Inorganic Nutrients
89
chromatographic isolation of the two hydrocarbons, and their quantitative measurement by a hydrogen-flame analyzer [e.g., Hardy et al. (1968) and Stewart et al. (1967)].
The C 2 H 2 -C 2 H 4 assay is 10 3 to 10 4 times more sensitive than 15N 2 methods. Ethylene
transformation by methane-oxidizing bacteria can occur in some natural systems and
sometimes confounds the method. This metabolism can be evaluated by controls in
which the rate of utilization of added ethylene is determined. Since the needed
instrumentation for the gas chromatography is now relatively simple, portable, and
inexpensive, in situ assays of N 2 fixation are readily possible.
PHOSPHORUS
Intense ecological interest in phosphorus stems from its major role in metabolism in the
biosphere. In comparison to the relatively rich supply of other major nutritional and
structural components of the biota (C, N, 0, S), phosphorus is least abundant and
commonly limits biological productivity in aquatic ecosystems.
The cycling of phosphorus is complex. Most of the phosphorus of fresh waters is in
the particulate phase of living biota, primarily algae. Labile compounds of low
molecular weight are secreted by this particulate phase as a transitory, high-molecular
weight collojdal fraction. Part of this colloidal fraction, as well as a portion of the
phosphorus of the particulate fraction, is lost from the productive zone by sedimentation, and part is hydrolyzed to soluble orthophosphate. The latter may be assimilated
rapidly by the biota; therefore the concentration of orthophosphate at any given time is
usually very low in the tropogenic zone offresh water [cf., Lean (1973)]. For this reason,
concentrations of orthophosphate are not very diagnostic for evaluating phosphorus
dynamics in aquatic ecosystems. Losses of colloidal and particulate phosphorus are
replaced by regeneration of solubilized phosphorus from decomposition; by release of
phosphorus from sediments, macrophytes, algae, animals, and bacteria; and by
phosphorus contained in precipitation (rain and snow) and in influents to lakes and
streams.
Phosphorus occurs in a number of inorganic and organic compounds in both
particulate and dissolved forms [see Strickland and Parsons (1968) for a detailed
discussion]. Differentiation offorms is based on their reactivity with molybdate, ease of
hydrolysis, and particle size. Only two fractions will be discussed here to illustrate
certain analytical techniques commonly used to determine phosphorus in aquatic
ecosystems. The biological significance of many of the phosphorus compounds is still
unclear and under investigation, and current theories are subject to modification.
Soluble Reactive Phosphate-Phosphorus (P0 4 -P)
In this procedure, the filtered water sample is allowed to react with a composite reagent
of molybdate, ascorbic acid, and trivalent antimony. The molybdic acids formed are
then converted by reducing agents to a blue-colored complex [Murphy and Riley
(1962)]. While the conditions used are specific for group Va elements (P and As), the
method cannot discriminate between phosphate (P0 4 -3) and arsenate (As0 4 -3)
compounds. Although arsenate occurs in concentrations much below phosphate in
most natural waters, the former is a common contaminant from pesticide treatment of
terrestrial vegetation. While this method is otherwise specific for P0 4 -P, it can
hydrolyze labile organic compounds containing phosphorus and hence overestimate
biologically available phosphorus.
89
chromatographic isolation of the two hydrocarbons, and their quantitative measurement by a hydrogen-flame analyzer [e.g., Hardy et al. (1968) and Stewart et al. (1967)].
The C 2 H 2 -C 2 H 4 assay is 10 3 to 10 4 times more sensitive than 15N 2 methods. Ethylene
transformation by methane-oxidizing bacteria can occur in some natural systems and
sometimes confounds the method. This metabolism can be evaluated by controls in
which the rate of utilization of added ethylene is determined. Since the needed
instrumentation for the gas chromatography is now relatively simple, portable, and
inexpensive, in situ assays of N 2 fixation are readily possible.
PHOSPHORUS
Intense ecological interest in phosphorus stems from its major role in metabolism in the
biosphere. In comparison to the relatively rich supply of other major nutritional and
structural components of the biota (C, N, 0, S), phosphorus is least abundant and
commonly limits biological productivity in aquatic ecosystems.
The cycling of phosphorus is complex. Most of the phosphorus of fresh waters is in
the particulate phase of living biota, primarily algae. Labile compounds of low
molecular weight are secreted by this particulate phase as a transitory, high-molecular
weight collojdal fraction. Part of this colloidal fraction, as well as a portion of the
phosphorus of the particulate fraction, is lost from the productive zone by sedimentation, and part is hydrolyzed to soluble orthophosphate. The latter may be assimilated
rapidly by the biota; therefore the concentration of orthophosphate at any given time is
usually very low in the tropogenic zone offresh water [cf., Lean (1973)]. For this reason,
concentrations of orthophosphate are not very diagnostic for evaluating phosphorus
dynamics in aquatic ecosystems. Losses of colloidal and particulate phosphorus are
replaced by regeneration of solubilized phosphorus from decomposition; by release of
phosphorus from sediments, macrophytes, algae, animals, and bacteria; and by
phosphorus contained in precipitation (rain and snow) and in influents to lakes and
streams.
Phosphorus occurs in a number of inorganic and organic compounds in both
particulate and dissolved forms [see Strickland and Parsons (1968) for a detailed
discussion]. Differentiation offorms is based on their reactivity with molybdate, ease of
hydrolysis, and particle size. Only two fractions will be discussed here to illustrate
certain analytical techniques commonly used to determine phosphorus in aquatic
ecosystems. The biological significance of many of the phosphorus compounds is still
unclear and under investigation, and current theories are subject to modification.
Soluble Reactive Phosphate-Phosphorus (P0 4 -P)
In this procedure, the filtered water sample is allowed to react with a composite reagent
of molybdate, ascorbic acid, and trivalent antimony. The molybdic acids formed are
then converted by reducing agents to a blue-colored complex [Murphy and Riley
(1962)]. While the conditions used are specific for group Va elements (P and As), the
method cannot discriminate between phosphate (P0 4 -3) and arsenate (As0 4 -3)
compounds. Although arsenate occurs in concentrations much below phosphate in
most natural waters, the former is a common contaminant from pesticide treatment of
terrestrial vegetation. While this method is otherwise specific for P0 4 -P, it can
hydrolyze labile organic compounds containing phosphorus and hence overestimate
biologically available phosphorus.