Large Bio-Geographical Shifts in the North-Eastern Atlantic Ocean: from the Subpolar Gyre, Via Plankton, to Blue Whiting and Pilot Whales
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Progress in Oceanography 80 (2009) 149–162 Contents lists available at ScienceDirect Progress in Oceanography journal homepage: www.elsevier.com/locate/pocean Large bio-geographical shifts in the north-eastern Atlantic Ocean: From the subpolar gyre, via plankton, to blue whiting and pilot whales H. Hátún a,b,*, M.R. Payne c, G. Beaugrand d, P.C. Reid e,f, A.B. Sandø b,g, H. Drange g,h, B. Hansen a, J.A. Jacobsen a, D. Bloch i a Faroese Fisheries Laboratory, Box 3051, FO-110, Tórshavn, Faroe Islands b Nansen Environmental and Remote Sensing Center, N-5006 Bergen, Norway c Technical University of Denmark, National Institute of Aquatic Resources, 2920 Charlottenlund, Denmark d Centre National de la Recherche Scientifique, Laboratoire d’Océanologie et de Géosciences, CNRS UMR LOG 8187, Station Marine, Université des Sciences et Technologies de Lille – Lille 1, BP 80, 62930 Wimereux, France e Marine Institute, University of Plymouth, Plymouth PL4 8AA, UK f Sir Alister Hardy Foundation for Ocean Science, and The Marine Biological Association, Plymouth PL1 2PB, UK g Geophysical Institute, University of Bergen, N-5007 Bergen, Norway h Bjerknes Centre for Climate Research, 5007 Bergen, Norway i Museum of Natural History, FO-110, Tórshavn, Faroe Islands article info abstract Article history: Pronounced changes in fauna, extending from the English Channel in the south to the Barents Sea in the Received 16 December 2008 north-east and off Greenland in the north-west, have occurred in the late 1920s, the late 1960s and again Received in revised form 6 March 2009 in the late 1990s. We attribute these events to exchanges of subarctic and subtropical water masses in the Accepted 6 March 2009 north-eastern North Atlantic Ocean, associated with changes in the strength and extent of the subpolar Available online 16 March 2009 gyre. These exchanges lead to variations in the influence exerted by the subarctic or Lusitanian biomes on the intermediate faunistic zone in the north-eastern Atlantic. This strong and persistent bottom-up bio- Keywords: physical link is demonstrated using a numerical ocean general circulation model and data on four trophi- Plankton cally connected levels in the food chain – phytoplankton, zooplankton, blue whiting, and pilot whales. Fisheries Spatial The plankton data give a unique basin-scale depiction of these changes, and a long pilot whale record Long-term from the Faroe Islands offers an exceptional temporal perspective over three centuries. Recent advances Climate variability in simulating the dynamics of the subpolar gyre suggests a potential for predicting the distribution of the Subpolar gyre main faunistic zones in the north-eastern Atlantic a few years into the future, which might facilitate a North Atlantic Ocean more rational management of the commercially important fisheries in this region. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction called Russell cycle (Russell et al., 1971). This cycle showed an in- creased influence of Lusitanian waters in the Channel between The north-eastern North Atlantic ecosystem is characterised by 1925 and 1935 with a reversal to a more boreal dominated ecosys- cold subarctic fauna within the subpolar gyre, warm Lusitanian fau- tem between 1965 and 1975 (Southward, 1980). A major north- na from the Bay of Biscay (Fraser, 1958) and, between these, a cold- ward and westward expansion of the boundary between the temperate boreal fauna in the mixing area west of the British Isles subarctic and the boreal zones was also documented during the and south of Iceland (Ekman, 1953). The faunistic boundaries are, period 1925–1935, with an influx of several boreal fish species into however, ill-defined because of very strong annual and inter-an- both Icelandic (Fridriksson, 1949) and Greenlandic (Tåning, 1948) nual temperature variability in the boreal region. The cold and waters. Several of the ecosystem changes observed during the warm flanks of the boreal zone are here referred to as Arctic-boreal 1920s and 1930s in the northern North Atlantic appear to have re- and Lusitanian-boreal respectively (Ekman, 1953). peated themselves in recent years (Drinkwater, 2006). After 1995, Shifts between Lusitanian and boreal regimes near the Western many new fish records and the detection of rare southern species English Channel (Fig. 1) have previously been described by the so- were reported in Icelandic (Astthorsson and Palsson, 2006) and Faroese (Rógvi Mouritsen, personal communication, 2008) waters. * Corresponding author. Address: Faroese Fisheries Laboratory, Box 3051, FO-110, Similarly, the marked northward expansion of pipefish into the Tórshavn, Faroe Islands. Tel.: +298 353900; fax: +298 353901. E-mail address: [email protected] (H. Hátún). 