Methods for orthophosphate determination and the problems of interferences are reviewed.An important group of methods utilizes the phosphomolybdate complex. The complexation step, the reduction step and the extraction step are treated separately and alternative procedures compared.Another group of methods uses ion association complexes; they are primarily used in physiology and not commonly used in water analyses today.Enzymatic methods for orthophosphate analysis in natural waters have been developed lately and are ready for application in selected waterbodies.Flame spectroscopic, fluorometric, gas chromatographic, ion exclusion chromatographic, inductively coupled plasma and other methods are also shortly presented.Radiobiological bioassays for orthophosphate are also available. In conclusion it was emphasized that the most common and reliable technique still is the molybdenum blue method as modified by Murphy & Riley (1962).The need for more specific and sensitive methods is particularly strong in investigations of phosphorus turnover and phosphorus limitation in natural waters. For these purposes the enzymatic phosphatase methods has advantages due to their specificity for orthophosphate and they might offer an alternative to the molybdenum blue method.
Experiments involving low-dose additions of phosphate, ammonium, nitrate and ADP, one by one and in combination, were performed in small (350 litre) in-sit11 enclosures in a moderately acid (pH 5.4) lake. Before manipulation, all large filter-feeding animals were removed by filtration. Phytoplankton responded t o the nutrient additions only when both phosphorus and nitrogen were added, thus indicating a close balance between phosphorus and nitrogen limitntion in the system.Variation of the inorganic nitrogen-source resulted in species-specific responses by phytoplankton. With ammonium as the nitrogen source Merismopedia, tenuissima was favoured, regardless of whether this species was dominant in the phytoplankton community at the beginning of the experiment or not. With nitrate as nitrogen source Peridinium inconspicuum, which was never particularly common a t the beginning of the experiments, was favoured. No other species of phytoplankton present in the bags was able to outcompete these two species as long as inorganic nutrients were added. With ADP tis phosphorus source together with nitrate, a third species, Dictyosphaerium cf. botrytella, was favoured and reached dominance. The zooplankton community remaining in the bags, dominated by rotifers and calanoid nauplii, did not respond to the fertilization-induced increases in the total biomass of phytoplankton.
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