Atlantic Meridional Transect (AMT) Programme from 1995 to 2005 (Robinson et al.
2006). The field sampling was conducted twice per year and covered 13,500 km
between 50°N and 52°S. We compare the physical, chemical, and biological
characteristics of the Pacific and Atlantic equatorial systems in Table 11.7.
Macronutrients (NO 3 −, PO 4 3−, and Si(OH) 4 ) are higher in the Pacific, while iron is
much higher in the Atlantic. Although > 1 μM nitrate is sometimes found in the
westward-flowing Atlantic equatorial “cold tongue”, during the winter (weak
upwelling season) nitrate in the surface water typically decreases to <0.1 μM, and
nitrogen limitation of phytoplankton growth is likely. Surface chlorophyll is higher in
the equatorial Atlantic, but vertically integrated chlorophyll is similar in the two
oceans. Primary productivity and the assimilation ratio also appear to be similar, but
they both show significant temporal and spatial variability that may not be adequately
sampled for precise comparisons.
Table 11.7 Comparison of eastern equatorial Pacific and equatorial Atlantic upwelling
regions.
Phytoplankton in both equatorial Atlantic and Pacific are dominated by
picoplankton, and heterotrophic protists are the dominant grazers. Mesozooplankton
standing stocks are also similar. Export of particulate organic carbon is approximately
the same, but variability is higher in the Atlantic, possibly due to spatial or seasonal
differences.
(^) In a series of eight AMT cruises, Tyrrell et al. (2003) found high concentrations of
Trichodesmium spp. between 0° and 15°N at 20°W. The concentrations of these
nitrogen-fixing cyanobacteria were correlated with the presence of a shallow mixed
layer and high estimated deposition of iron to the surface ocean. “Tricho” abundance
was not correlated with temperature, nitrate concentrations, or total dissolved iron in
the seawater. The region where Trichodesmium is abundant coincides with an area of