Pp

Water-extractable PP IMRP

NH4Cl-extractable PP

Citrate-dithionate-extractable PP NaOH-reactive P

NH4Cl-P

Na2S2O4-P

NaOH-rP

Non-alkali-reactive PP NaOH-nrP

HCl-reactive P HCl-P

non-HCl-reactive resP residue

HClO4-digestible P TP

Source: Based on Table 1 of Reynolds and Davies (2001), compounded from various sources.

(Lijklema, 1977). On the other hand, the mechanisms favouring increased bioavailability of phosphorus are more active in environments that are already relatively enriched with respect to this and other nutrients. These are not exceptional or uncommon conditions among shallow, enriched lakes. However, the wide acceptance that a majority of lakes and many seas conform to a model of pristine conditions characterised by low phosphorus availability is well justified. Such habitats frequently carry a total-phosphorus concentration (TP, being the aggregate of all dissolved, min eral and biogenic particulate phosphorus in the water) generally under 1-2 |M. Moreover, only a small proportion of this TP may be in solution or be so readily soluble to be measurable by the standard molybdenum-blue method of Murphy and Riley (1962). Most of the balance will already be constituent in pelagic biomass or in non-bioavailable colloids and fine particles.

These various fractions are potentially separable by serial assays, each step using a progressively more aggressive chemical cleavage (see Table 4.1). Prior to these methods being developed, the understanding of phosphorus dynamics was poised between the detection of small amounts of molybdate-reactive phosphorus (MRP), with poor sensitivity, and the MRP content of companion samples after digestion with powerful oxidants, supposedly corresponding to the TP concentration. Neither necessarily affords a clear notion of the supportive capacity of the bioavailable forms (BAP): the MRP content, if reliably measurable at all, is an underestimate of BAP, with some or most of what is available having already been biologically assimilated. The TP determination is always likely to include fractions that are chemically immobilised and, in the short term, biologically inert. The whole issue of what is or is not bioavailable is complex and requires a different approach (see Section 4.3.2). Yet it is perfectly clear, from studies of systems as far apart as Windermere and Lake Michigan, that vernal 'blooms' of planktic diatoms, featuring significant increments in chlorophyll concentration and a > 30-fold increase in the concentrations of cells in suspension take place against only small changes in MRP concentration. As revealed by conventional chemical analyses, these scarcely exceed 0.1 |M (i.e. ~3 mg P m-3: Reynolds, 1992a). Some planktic algae and bacteria, at least, are sufficiently well adapted to gather phosphorus to fund several cell doublings despite chronically low ambient MRP concentrations.

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