Macroand micronutrients

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A variety of nutrients are essential for the growth of phytoplankton (Arrigo 2005). These can be divided into macronutrients such as nitrate and phosphate that are required in relatively high concentrations, and micronutrients such as iron and zinc

Table 8.1 Elements important for oceanic new primary production and their principal supply routes. (Elements that generally limit new production in some areas are in italics. The concentration limiting productivity is not a fixed value as co-limitation is a frequent occurrence during which a low concentration of one nutrient can render the community more sensitive to limitation by another. Silicon does not affect the total annual new production directly but will alter the temporal trend of new production during the growing season (earlier peak in presence of silica). However, the presence or absence ofSi will affect the community structure which will probably affect the remineralization length scale of settling particles. This, in turn, will affect the vertical distribution of nutrients in the mesopelagic, which will then change subsequent new production levels. Note that in contrast to terrestrial ecosystems, carbon (as DIC) is not usually a limiting nutrient.)

Upwelling Cone, range in and mixing of Atmospheric Atmospheric Cone, limiting the oceanic deep water supply as gas supply as dust productivity euphotic zone

Upwelling Cone, range in and mixing of Atmospheric Atmospheric Cone, limiting the oceanic deep water supply as gas supply as dust productivity euphotic zone

Phosphorus

♦♦♦♦

< 0.01 |iM

0.005-2.0 |M

Nitrogen

♦♦♦

< 0.02 |M

0.002-30.0 |M

Silicon

♦♦♦♦

0.2I M

0.05-130 I M

Iron

♦♦♦

♦♦

0.2 nM

0.005-1.0 nM

Zinc

♦♦♦♦♦

0.01-1.0 nM

Carbon

♦♦♦♦

that are required in much smaller quantities. Some elements such as silicon are essential for growth of certain phylogenetic groups (diatoms) but do not necessarily limit overall production. Nutrients are supplied to the euphotic zone by a variety of mechanisms (Table 8.1) all of which are relevant to this discussion.

Macronutrients below the euphotic zone generally occur in a constant 'Redfield ratio' of N: P of 16: 1 (Redfield 1934) and the elemental ratio of particulate matter in surface waters often does not deviate far from this (e.g. Chen et al. 1996). With regard to carbon, the vast majority is DIC but the particulate matter in the surface and subsurface has a ratio of C : N: P of approximately 106: 16 : 1, while dissolved organic matter (DOM) has a ratio of 199:20: 1 (Hopkinson & Vallino 2005). The implication from this is that ifnutrients were provided solely from deep water, and if the settling biogenic particles (or DOM) had the same composition as the upwelled water, sequestration could not be enhanced in any sustained way. However, such simple first-order statements are not precisely correct and second-order effects allow some scope for sequestration by artificial ocean fertilization. For example, as seen from Table 8.1, some nutrients are not associated with carbon such as the nitrogen gas from the atmosphere and furthermore settling particles do not always have a Redfield composition.

The availability of nutrients in the oceans and their means of supply vary considerably from one region to another due largely to differences in physical characteristics. For example, approximately 25 per cent of the ocean surface has consistently high concentrations of macronutrients but still the plant biomass (as defined by chlorophyll) is low. The production in these high-nutrient low-chlorophyll (HNLC) waters is primarily limited by micronutrients especially iron. By contrast, low-nutrient low-chlorophyll (LNLC) waters can be found in the subtropical gyre systems of the oceans. These oligotrophic regions comprise approximately 40 per cent of the ocean surface and are characterized by wind-driven down-welling and a strong thermocline (both of which impede the nutrient supply from deeper water by vertical mixing) and hence exhibit very low surface water nutrient concentrations. To overcome the deficiency of nitrogen, fixation of nitrogen gas (diazotrophy) by cyanobacteria forms a crucial component of the biogeochemical cycle in many of these waters as it provides a major source of available nitrogen. In effect, diazotrophy ultimately prevents the ocean from losing the nitrogen required for photosynthesis (Falkowski 1997; Tyrrell 1999). For phosphorus, there is, however, no alternative supply route and it can therefore be considered as the ultimate limiting macronutrient (Tyrrell 1999). The only sources available to fuel primary production are the stocks in deep water or those supplied from rivers or on airborne dust and unless such sources exist, productivity will cease once local production exhausts the upper ocean pool.

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