The biological importance of the major ocean currents

Transcription

1 The biological importance of the major ocean currents

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4 Squid and the western boundary currents Illex illecebrosus, the short-finned squid

5 Squid use the Gulf Stream to facilitate their migration. The center of the spawning area is the Blake Plateau, off Cape Canaveral, Florida The larvae and juveniles are transported northward by the Gulf Stream up to 1000 km per week, but many are caught up in frontal eddies that delay their northward progress. From Jan. to early Mar. larvae occur in slope water along the northern edge of the Gulf Stream, roughly from Cape Hatteras to the latitude of Cape Cod In May juveniles are found in warm waters along the southern slope of the Grand Banks, and from there they migrate to Canadian inshore waters in late June and July Adults return south to spawn on the northern edge of the Gulf Stream by moving into the southward-flowing coastal currents. Warm-core rings may play an important part in this shoreward movement

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7 North of Cape Hatteras the Gulf Stream meanders cause convergence and downwelling on their leading edges, but divergence and upwelling of nutrient-rich water on their trailing edges. Hence, the frontal processes of the Gulf Stream will tend to support both enrichment of the food web in the areas of divergence and concentrations of food organisms in the areas of convergence.

8 Overall the successfulness of the recruitment might be influenced by the following three classes of environmental factors: Enrichment of the food web by physical processes (upwelling, mixing, etc.); Opportunity for concentrated patches of food particles to accumulate (stable structure, convergent flow patterns, frontal formations, etc.) Flow patterns that enable a population to maintain itself, through adaptive responses, in a continually moving fluid medium

9 Eels and the North Atlantic gyre

10 Recruitment of American and European eels as represented by the number of juveniles passing a counting ladder on the St. Lawrence River, Canada, and the catch per unit effort of elvers in the Den-Oever estuary of the Netherlands. However, catch-per-unit-effort data from the Bay of Fundy and in Chesapeake Bay do not indicate a drastic decline in stocks at the same period.

11 Drinkwater and Myers (1993) hypothesize that the slowing of the Gulf stream was the most likely cause of the drastic decline. Prolongation of the time for transit to Europe might mean that the European eels missed the optimum time for metamorphosis. Eels migrating to the northern rivers of North America might be similarly affected, while those migrating to the more southern rivers were able to metamorphose successfully, in spite of the slower currents In addition, slowing of the Gulf Stream is associated with more frequent formation of warm core rings, which may cause more larvae to be advected out of the Gulf Stream and into the southward-flowing coastal waters

12 Salmon and the Alaskan gyre

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16 Biology of eddies and rings associated with major currents

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18 Gulf stream frontal eddies Formation of these structures is a common event south of Cape Hatteras

19 These cold cores differ from that contained in cold-core rings formed further north in being formed by upwelling of North Atlantic central water from deep in the Gulf Stream. A simplified explanation is that wherever the Gulf Stream in its meandering moves away from coast, water from deep in the Gulf Stream upwells in the space created.

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21 Cold-core ring

22 Warm-core ring

23 Ecology of Rings Cold-core Ring Counter-clockwise circulation Upwelling at the center Water of the core from continental slope Nutrient rich High production Warm-core Ring Clockwise circulation Downwelling at the center Water of the core from Sargasso Sea Nutrient depleted Lifetime productivity about the same to the surrounding shelf water Mechanisms for the not so low pp in the warm-core ring: 1) Upwelling at the periphery. 2) Convective mixing caused by the cooling of surface water as the ring moves north of the Gulf Stream.

24 Ecology of the central gyres Previous view: biological deserts steady state with low pp and low f ratio Current view pp much higher (~ 4 times higher than pre-1984 data) Using of trace-metal-clean techniques (Fitzwater et al. 1982) episodic mixing event important mesoscale eddies and Rossby waves enhance productivity

25 HOT and BATS hahana.soest.hawaii.edu /hot/hot_jgofs.html /bats/bats.html

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27 Time series at Station ALOHA of (a) euphotic zone chlorophyll a, (b) integrated primary productivity (0-200 m), with a 3-point running mean showing the recurring seasonal pattern, and (c) assimilation numbers at 5 m depth (from Karl et al. 2001)

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30 (A) silicic acid (mm), (B) total chlorophyll a (TChl a, mg m 3), (C) fucoxanthin (mg m 3), and (D) photochemical energy conversion efficiency (Fv /Fm).

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34 Maranon et al. 2001

35 Gallienne and Robins (1998) Total biovolume of zooplankton estimated by optical plankton counter (OPC), along an Atlantic Meridional Transect (from 50ºN to 50ºS) in May, plotted on a logarithmic scale and on a linear scale along with sea surface temperature (SST) and mean equivalent spherical diameter (ESD). A, in Falklands current; B, in oligotrophic South Atlantic subtropical gyre; C, in Equatorial Current; D, in West African upwelling; E. in oligotrophic Canary Basin; F, in temperate northeast Atlantic

36 Subarctic Gyres Smaller, cyclonic gyres north of the main anticyclonic subtropical gyres in both the Atlantic and Pacific Oceans

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38 Schematic of the subpolar gyre. Orange/yellow arrow denotes northward flow of warm Gulf stream waters; blue/yellow arrow southward flow of fresh, cold waters, and light blue line the southward flow of deep, dense waters. Dark blue shading is water deeper than 3000 m, light blue deeper than 1500 m, and white is shallower than 200 m.

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40 North Atlantic Alaskan gyre subarctic gyre Spring bloom Yes, in April no Change of phytoplankton biomass 10 2 between winter and summer Winter depth of mixed layer >200 m ~100 m Dominant copepod Calanus finmarchicus Neocalanus spp. and Calanus pacificus Iron-limited HNLC region No Yes

41 Variability in ocean circulation and its biological consequences ENSO and other long-term climate variations ENSO: El Niño Southern Oscillation

42 El Niño and Southern Oscillation are often connected, occurring every 3-7 years An El Niño Year A Non-El Niño Year In an El Niño year, when the Southern Oscillation develops, the trade winds diminish and then reverse, leading to an eastward movement of warm water along the equator. The surface waters of the central and eastern Pacific become warmer, and storms over land may increase. In a non-el Niño year, normally the air and surface water flow westward, the thermocline rises, and upwelling of cold water occurs along the west coast of Central and South America.

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44 January 1982 January 1983

45 Water depth (m) Temperature Plankton Dissolved nutrients Water depth (m) Water depth (ft) Increasing temperature Normal Conditions Increasing temperature During El Niño Water depth (ft)

46 SOI Southern Oscillation Index ALPI Aleutian Low Pressure Index PDO Pacific Decadal Oscillation NAO North Atlantic Oscillation The SOI is based on the monthly anomaly of the sea-level pressure difference between Tahiti and Darwin. (x10) ALPI is the area in km 2 of the low defined by a se-level pressure of kpa The PDO is defined by the first principal component of the variations in sea surface temperature north of 20 N in the Pacific Ocean

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50 Total landings of sockeye, chum and pick salmon for the North Pacific also showed a peak landings in the 1940s and 1980s.

51 Commercial salmon harvest of Alaska, (top). Data are from the Alaska Dept. of Fish and Game. (Redrafted from Pearcy, 1992). Historical catches in the sardine fishery (bottom) of Japan, California, and Peru-Chile. (Modified from Kawasaki, 1992). Note different ordinate scales.