changes in abundance of the components of the carbonate system in seawater can alter
the rate of photosynthesis. The quantities of fixed nitrogen and phosphorus in the
ocean at great depth are in the rough ratio 16 : 1. The reason is that those elements are
supplied to the deep sea primarily by decay of organic matter descending from above.
Phosphorus and nitrogen are required for production of organic matter in roughly the
Redfield ratios, and since they are often in limited supply (rate of supply < (rate of
demand for maximum growth rates), they partly control rates of phytoplankton
production in the sea. They are known as “major nutrients”. Silicon is also a major
nutrient for diatoms and silicoflagellates. Each of these major nutrients shows
characteristic vertical distributions in oceanic waters (Fig. 3.13) with low to non-
detectable concentrations in the surface water, and increasing concentrations with
depth, but with some differences in the location of the nutriclines (steepest gradients
in concentration) depending on the depth and rates of remineralization processes.
Fig. 3.13 Vertical nitrate profiles from various oceanic regions.
(^) (After Beckmann & Hense 2009.)
(^) In addition, all phytoplankton have small, but important, requirements for a wide
array of other elements. Particularly important are transition metals and common ions.
Growth of phytoplankton could be limited by the supply of any of these. However,
many (Na+, Cl−, SO 4 2−, Mg2+, Ca2+) are present far in excess of need. Thus,
constituent elements important in minor quantities (trace elements) are Fe, Zn, Cu,
Mn, Mo, and Co, termed “minor nutrients”. There is evidence that at least some of
these trace metals must be in organically chelated form before they can be used, but,
on the whole, availability of many trace metals for uptake by cells is proportional to
the free ion activity. Interference of trace metals with one another (some can occupy
and block the uptake sites for the others) is another important aspect of phytoplankton
nutrition. Finally, some of the vitamins needed for human nutrition are required as
growth supplements by many species of phytoplankton. Such vitamins, particularly