Macronutrient element availability

In addition to nitrogen, algae also have an absolute requirement for phosphorus. The principle form of phosphorus available to algae is orthophosphate. Some classes of algae, most notably diatoms, have an absolute requirement for silicate which they use in cell walls and other structures. 

Under the low nutrient summer conditions that follow the spring plankton bloom, the single largest pools of fixed nitrogen and phosphorus in coastal 

Dissolved organic phosphorus can be accessed via the enzyme alkaline phosphatase, and the analysis of levels of this enzyme has shown that DOP utilisation may be quantitatively significant in the coastal zone (Suzumura etal., 1998 Gambin etal., 1999). 

The strength of this sorption is defined by the distribution coefficient, Kd. Values of around 50000Lkg 1 are typical for phosphate implying that the equilibrium between the dissolved and particulate phase is heavily biased 

Very high removal of P is seen in the low salinity reaches of many estuaries (Prastka etal., 1998). However, the fate of the P-enriched material is important, since burial will remove P from the system and particle flushing may result in desorption in more saline waters where the particulate load is lower. One consequence of industrialisation is that not only are the particulate loads higher but also the sedimentation is lower, thus flushing is the likely fate. The desorption of large amounts of P in offshore regions means that the local D AIN DIP ratio may be a poor indicator of the potential limiting factors. 

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We focus on temperate shelf seas because these are not only best studied, but also most subject to anthropogenic influence. Most of our examples are taken from studies in northwest European waters, because it is these that we know best. The aim of this chapter is to review recent improvements in knowledge of the sources and sinks of nutrients in the shelf seas of northwest Europe, and to consider, especially, the influence of ratios of nutrient elements on the floristic composition of the phytoplankton in these seas. We deal mainly with the macronutrient elements nitrogen, phosphorus and silicon, and to some extent with iron. The acronym DAIN (Dissolved Available /norganic Mtrogen) is a convenient way to refer to nitrate, nitrite and ammonium, excluding di-nitrogen which is not available to most phytoplankters. 

Micronutrients - Other elements that are needed by plants in much lower amounts than the macronutrients are called micronutrients or trace elements (Table 2.2). They are as essential to plant growth as the macronutrients because they perform very essential and specific roles, particularly in molecules involved with energy transfer processes, hormones, and enzymes. When deficiencies of micronutrients are properly diagnosed and corrected, spectacular growth increases can occur by making small amounts of the limiting element available to the crop. 

The rate of growth and reproduction of organisms depends not only upon the availability of carbon, water and energy but also upon a variety of essential mineral nutrients. In Chapter 2 we saw that a number of elements are important, such as N in chlorophyll and amino acids, P in ATP and phospholipids, Si in diatom tests and Ca in coccoliths. Some of these essential elements (e.g. N, P, Ca and Si) are generally abundant, and so can be termed macronutrients, whereas others (e.g. Fe and Mg) are required by organisms in only trace amounts and are called micronutrients. 

Abundant supplies of the macronutrients C, H and O are available from water and atmospheric carbon dioxide, while nearly all the other essential elements are supplied via the soil solution supplied by the root system. Commercial fertilisers are used primarily to supply the elements N, P and K. Soils are not usually deficient in Ca, Mg and S, but these elements are in any case present in many types of commercial fertiliser. Fertiliser compounds of leading importance are... 

Stable isotopes are non-radioactive atoms of the same chemical element, which differ only in their number of neutrons [19]. Many elements also have radioactive (non-stable) isotopes. The most commonly used stable isotopes in studies of macronutrient metabolism are (D or deuterium), C, and 0, while Mg, Mg, a, Ca, Ca, Fe, Fe, Zn, and °Zn are the most commonly used stable isotopes for studies of mineral metabolism. The most commonly used radioactive isotopes are and (tritium) [19]. More than 6(X)0 stable isotope-labeled compounds (tracers) are commercially available for use in metabolic studies. Examples for some of these tracers are [1- C] leucine, [1- C, N] leucine, [ring- Hj] phenylalanine, and [6,6]-D2 glucose. It is currently accepted that these compounds have neghgible biological side-effects, which renders them ethically acceptable for use in children [20]. 

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