Major Nutrients in the Sea Nitrogen and Phosphorous

The synthesis of organic matter by primary producers can be viewed as a kind of stoichiometric reaction (in the sense that the reactants should be present at some definite proportions to give the final product). The following equation is often used as a reference for plankton production:

IO6CO2 + 9OH2O + 16NO- + PO3-^ Ci06Hi80O54Ni6P + 150O2 (1)

According to the average elemental composition of seagrasses (Duarte, 1990), Eq. (1) could be roughly rewritten:

^ C435H790O305N20P + 522O2 (2)

A. WD. Larkum et al. (eds.), Seagrasses: Biology, Ecology and Conservation, pp. 227-254. © 2006 Springer. Printed in the Netherlands.

to obtain an oxygen release/carbon assimilation molar quotient of 1.2.

This equation will not be discussed fUrther, as it is merely an orientation and disregards some basic facts, such as the (frequent) assimilation of ammonium instead of nitrate or the participation of the other essential elements. But it has the advantage of reminding us that plant growth is, to a certain extent, a process requiring specific 'reagents' at fixed proportions. It also highlights what we can consider the three major nutrients in the sea: carbon dioxide, nitrate (or the reduced form of inorganic nitrogen, ammonium), and phosphate. Inorganic carbon is, at least in bulk concentration, much more abundant than the other two in marine waters. Despite this, the importance of the dissolved inorganic carbon supply and its role in limiting or controlling plant growth is far from being completely elucidated. However, and since this topic is addressed elsewhere in this book (Larkum et al., Chapter 14), nutritional aspects related with carbon will not be further discussed here. Other elements considered as 'macronutrients' in agriculture or terrestrial ecology (K, S, Ca, and Mg) are probably not limiting in the marine environment, given the high concentration at which they occur. In any case, any reports examining their role in seagrass production are not known to us.

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