Pacific is evident in standing stocks of upper trophic levels and in sinking fluxes of
organic material. Planktivorous seabirds (Oceanodroma: Leach’s and Galapagos
storm petrels) accumulate at the equatorial front (the boundary between the equatorial
cold tongue and the warm subtropical water to the north) in densities more than an
order of magnitude greater than seen on either side in the countercurrent flows to the
north and south of the equator (Table 11.6). An important feature in the eastern
tropical Pacific is the tuna–dolphin–seabird assemblage comprising yellowfin tuna
(Thunnus albacares), spotted and spinner dolphins (Stenella attenuata and S.
longirostris), and petrels (Pterodromo spp.) (Ballance et al. 2006). The shallow
thermocline there and equatorial upwelling may contribute to the geographical
location of this assemblage, which forms the basis of the world’s largest yellowfin
tuna fisheries.
Table 11.6 Changes in oceanographic conditions and seabird density (birds per km^2 )
when passing across the equatorial front from the South Equatorial Current (SEC) to
the North Equatorial Countercurrent (NECC) on 11 October 1998 at 3°, 34′N, 117°,
37′W.
(Table from Ballance et al. 2006.)
Buesseler et al. (1995) estimated high levels of POC flux (3–5 mmol C m−2 d−1) to
depths of 100 m along the equatorial band, and these were about twice as high as seen
north and south of the equator. Honjo et al. (1995) measured POC fluxes to the deep
ocean (1000–3000 m), and these ranged from 0.2 to 1.0 mmol C m−2 d−1. Although
Honjo et al. (1995) concluded that deep fluxes in the equatorial Pacific are less than in
the subarctic Pacific, Honjo et al. (2008) report similar rates: 158–194 mmol C m−2 yr
−1 in the equatorial Pacific and 163 mmol C m−2 yr−1 at Station P (50°N, 145°W).
Atlantic Equatorial Upwelling
(^) The main circulation in the equatorial Atlantic is also driven by the trade winds, and
results in divergence of the surface water. Tropical instability waves produce
undulations in the tongue of upwelled water visible in remotely sensed temperature
measurements (Plate 11.4). The TIWs are most apparent from June to October (five
months), as opposed to the nine-month seasonal duration in the Pacific. Integrated
studies of the Atlantic equatorial upwelling region were conducted as part of the