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

Cellular nutrient ratios are often applied as indicators of nutrient limitation in phytoplankton studies, especially the so-called Redfield ratio. For periphyton, similar data are scarce. We investigated the changes in cellular C:N:P stoichiometry of benthic microalgae in response to different levels and types of nutrient limitation and a variety of abiotic conditions in laboratory experiments with natural inocula. C:N ratios increased with decreasing growth rate, irrespective of the limiting nutrient. At the highest growth rates, the C:N ratio ranged uniformly around 7.5. N:P ratios <13 indicated N limitation, while N:P ratios > 22 indicated P limitation. Under P limitation, 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 benthic microalgae, which is slightly higher than the Redfield ratio (106:16:1) considered typical for optimally growing phytoplankton. The optimal ratio was stable against changes in abiotic conditions. In conclusion, cellular nutrient ratios are proposed as an indicator for nutrient status in periphyton.