Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawaii
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ORTHOPHOSPHATE AND DISSOLVED ORGANIC PHOSPHORUS
SUMMARY: Seawater is collected from known depths using CTD- rosette sampling protocols. Subsamples are drawn and stored in acid-washed polyethylene bottles. Soluble reactive phosphorus (SRP) is measured spectrophotometrically following the formation of phosphomolybdic acid. Total dissolved phosphorus (TDP) is measured in a separate sample after exposure to ultraviolet (UV) light and dissolved organic phosphorus (DOP) is estimated by difference.
Phosphorus (P) is one of several macronutrients required for the growth of marine organisms. In open ocean marine ecosystems, P is often present in low and, perhaps, limiting concentrations for microalgal and bacterial populations. Therefore, P is a central element in oceanic biogeochemical cycles.
In seawater, inorganic phosphorus (also referred to as orthophosphate or soluble reactive phosphorus, SRP) occurs chiefly as ions of HPO42-, with a small percentage present as PO43-. Dissolved organic phosphorus (DOP) exists in a variety of forms (primarily P-esters) which result from excretion, decomposition, death and autolysis.
In this analytical procedure, we make direct measurements of SRP and total dissolved phosphorus (TDP). The latter includes all organic and inorganic phosphorus compounds. Bound phophorus is released from organic matter by ultraviolet light (UV) oxidation and the liberated orthophosphate is reacted with an acidified molydate reagent and potassium antimonyl tartrate. The resulting compound, a heteropoly acid (phosphomolybdic acid), is reduced to the intensely colored molybdenum blue by ascorbic acid and measured spectrophotometrically. SRP samples are prepared in the same manner as TDP samples, but without the prior UV oxidation step. DOP is calculated by difference (i.e., TDP-SRP).
Contamination is the primary concern with P determinations. This is particularly true with samples collected from the euphotic zone, where SRP concentrations are extremely low (<0.2 µM). In order to avoid contamination, sample bottles must be meticulously cleaned with dilute HCl and rinsed with deionized distilled water (DDW) before use.
3. Sample Collection and Storage
4. Sample Analysis
Currently, GOFS and WOCE nutrient samples collected during the Hawaii Ocean Time Series cruises are analyzed by the Hawaii Institute of Marine Biology Analytical Facility. Mr. Ted Walsh has provided us with the following procedure for the analysis of phosphate.
Phosphorus analyses are performed on a four-channel Technicon Autoanalyzer II continuous flow system. The automated wet chemistries generally follow the standard methods of seawater analysis as given by Technicon (1973). Slight modifications have been incorporated to achieve the optimum range and sensitivity for each nutrient at concentration levels specific for the Station ALOHA seawaters.
5. Calibration, Data Reduction and Calculations
6. Accuracy and Precision
The detection limit for phosphorus is approximately 0.02 µM with a coefficient of variation for field-collected replicates of 0.3% For DOP the detection limit is 0.02 µM with a coefficient of variation of 1%.