photosynthesis in freshwater systems but not in marine ones. That is because fresh
water is always closely associated with land, and thus is better supplied with iron. Iron
enables nitrogen fixation in cyanophytes, which can then carry on photosynthesis until
phosphate is exhausted. In coastal marine systems, nitrate simply runs out first, which
Tyrrell (1999) shows with a graph based on widely dispersed NO 3 − and PO 4 3−
observations (Fig. 3.15). At low levels, phosphate is often positive when nitrate is at
zero, usually not the opposite.
Fig. 3.15 A scatter plot from a series of global sampling sections (GEOSECS) of
nitrate (NO 3 −) vs. phosphate (PO 4 3−). Some phosphate is usually left when nitrate is
depleted below levels detected by standard techniques.
(^) (After Tyrrell 1999.)
(^) Nitrogen recycling is fast enough for production to continue at reduced levels
without forcing the system to dependence on nitrogen fixation. In oceanic waters
several factors work against nitrogen fixation. Subarctic, subantarctic and seaward
antarctic ecosystems are colder than the adaptive range of nitrogen-fixing
cyanobacteria. Moreover, except for the spring-blooming North Atlantic, nitrogen is
usually not reduced to strongly limiting levels because of iron limitation. The huge
oligotrophic sectors in equatorial belts and the subtropics do have significant levels of
nitrogen fixation, which can be limited by either phosphorus or iron (Hutchins & Fu
2008). Van Mooy and Devol (2008) assessed nutrient limitation in the North Pacific
subtropical gyre by measuring RNA synthesis (which requires a source of
phosphorus) after additions of NH 4 + and PO 4 3−. The NH 4 + additions resulted in
increased rates of RNA synthesis, while PO 4 3− had no effect, suggesting that the