The nutrient stoichiometry of benthic microalgal growth: Redfield proportions are optimal

Cellular nutrient ratios are often applied as indicators of nutrient limita tion in phytoplankton studies, especially the so-called Redfield ratio. For periphyton, similar data are scarce. We investigated the changes in cellul ar C:N:P stoichiometry of benthic microalgae in response to different level s and types of nutrient limitation and a variety of abiotic conditions in l aboratory experiments with natural inocula. C:N ratios increased with decre asing growth rate, irrespective of the limiting nutrient. At the highest gr owth rates, the C:N ratio ranged uniformly around 7.5. N:P ratios <13 indic ated N limitation, while N:P ratios > 22 indicated P limitation. Under P li mitation, the C:P ratios increased at low growth rate and varied around 130 at highest growth rates. For a medium with balanced supply of N and P, an optimal stoichiometric ratio of C:N:P = 119:17:1 could be deduced for benth ic microalgae, which is slightly higher than the Redfield ratio (106:16:1) considered typical for optimally growing phytoplankton. The optimal ratio w as stable against changes in abiotic conditions. In conclusion, cellular nu trient ratios are proposed as an indicator for nutrient status in periphyto n.