Abstract

A series of laboratory experiments was designed to investigate the influence of nitrogen, phosphorus and photon irradiance on the protein to carbohydrate ratio and growth rate of Scenedesmus quadricauda. These parameters responded to nutrient additions and depletions in a predictable manner. As nutrients were added the ratio rose to a maximum of 5, but as nutrients were depleted from the medium the ratio fell to < 1 during long-term experiments. The ratio also showed that diurnal fluctuations in cultures grown under a 12 h light : 12 h dark cycle were similar in magnitude to the long-term changes. The extent of these diurnal changes was dependent upon the nutrient status of the cells. In nutrient-saturated medium the ratio showed maximum change (4.2 to 1.1), but in nutrient-depleted medium the change was small (0.98 to 0.81). In addition to these responses to nutrients, the ratio was also influenced by photon flux. At high photon fluxes the ratio showed a marked diurnal change (0.91 to 4.05), but this change decreased with decreasing photon fluxes. A general relationship existed between growth rate and the protein to carbohydrate ratio for ratios >2. Below this value, however, growth rate was independent of the protein to carbohydrate ratio.

Criteria based upon these laboratory experiments were derived to determine the nutrient status of field populations of phytoplankton. Using these criteria, the phytoplankton of Mount Bold Reservoir were shown to pass from a condition indicating nutrient sufficiency in November to a condition that implied varying degrees of nutrient deficiency for the period December to March. This pattern of nutrient sufficiency based upon protein to carbohydrate ratios agreed well with the results of nutrient-enrichment trials. This pattern would not have been discernible by the rigid application of previously determined critical protein to carbohydrate ratios.