Nutrient limitation bioassay experiments

A more direct indication of nutrient limitation than is available from nutrient ratios can be gained from bioassay experiments. In this procedure a small volume of natural lake water is enclosed and various known concentrations of potentially limiting nutrients are added (145-147). A growth response (usually measured as an increase in biomass) in treatments containing an added nutrient constitutes evidence of limitation by that nutrient. The results of such experiments are available for only a few selected nutrient-poor lakes, however. They indicate a variety of responses, including strong P limitation (148), limitation by P and iron (147), simultaneous N and P limitation in which the two nutrients are so closely balanced that addition of one alone simply leads to limitation by the other (147), and limitation primarily by N (142, 149). No clear pattern of N or P limitation develops from an examination of these few studies. 

The strength of the bioassay approach is that it directly estimates the fraction of natural DOC that can be used by a natural microbial assemblage under defined conditions. However, there are numerous manipulations of water samples during bioassay incubations, and the effects of these manipulations on the measured parameters are not well known. For example, containment of water samples can rapidly alter microbial population structure. Nutrients, rather than carbon, can be limiting for microbial utilization of DOM. Moreover, there are no standard protocols for bioassay experiments. Different indicators of DOM utilization are measured by different investigators, and many of the measured parameters rely on conversion factors that are also quite variable. The extent of DOM utilization also depends upon the duration and temperature of the bioassay experiment. Despite these shortcomings, the bioassay experiment remains the best approach for estimating the bioavailability of DOM. 

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