initially part of the JGOFS program, we have substantial data describing the North
Pacific subtropical gyre (NPSG). The JGOFS BATS time-series (Steinberg et al.
2001) and earlier time-series work near Bermuda provide a Sargasso Sea comparison.
The ESTOC time-series in the eastern subtropical North Atlantic (Neuer et al. 2007)
and many process studies in the Sargasso Sea and elsewhere provide additional
insight. HOT (100 km from Oahu) may have been affected somewhat by proximity to
the Hawaiian Rise, and ESTOC may have modest effects from proximity to both the
Canary Islands and Africa. They are useful nevertheless. Comparable ecosystem
features and processes are found in the three southern hemisphere gyres, all of which
have been studied on a more occasional basis.
Basis of Oligotrophy in the Subtropics
(^) The key feature of the subtropical gyres is water-column stability. Examination of
north–south sections of temperature or σt show flat isopleths in the gyres for
thousands of kilometers. The system is said to be “barotropic”. This results from the
anticyclonic direction of gyral flow, which tends to be convergent, piling water in the
center under the tropical sun for stabilizing warming. There is a zone of moderately
strong stratification (Fig. 11.22) from approximately 120 m down to about 1200 m,
the permanent pycnocline. This is a barrier to mixing which ensures very slow
provision of nutrients upward. The surface layer exhibits some temperature cycling
(∼18°C winter to ∼25°C summer) over a seasonal thermocline at about 50–70 m.
Because of water clarity, net photosynthesis is possible to about 125 m (Fig. 11.23)
and nutrients become depleted down to just above that level. Nitrate and phosphate in
surface layers are usually removed to levels undetectable by standard analyses
(sensitive to ∼0.1 μM). However, preanalysis concentration and ultrasensitive
procedures have been invented to measure these macronutrients at nanomolar levels,
albeit with some interference from readily hydrolyzed dissolved organic forms.
Primary production runs mostly on recycled nutrients (f-ratio ≈ 0.05 to 0.1) with some
addition from nitrogen fixation. Subtropical phytoplankton have extreme affinity for
ammonium and carry surface enzymes for stripping phosphate from organic
complexes (e.g. Beversdorf et al. 2010; Duhamel et al. 2010). Since there are always
losses of organic matter to depth, however, the system cannot run entirely on recycled
nutrients. At 125 m or just above is the top of a nutricline, which is established at the
deepest penetration of light that can support photosynthesis to provide energy for
nutrient uptake. The downward increase in available nitrate, phosphate and silicate is
supplied by the slow upward mixing through the permanent pycnocline from high
concentrations at depth (16 μM nitrate in the North Atlantic, 40 μM in the North
Pacific). Thus, mixing rate ultimately sets the rate of primary production.
Fig. 11.22 T, S and (nitrate + >nitrite) = N profiles on 16 March 2006 from the