Nutrient Cycles: Phosphorus, Nitrogen, and Sulfur
Nutrients have been discussed throughout this book in terms of limiting factors for producers, consumers, and decomposers, as electron donors for microbial decomposers, and in a number of other roles. It is clear, then, that nutrients are inextricably linked to almost all other ecological processes. In this chapter we look at the transformations, exchanges, and general dynamics of three nutrients: phosphorus, nitrogen, and sulfur. The dynamics of these three elements illustrate how nutrients interact with sediments and water, and the great significance of oxidation state and of the interactions with organisms.
KeywordsWater Column Salt Marsh Inorganic Nitrogen Nutrient Cycle Overlie Water
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- Orthophosphate refers to any salt of H3PO4, phosphoric acid, which can also be described as 3H2O • P2O5. Water molecules can be removed from orthophosphate; the various resulting condensed compounds are named polyphosphates (Van Wazer, 1958).Google Scholar
- The standard reference for methods of analysis for nutrients in seawater is Strickland and Parsons (1968), which includes procedures for the various forms of phosphorus as well as nitrogen and carbon. A critique of methods of measuring phosphorus is given by Chamberlain and Shapiro (1973), and for measurement under anaerobic conditions see Bray et al. (1973) and Loder et al. (1978) .Google Scholar
- Alkalinity is a measure of the bases that are titratable with strong acid. It can be thought of as the acid-neutralizing capacity. It is measured in milliequivalents, and in seawater is due primarily to the presence of bicarbonate (HCO- 3). Various forms of borate and any other base in seawater add to the alkalinity. Edmond (1970) and Morel and Morgan (1972) discuss advanced methods to determine alkalinity in seawater.Google Scholar
- In this method the reduction of acetylene to ethylene is measured, and the rates are transformed to N2 fixation using a 3 : 1 ratio. This assumes that the enzymes used to attack the triple bonds in acetylene (HCCH) and N2 are the same.Google Scholar
- The cyanobacteria are the principal organisms that fix nitrogen in aquatic environments. Blue-greens are most abundant in lakes with N:P = 29 (Smith 1983). The scarcity of bluegreens in seawater relative to fresh water is curious, since the low supply of nitrogen in seawater would make fixation advantageous. Recent work has shown an abundance of small cyanobacteria in seawater reaching densities of 103-105 cells ml-1 (Waterbury et al., 1980; Johnson and Sieburth, 1979). These small blue-greens, however, do not fix nitrogen (J. Waterbury, personal communication), so the higher fixation of nitrogen in freshwater compared to seawater still remains to be explained. Several possibilities have been suggested. Blue-green bacteria are most effective at obtaining CO2 at low concentrations (high pH) (King, 1970), do best at taking up PO- 4 at high PO- 4 concentrations (Shapiro, 1973), do best in warm waters (Clendenning et al., 1956), and at high densities may secrete enough inhibitory substances to deter growth of other phytoplankton (Keating, 1978). These properties do not seem well suited for life in the sea, where pH is buffered at about 8, nutrient concentrations are low, waters are as a rule colder, and cell densities are lower than in freshwaters. One other possibility is that nitrogen fixation by marine blue-greens is limited by the availability of molybdenum, an element that is part of the enzymes involved in nitrogen fixation. R. Howarth and J. Cole (personal communication) suggest that the abundant sulfate in seawater interferes with the uptake of molybdenum by fixers, and therefore reduces their abundance and the rates of nitrogen fixation in seawater. All these explanations need substantiation.Google Scholar
- † The term “new” nitrogen was first used by Dugdale and Goering (1967) to differentiate allochthonously provided nitrogen from “old” or recycled nitrogen. The idea that these are separable is perhaps applicable only to deep water, since some nitrate is regenerated from sediments in shallow water (Fisher et al. , 1982).Google Scholar
- The increased acidity of rainfall associated with industrial regions is due to the sulfuric and nitric acids released into the air. Freshwater is usually not well buffered and its pH can be changed by external addition of CO2 or other acids; thus acid rain is a serious problem, and many lakes and ponds in North America and Europe have been acidified enough to lose most of their fauna. Acid rain is a far less severe problem in the more strongly buffered seawater.Google Scholar
- The mechanism behind preference for ammonia is not well known. Either actual uptake or reduction by nitrate reductance may be inhibited by high ammonium concentrations. The ultimate reason for the preference for ammonium may be that energy is required to reduce nitrate to the amino form. Uptake of ammonia thus avoids the need to spend energy, since the reduced nitrogen taken up as NH+ 4 can be directly incorporated into proteins.Google Scholar
- Notice that this discussion deals with inputs and outputs to the ecosystem. Internal transformations due to biological activity—for example, benthic regeneration in Narragansett Bay (Table 11–2)—can be larger than inputs or outputs.Google Scholar
- Anaerobic conditions occur in sediments or in the water column; Figure 11–14 is more representative of a coastal environment, where anaerobic sediments are common. In the Black Sea (cf. Fig. 1–10), fjords, and in certain coastal ponds, water below the mixed layers is anaerobic.Google Scholar