeaugrand 2004

Gadoid recruitment

Calanoid copepods

SST

Northern hemisphere temperature anomalies

Beaugrand. 2004. Progr. Oceanogr. 60

North Sea regime shift: 1982 –1988 2 periods: 1982 – 1985 SST increase, change in wind intensity and

direction, change in biogeographical boundary

1987 – 1988 large-scale hydroclimatic forcing

Hypotheses

• displacement of oceanic biogeographical boundary (prior to 1987)• change in hydro-meteorological forcing (after 1987) NAO• increase in oceanic inflow (after 1987)

Some indicators showed evidence of a regime shift, while some did not. current debate about existence of regime shift

Compare to Baltic Sea processes

1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

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NAO winter index, annnual temperature minimum in the Intermediate Winter Water (IWW) of the Bornholm Basin, Station K2, (solid line) and annual SST anomalies at Helgoland Roads (dashed line)

1 2 3 4 5 6 7 8 9 10 11 12

M onth

M onth ly m ean tem perature difference [°C ] in the centra l Bornholm Basin betw een the periods 1970-1987 and 1988 - 2003

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Dep

th [m

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Smoother & confidence interval

Smoother Upper limit Low er limit

1999199719951993199119891987198519831981

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Smoother & confidence interval

Smoother Upper limit Low er limit

1999199719951993199119891987198519831981

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Fig. 1: Trend in diatom biomass (in mg m-3, given as log10-data, means March-May) in Gotland Sea

Fig. 2: Trend in dinoflagellate biomass (in mg m-3, given as log10-data, means March-May) in Gotland Sea

WASMUND and UHLIG: ICES J. Mar. Sci. 60 (2003), 177-186,

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Southern Gotland Sea(d)

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80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99

Year

Si

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ol/

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Si conc. before bloomSi consumption by bloom

Bornholm Sea(c)

Silicate consumption proxy for diatom biomass

Acartia

1960 1970 1980 1990 2000

biom

ass

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aly

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Temora

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Pseudocalanus

1960 1970 1980 1990 2000

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Sprat

1980 1985 1990 1995 2000

recr

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ent-

at-a

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Sprat

1980 1985 1990 1995 2000

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ass

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aly

-1.5e+6

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north-west, mud, 83 m

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week

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10

³) m-2 w

k-1 4 °C 10 °C

Hatching of Acartia spp. nauplii from sediments of Bornholm BasinDutz, Mohrholz, Peters, Renz, Alheit. 2004. GLOBEC Newsletter.

Comparison of Regime Shifts

North Sea Baltic Sea

Phytoplankton + +

Zooplankton + +

Fish + +

Benthos + ?

Conclusions

• teleconnection between regime shifts in both systems

• timing in Baltic Sea: initiated after 1987 suggests that North Sea shift also initiated after 1987

• cause in Baltic Sea: drastic change in NAO suggests that North Sea shift also caused primarily by NAO less importance of biogeographical shift and oceanic inflow