0079-6611/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.pocean.2009.03.001 150 H. Hátún et al. / Progress in Oceanography 80 (2009) 149–162 Longitude et al., 1998). The subpolar front, which outlines the periphery of 34°W 30°W 26°W 22°W 18°W 14°W 10°W 6°W 2°W the subpolar gyre, acts as a boundary for several copepod taxa Green- Nordic 66°N (Gaard et al., 2008) and large shifts of this front are therefore likely Seas land to lead to changes in copepod communities. However, little is Iceland known about the long-term variability of the zooplankton commu- 64°N Faroe nity in the waters south of Iceland. Islands Such changes in the plankton community have consequences 62°N for organisms at higher trophic levels, e.g. planktivorous fish, such Irminger as the small pelagic gadoid, blue whiting (Micromesistius Sea 60°N poutassou). This large fish stock spawns west of the British Isles Iceland Basin Faroe- and migrates past the Faroe Islands to its main feeding grounds Latitude Reykjanes Ridge Shetland Ch. 58°N Rockall in the Nordic Seas (Bailey, 1982) and is of considerable importance Plateau for both regional fisheries and as a food source for piscivorous fish, 56°N squid, seals, and pilot whales. The blue whiting stock biomass tri- Rockall Trough pled in the late 1990s due to a succession of good recruitment 54°N Ire- years after 1995 (ICES, 2007), but little is known about the under- land lying causes of these changes. 52°N At even higher trophic levels, piscivorous species such as the Western English Ch. 50°N long-finned pilot whale (Globicephala melas), may also be influ- enced by changes in food availability. This species occurs Fig. 1. Map of the north-eastern Atlantic Ocean. The 500 m, 1000 m (thick line) and year-round in Faroese waters, where it feeds primarily on the squid 2000 m isobaths are shown. species Todarodes sagittatus and Gonatus sp., and blue whiting when squid abundances are low (Desportes and Mouritsen, North and Nordic Seas (Kirby et al., 2006) illustrates post-1995 1993). Squid, in turn, also prey heavily on blue whiting in the changes in the distribution of warmer water fish species. north-eastern Atlantic (Gaard, 1988). The number of pilot whales The physical oceanography of this region is dominated by the landed in the Faroe Islands has previously been related to both dynamics of the North Atlantic subpolar gyre; a large counter- the biomass of blue whiting in the Norwegian Sea during the clockwise rotating body of relatively cold and low-saline subarctic water in the central northern North Atlantic (Fig. 2). During the early 1990s the circulation of the gyre was intense, but declined AMO substantially after 1995 (Häkkinen and Rhines, 2004), leading to 2 a rapid warming and salinification of the north-eastern Atlantic (Hátún et al., 2005b). These changes had a particularly large im- 1 print in the Iceland Basin (Hátún et al., 2005b)(Fig. 2), where the SST cold and fresh subarctic water masses (Bersch, 2002; Pollard 0 et al., 2004) were replaced by warmer and more saline water CET GI masses from the south (Bersch et al., 2007). -1 It is tempting to suggest that these simultaneous physical and Anomaly (°C) Temperature NAO ecological changes are related. Such links have been reported pre- -2 viously, e.g. half century-long time series of the biomass of saithe in Faroese waters (Steingrund and Hátún, 2008) and puffin catches 1850 1875 1900 1925 1950 1975 2000 2008 Year in Iceland (Freydís Vigfúsdóttir, personal communication, 2008) have both been related to the dynamics of the subpolar gyre, Fig. 3. Climate indices. Annual averages of: the Atlantic Multi-decadal Oscillation although the causal mechanisms have not, as yet, been clarified. (AMO) (Enfield et al., 2001), the SST west of the British Isles (Rayner et al., 2006) Any such link between the marine climate and biota is likely to in- (see text), the inverted gyre index (GI) (Hátún et al., 2005b) and the North Atlantic Oscillation (NAO, inverted) (http://www.cdc.noaa.gov) are shown. The Central volve phytoplankton and zooplankton, the basic food resource for England Temperature (CET) (Parker and Horton, 2005) during the spring months all higher marine ecosystems. Phytoplankton abundance in the off- March and April has been plotted over the SST series. The CET is low-pass filtered shore waters between Iceland and the Faroe Islands are known to using an 8-year filter width. The two temperature time series (black) are to scale, have declined during the period from the late 1950s to 1995 (Reid the others are not. a) Nordic Seas b) Greenland Iceland MNAW MNAW Subpolar Gyre NAC NAC ENAW ENAW Fig. 2. Water masses. Simulated upper layer (Hátún et al., 2005b) temperatures in the northern North Atlantic Ocean. Annual averages for a cold period (a, 1993) and a warm period (b, 1998) are presented to highlight the large shift taking place. Red and blue colours represent temperatures above 9 °C and below 7 °C, respectively.