al. (1991) used argon and N 2 to estimate the effects of temperature and bubble
processes on gas exchange rates between surface seawater and the atmosphere in the
subarctic Pacific during the summer. The difference in saturation state between argon
and oxygen indicates the biological component of O 2 supersaturation. Net oxygen
production rates agreed reasonably well with ^15 N estimates of new production for the
summer samplings. To determine seasonal and annual estimates of net oxygen
production, Emerson et al. have used in situ measurements of O 2 and N 2 in surface
waters. Nitrogen gas can be used as an “inert” gas because rates of nitrogen fixation
only change the concentration by ∼0.1%. By measuring temperature, salinity, oxygen,
and total dissolved gas pressure every two hours on a mooring at the Hawaii Ocean
Time series, Emerson et al. (2008) determined a net biological oxygen production in
the surface mixed layer of 4.8 ± 2.7 mol O 2 m−2 yr−1. Emerson and Stump (2010)
used the same method to measure net oxygen production in the subarctic Pacific.
Measurements taken every three hours for nine months on a surface mooring
indicated a mean summertime oxygen production of 24 mmol O 2 m−2 d−1, and very
little net oxygen production during the winter. Net oxygen production can be scaled to
carbon production using a photosynthetic quotient (mol O 2 evolved : mol CO 2
consumed) of 1.45. For both of these studies, net oxygen production rates agreed well
with other estimates of new production, but lack of precision in the estimates of the
physical processes results in an uncertainty of about 40% for the rates of net oxygen
production.
Phosphate
(^) Phosphorus, available in the ocean as phosphate (PO
4
3−), is incorporated in many
biological molecules, for example, nucleic acids, and adenosine di- and triphosphates
(ADP and ATP). Esterification of a phosphate group to a small molecule creates a
diffusible, high-energy “currency” suitable for enzyme binding at sites adjacent to
binding of substrates. Hydrolysis of the ester bond (dephosphorylation) then provides
energy (−62 kJ mol−1) for substrate transformations. Phosphate in the ocean comes
only from rocks, and it is ultimately removed from the ocean by incorporation in
sediments. In the meantime, it cycles as a nutrient much as fixed nitrogen, but without
an informative variety of oxidation states. In highly oligotrophic environments
(central gyres), a significant fraction of dissolved phosphate is in small
organophosphate compounds, and at least some phytoplankton in those regions bear
surface enzymes which can remove phosphate from organophosphorus and
incorporate it.
(^) In general, and despite debates about the matter (Falkowski 1997; Cullen 1999;
Tyrrell 1999), phosphate is the principal limiting nutrient for planktonic