Tions of phytoplankton

The word 'limitation' has been used, variously to explain the control of phytoplankton growth dynamics, the poverty of plankton biomass and the dearth of the supportive nutrients. All non-toxic environments have a finite supportive capacity which is generally based upon the notion that available resources are deployed in the assembly of biomass, ideally in quasi-fixed quotas, up to a maximum, Bmax = K/qo, where = Ki is the steady-state concentration of the ith resource and q0 is the minimum cell quota in the biomass, supposing a uniform, Redfield-type composition, or the minimum cell quota in the biomass of the jth species. In this usage, the limiting capacity is the lowest of the individual supportive capacities, Ki. By implication, the ratios among the components of the cell will show supra-ideal values to the one that is sub-ideal and, thus, biomass limiting (Reynolds, 1992a; Reynolds and Maberly 2002). The capacity limitation by the factor least available relative to demand is the expression of von Liebig's 'Law of the Minimum' (von Liebig, 1840). In practical terms, the identity of the capacity-limiting factor is revealed by the magnitude of the response to its augmentation. Gibson (1971) usefully deduced that a substance is not capacity-limiting if an increase in that factor produces no stimulation to the biomass that can be supported.

Growth dynamics may also limited, in the sense that the rate of biomass elaboration is determined by the rate of resource supply Moreover, the rate-limiting factor is the one upon by whose rate of supply determines the rate of elaboration. Analysing data on the growth of the diatom Asterionella in Windermere over a period of 50 years, Reynolds and Irish (2000) were able to confirm Lund's (1950) view that the biomass capacity was set by the winter concentration of soluble reactive silicon. However they also showed that the rate of its attainment had been phosphorus limited and that the timing of the silicon-limited maximum had advanced over the period of the documented enrichment of available phosphorus in this lake.

'Competition' is used inconsistently by biologists. However, the term 'competitor' is applied by aquatic ecologists, with great consistency, to refer to species that eventually rise, tortoise-like, to a steady-state dominance. Unfortunately in the parlance of terrestrial plant ecologists, a good competitor is dynamic, fast-growing and applicable to Aesop's fabled hare. Mindful of the place that competition theory occupies in ecological and evolutionary theories, it seems important to have robust definitions. In this book, I use 'competition' in the sense of Keddy's (2001) definition as 'The negative effects that one organism has upon another by consuming, or controlling access to, a resource that is limiting in its availability' Thus, a competitive outcome has only transpired if the activities of species 1 denies access to the resources required to nourish the activities of species 2. Being able to grow faster when fully resourced does not, by itself,make species 1 'more competitive' than species 2. It is merely more efficient in converting adequate resources into biomass. On the other hand, the behavioral or physiological flexibility of species 2 to better exploit a critically limiting resource affords a significant competitive advantage over species 1, at such times when that resource limitation is operative.

as fluorapatite and hydroxylapatite, and the amorphous phosphorite. All are forms of calcium phosphate, which has a low solubility in water at neutrality, and the bioavailability of phosphorus in drainage waters tends to be low (Emsley, 1980). Terrestrial plants and the ecosystems of which they are part share analogous problems of phosphorus sequestration. Not surprisingly, forested catchments, especially, remove and accumulate much of the modest quantities of inorganic phosphorus with which they are supplied, leaving little in the export to receiving waters save as organic derivatives of biogenic products. The losses of inorganic phosphorus to water can be greatly enhanced through anthropogenic activities (quarrying, agriculture and tillage and, especially, the treatment of sewage) but the general condition of natural waters draining from any but desert catchments and/or ones with an abundant occurrence of evaporite minerals is to be moderately or severely deficient in inorganic phosphorus (Reynolds and Davies, 2001).

