Nitrogen storage and use of biochemical indices to assess nitrogen deficiency and growth rate in natural plankton populations

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Abstract
Various newly developed indicators of N deficiency, physiological state (approximate growth rate), and N source for growth were measured during five cruises to Dabob Bay, Washington from early spring to summer. Although nitrate and ammonium in the surface layer were depleted early in the spring, the plankton populations never became extremely N deficient, as indicated by high intracellular amino acid/protein ratios. However, growth rates, estimated from protein/DNA or RNA/DNA ratios, were usually low unless nitrate concentrations were high or had recently been high, as indicated by large intracellular nitrate pools or high nitrate reductase activities. High growth rates were observed during the spring bloom or as a result of the sporadic supply of nitrate to the euphotic zone, which was inferred from measurements of biochemical indicators on several cruises after the spring bloom. The sporadic supply of nitrate could account for the lack of N deficiency in these populations and mask diel periodicity in N utilization. These results demonstrate that biochemical indicators can be easily measured in the field and that variations in indicators such as intracellular amino acid/protein, protein/DNA, RNA/DNA ratios, NR activities and intracellular nitrate concentrations are an aid in understanding plankton dynamics.