Growth, nutrient storage, and release of dissolved organic nitrogen by Enteromorpha intestinalis in response to pulses of nitrogen and phosphorus

Enteromorpha intestinalis was subject to nitrogen (N) and phosphorus (P) enrichment supplied in four weekly pulses in a 10:1 molar ratio with six concentrations ranging from no addition to 1000 μM <F>NO3+100</F> μM PO4. The alga reduced inorganic N to very low concentrations (<3.5 μM) across all nutrient additions while a larger proportion of the added P remained in the water at the end of the experiment (up to 20 μM in highest addition). Growth of E. intestinalis increased in proportion to enrichment across the lower four of the six treatments; range of growth was −6 to 60% change from initial wet wt. N concentration in the tissue decreased at the four lower loading rates due to dilution by growth, but increased greatly (from an initial of 2.7–5% dry wt.) in the highest loading rate partially due to lack of growth in this treatment. In contrast, the mass of N in the tissue increased linearly (<F>slope=0.012</F>, <F>r2=0.779</F>, <F>P=0.0001</F>) over all treatments, showing that uptake and storage were not always coupled to growth. Patterns of tissue P concentration and accumulation were similar to N. Significant amounts of dissolved organic N accumulated in the water, especially in the highest N pulses. Net retention of N in algal tissue ranged from 73 to 98% while retention of P ranged from 79 to 88%; the % lowest retention occurred when nutrient pulses were either very high or low. We hypothesize that this opportunistic, bloom-forming alga is adapted to pulses of nutrients, such that, when pulses are of very high concentration, E. intestinalis can delay growth in favor of saving energy to maximize nutrient uptake and storage. [Copyright &y& Elsevier]