In all its biologically available (or 'bioavail-able') forms, phosphorus occurs in combination with oxygen in the ions of orthophosphoric acid, OP(OH)3 (Emsley, 1980). Orthophosphoric acid itself is a weak tribasic acid and is freely water soluble. The relative proportions of the various anions (PO4-, HPO4- and H2PO-) vary with pH. The hydrogen radicals are all replaceable by metals. The orthophosphates of the alkali metals (except lithium) are also soluble but those of the alkaline earth metals and the transition elements are quite insoluble. Three of these - calcium, aluminium and iron - are especially relevant to the consideration of phosphorus availability and plankton behaviour. The precipitation of calcium phosphate effectively removes orthophosphate ions from solution, in stoichiometric proportions. The bioavailability of orthophosphate ions can be significantly affected through exchange with the hydroxyl ions that are otherwise immobilised, in large, non-stoichiometric numbers, on the surfaces of aluminium oxides; this sorption of the orthophosphate ions effectively renders them biologically unavailable. A similar behaviour characterises the reactions of phosphate with the precipitated hydroxides of iron (and man ganese), although there is the further complication of their redox sensitivities. At redox potentials below +200 mV, the higher-oxidised ion, Fe3+, is reduced to the divalent Fe2+. Whereas the hydrolysis of the trivalent ion leads to the precipitation of insoluble ferric hydroxide, divalent ferrous ions remain in solution. Raising the redox potential favours the opposite reaction (Fe2+ -e ^ Fe3+, although it is usually enhanced by microbial oxidation): the floccular ferric hydroxide precipitate scavenges orthophosphate ions, again in exchange for hydroxyls. At close to neutrality, the orthophosphate ions are substantially immobilised ('occluded') to the extent that they are scarcely any longer available to algal or micro-bial uptake. Only a further change in redox or an increase in the ambient alkalinity of the medium alters this position. The phosphate ions that are released into solution are, potentially, fully bioavailable (Golterman et al., 1969).

Redox-mediated changes in phosphate solubility in sediment water and in limnetic hypolimnia were described over 60 years ago (Einsele, 1936; Mortimer, 1941, 1942). Since then, many of the fears about the impacts of phosphorus enrichment on aquatic ecosystems have continued to be dominated by the renewed bioavailability to phytoplankton of sediment phosphorus. Of course, there still needs to be phytoplankton access to these phosphorus sources. Though the 'release' of orthophosphate to the water seems just as likely an occurrence, most of this should be re-precipitated with ferric iron, once the redox is raised sufficiently (>+200 mV). Under severe reducing conditions (redox potential < -200 mV), however, sulphate ions are reduced to sulphide ions. These readily precipitate with ferrous iron, thus scavenging the water of Fe2+ ions. The consequence then is that, on re-oxidation, the residual iron content will be diminished, less ferric hydroxide will precipitate and less phosphate may be scavenged. High phosphate levels in eutrophic systems may be more influenced by the redox transformations of sulphur than by those of iron.

Certainly, the solubility transformations at high pH and the behaviours of other elements at low redox can have profound effects on the bioavailability of phosphate in natural waters

Table 4.1 Phosphorus-containing fractions in water: nomenclature and availability

Chemical sensitivity and bioavailability

Free orthophosphate ions, some in combination with organic derivatives. Assumed to be freely bioavailable DP + fine colloidal organic material.

Demonstrably bioexhaustible and supposed to be freely bioavailable Phosphorus not in solution or in fine colloids but bound to suspended solids; fraction subdivisble as: Phosphorus moves into solution in irrigating water; most frequently encountered in intact sediments, where it is mainly from the interstitial and is conditionally bioavailable Particle-bound phosphorus, ion exchangeable and conditionally bioavailable Iron-bound phosphorus. Scarcely bioavailable, dependent upon low redox or high pH Iron- and aluminium-bound phosphorus, sensitive to high pH. Otherwise scarcely bioavailable

PP that is not soluble in strong alkali; fraction includes:

Phosphorus in compound with alkaline metals, esp. apatite. Scarcely bioavailable (Organic) PP soluble only in powerful oxidant

(e.g. perchloric acid). Not bioavailable Perchloric acid digestion releases all known combinations of phosphorus. Phosphorus quoted as 'TP' is only partially bioavailable as roughly determined by serial analysis of the above sequence


Dissolved P


Was this article helpful?

0 0

Post a comment